Section XIII - The Vitamins docx

In the case of several water-soluble vitamins, activation includes phosphorylation thiamine, riboflavin, nicotinic acid, pyridoxine and also may require coupling to purine or pyridine n

Section XIII The Vitamins

Overview

The diet is the source of some 40 nutrients for human beings These classically are divided into energy-yielding dietary components (carbohydrates, fats, and proteins), sources of essential and nonessential amino acids (proteins), essential unsaturated fatty acids (fats), minerals (including

trace minerals), and vitamins (water-soluble and fat-soluble organic compounds) (see Shils et al. , 1999)

Vitamins, despite their diverse chemical composition, can be defined as organic substances that

must be provided in small quantities from the environment because either they cannot be

synthesized de novo in human beings or their rate of synthesis is inadequate for the maintenance of health [e.g., the production of nicotinic acid (niacin) from tryptophan] In most cases, the

environmental source is the diet, but an obvious exception to this general rule is the endogenous synthesis of vitamin D under the influence of ultraviolet light This definition differentiates vitamins

from essential trace minerals, which are inorganic nutrients needed in small quantities It also

excludes the essential amino acids, which are organic substances needed preformed in the diet in

much larger quantities The term vitamin is restricted here to include only organic substances

required for the nutrition of mammals; substances required only by microorganisms and cells in

culture should be defined as growth factors, to prevent scientifically unsound claims for their

therapeutic benefit as vitamins for human beings When the vitamin occurs in more than one

chemical form (e.g., pyridoxine, pyridoxal, pyridoxamine) or as a precursor (e.g., carotene for

vitamin A), these analogs sometimes are referred to as vitamers

Although the individual vitamins differ widely in structure and function, some general statements

do apply Water-soluble vitamins are stored to only a limited extent, and frequent consumption is necessary to maintain saturation of tissues Fat-soluble vitamins can be stored to massive degrees,

and this property confers upon them a potential for serious toxicity that greatly exceeds that of the water-soluble group As consumed, many vitamins are not biologically active and require

processing in vivo In the case of several water-soluble vitamins, activation includes

phosphorylation (thiamine, riboflavin, nicotinic acid, pyridoxine) and also may require coupling to purine or pyridine nucleotides (riboflavin, nicotinic acid) In their major known actions, water-soluble vitamins participate as cofactors for specific enzymes, whereas at least two fat-soluble vitamins, A and D, behave more like hormones and interact with specific intracellular receptors in their target tissues

Vitamin Requirements

Dietary Reference Intakes

In many countries throughout the world, scientific committees periodically assess the evidence about the requirements of the population for individual nutrients In the United States, the Food and Nutrition Board of the Institute of Medicine, National Academy of Sciences, with active

involvement of Health Canada are taking a new approach to the Recommended Dietary Allowances (RDAs) that have been published since 1941 The development of Dietary Reference Intakes (DRIs) expands and replaces the RDA DRIs are a family of reference values that are quantitative estimates

of nutrient intakes designed to be used for planning and assessing diets for healthy people They include RDAs as goals for intake of individuals, but also present three new types of reference

values These include Adequate Intake (AI), the Tolerable Upper Intake Level (UL), and the

Estimated Average Requirement (EAR) (Yates et al. , 1998 )

The Food and Nutrition Board has embarked on a multiyear project to expand the framework for quantitative recommendations regarding nutrient intake, which includes evaluating both nutrients and other food components for impact on health The review goes beyond criteria needed to prevent classical deficiencies and includes review of data related to risk of chronic diseases

Current recommendations for males and females of different ages are summarized in Tables XIII–1, XIII–2, and XIII–3 Table XIII–1 contains RDAs for those nutrients yet to be reviewed by the DRI committee Table XIII–2 contains the newly revised recommended intakes Age groupings have been changed from the earlier RDA publications Finally, Table XIII–3 contains the ULs for the newly revised intakes The RDA for a given nutrient, which is an individual intake goal, represents the intake at which the risk of inadequacy is very small, about 2% to 3% of the population Those with intakes below the recommended allowance will not necessarily develop a deficiency; however, their long-term risk of deficiency rises in proportion to the degree to which the recommended allowance is not met

Intakes at the level of RDAs or AIs would not necessarily be expected to replete an undernourished individual, nor would it be adequate for disease states which lead to increased requirements

Because the DRIs are based on data from the U.S and Canada, they may not apply globally where food and indigenous practices may result in different bioavailability of nutrients

The tolerable upper intake level (ULs) is the highest level of daily intake that is likely to pose no risk of adverse health effects to most individuals ULs are useful because of increased interest in and availability of fortified foods and continued use of dietary supplements

As the standing committee on the scientific evaluation of DRIs of the Food and Nutrition Board completes the review of each set of nutrients, reports are issued For up-to-date information about these reports visit the Food and Nutrition Board home page at http://www.nas.edu/iom/fnb

Federal Regulations on Vitamins and Minerals

The United States Food and Drug Administration (FDA), under the authority of the Federal Food, Drug, and Cosmetic Act, regulates the labeling of vitamin and mineral products sold as foods or drugs The Nutrition Labeling and Education Act of 1990 (NLEA), with the final rules published in

the Federal Register in early 1993, has led to nutrition labeling on virtually all packaged food, new nomenclature for declaring nutrient contents using the term Daily Values (DVs), and a series of

disease-specific health claims The FDA has only limited authority to control the nutrient content of supplements, except those intended for use by children under 12 years of age and by pregnant or lactating women However, because of uniform labeling procedures, the purchaser can determine what proportion of the recommended daily allowance for each nutrient is provided by a given amount of the food

The use of vitamins and other nutrients to treat disease comes under FDA review, either as foods for special dietary use, including food supplements, or as "over-the-counter" or prescription drugs, depending on the purposes for which the product is intended and the claims made for it Nutrient products designed specifically for special application in medical treatment, such as parenteral

solutions for hyperalimentation and so-called medical foods (e.g., defined formula diets), are

evaluated for safety and efficacy, as are "over-the-counter" drugs containing vitamins and minerals.

Dietary supplements are used by more than 50% of the U.S population (Report of the Commission

on Dietary Supplement Labels , 1997) The most commonly used supplements are vitamins and minerals Forty-seven percent of the U.S population takes a vitamin and/or mineral supplement (USDA's 1994–1996 Continuing Survey of Food Intakes by Individuals , 1999) The intense interest

in supplements by consumers and those who market them has put pressure on Congress to keep this area free of regulation The history of supplement regulation shows efforts by the FDA to regulate the potency and combinations of marketed nutrients and Congress taking action to prevent

regulation

The Dietary Supplement Health and Education Act (DSHEA) resulted in substantial deregulation of supplement marketing and the assertions that can be made about their benefits (Bass and Young, 1996) DSHEA broadens the definition of dietary supplements, which includes vitamins and

minerals, and maintains their regulation as foods Thus, a supplement must be safe under the

conditions recommended on the label or under ordinary conditions of use The responsibility for safety is placed on the manufacturer This changes the FDA regulating procedure for supplements

from one of preclearance to policing (see Chapter 3: Principles of Therapeutics).

Range of Intakes of Vitamins and Minerals

Many millions of individuals living in the United States regularly ingest quantities of vitamins vastly in excess of the RDA One reason some people take vitamin supplements is the erroneous belief that such preparations provide extra energy and make one "feel better." This evidence of widespread nutritional self-medication should be kept in mind when taking a medication history from a patient

The use of vitamin supplements is medically advisable in a variety of circumstances where vitamin deficiencies are likely to occur Such situations may arise from inadequate intake, malabsorption, increased tissue needs, or inborn errors of metabolism (see Position of the American Dietetic

Association, 1996) In practice, these causes may overlap, as in the case of the alcoholic, who may have both inadequate food intake and impaired absorption The patient who requires long-term total parenteral nutrition is absolutely dependent on supplemental vitamins added to the infusates

Unfortunately, a serious undersupply of parenteral multivitamin preparations in the United States has made it difficult to meet clinical demand

While gross vitamin deficiencies due to inadequate intakes are encountered in underdeveloped areas

of the world, few florid cases are seen in the United States Ongoing surveillance of dietary intake is conducted periodically by the United States government Mean intakes consistently exceed RDA for several major vitamins (vitamin A, thiamine, riboflavin, niacin, and ascorbic acid) Individuals living below the poverty level, particularly the elderly and ethnic minorities, may have a

substantially greater risk of inadequate intake of some vitamins, especially vitamins A and C

Certain individuals are exposed to deficient intakes of vitamins as a result of eccentric diets, such as food faddism, and the avoidance of food because of anorexia Intakes of vitamins less than those recommended also can occur in subjects on reducing diets and among elderly people who eat little food for economic or social reasons The consumption of excessive amounts of alcohol also can lead to inadequate intakes of vitamins and other nutrients

Malabsorption of vitamins also is seen in various conditions Examples include hepatobiliary and

pancreatic diseases, prolonged diarrheal illness, hyperthyroidism, pernicious anemia, sprue, and intestinal bypass operations Moreover, since a substantial proportion of vitamin K and biotin is synthesized by the bacteria of the gastrointestinal tract, treatment with antimicrobial agents that alter the intestinal bacterial flora inevitably leads to decreased availability of these vitamins.

Increased tissue requirements for vitamins may cause a nutritional deficiency to develop despite the ingestion of a diet that previously had been adequate For example, requirements for some vitamins may be altered by the use of certain antivitamin drugs, such as the interference with the utilization

of folic acid by trimethoprim (see Roe, 1981) Diseases associated with an increased metabolic rate, such as hyperthyroidism and conditions accompanied by fever or tissue wasting, also increase the body's requirements for vitamins

Finally, an increasing number of cases are recorded in which genetic abnormalities lead to an increased need for a vitamin This often is due to an abnormality in the structure of an enzyme for which the vitamin provides a cofactor, leading to a decreased affinity of the abnormal enzyme protein for the cofactor (Scriver, 1973)

The impact of disease on requirements for nutrients may vary according to its phase and intensity The need for therapy with vitamins may change throughout the course of the illness; eventually, cure should be associated with cessation of this therapy

Chapter 63 Water-Soluble Vitamins: The Vitamin B Complex and Ascorbic Acid

Overview

This chapter provides a summary of physiological and therapeutic roles of members of the vitamin

B complex and of vitamin C The vitamin B complex comprises a large number of compounds that differ extensively in chemical structure and biological action They were grouped in a single class because they originally were isolated from the same sources, notably liver and yeast There are traditionally eleven members of the vitamin B complex—namely, thiamine, riboflavin, nicotinic acid, pyridoxine, pantothenic acid, biotin, folic acid, cyanocobalamin, choline, inositol, and

paraaminobenzoic acid Paraaminobenzoic acid is not considered in this chapter, as it is not a true vitamin for any mammalian species but is a growth factor for certain bacteria, where it is a

precursor for folic acid synthesis Although not a traditional member of the group, carnitine also is considered in this chapter because of its biosynthetic relationship to choline and the recent

recognition of deficiency states Folic acid and cyanocobalamin are considered in Chapter 54: Hematopoietic Agents: Growth Factors, Minerals, and Vitamins because of their special function in hematopoiesis Vitamin C is especially concentrated in citrus fruits and thus is obtained mostly from sources differing from those of members of the vitamin B complex

The Vitamin B Complex

Thiamine

Thiamine, or vitamin B1, was the first member of the vitamin B complex to be identified Lack of thiamine produces a form of polyneuritis known as beriberi; this disease became widespread in East Asia in the nineteenth century due to the introduction of steam-powered rice mills, which produced polished rice lacking the vitamin-rich husk A dietary cause for the disease was first indicated in

1880, when Admiral Takaki greatly reduced the incidence of beriberi in the Japanese Navy by adding fish, meat, barley, and vegetables to the sailors' diet of polished rice In 1897, Eijkman, a Dutch physician working in Java where beriberi also was common, showed that fowl fed polished rice develop a polyneuritis similar to beriberi and that it could be cured by adding the rice

polishings (husks) or an aqueous extract of the polishings back into the diet He also demonstrated that rice polishings could cure beriberi in human beings

In 1911, Funk isolated a highly concentrated form of the active factor and recognized that it

belonged to a new class of food factors, which he called vitamines, later shortened to vitamins The

active factor subsequently was named vitamin B1; in 1926 it was isolated in crystalline form by Jansen and Donath, and in 1936 its structure was determined by Williams The Council on

Pharmacy and Chemistry adopted the name thiamine to designate crystalline vitamin B1

Chemistry

Thiamine contains a pyrimidine and a thiazole nucleus linked by a methylene bridge Thiamine functions in the body in the form of the coenzyme thiamine pyrophosphate (TPP) The structures of thiamine and thiamine pyrophosphate are as follows:

The conversion of thiamine to its coenzyme form is carried out by the enzyme thiamine

diphosphokinase, with adenosine triphosphate (ATP) as the pyrophosphate (PP) donor

Antimetabolites to thiamine that inhibit this enzyme have been synthesized The most important of

these are neopyrithiamine(pyrithiamine) and oxythiamine.

Pharmacological Actions

Thiamine is practically devoid of pharmacodynamic actions when given in usual therapeutic doses; even large doses produce no discernible effects Isolated clinical reports of toxic reactions to the long-term parenteral administration of thiamine probably represent rare instances of

hypersensitivity

Physiological Functions

The vitamins of the B complex function in intermediary metabolism in many essential reactions; some of these functions are summarized in Figure 63–1 Thiamine pyrophosphate, the

physiologically active form of thiamine, functions in carbohydrate metabolism as a coenzyme in the decarboxylation of -keto acids such as pyruvate and -ketoglutarate and in the utilization of pentose in the hexose monophosphate shunt; the latter function involves the thiamine

pyrophosphate–dependent enzyme transketolase Several metabolic changes of clinical importance can be related directly to the biochemical action of thiamine In thiamine deficiency, the oxidation

of -keto acids is impaired, and an increase in the concentration of pyruvate in the blood has been used as one of the diagnostic signs of the deficiency state A more specific diagnostic test for

thiamine deficiency is based upon measurement of transketolase activity in erythrocytes (Brin, 1968) The requirement for thiamine is related to metabolic rate and is greatest when carbohydrate

is the source of energy This fact is of practical significance for patients who are maintained by parenteral nutrition and who thereby receive a substantial portion of their calories in the form of dextrose Such patients should be given a generous allowance of the vitamin

Figure 63–1 Some Major Metabolic Pathways Involving Coenzymes Formed from Water-Soluble VItamins (Abbreviations are defined in the text throughout

this chapter.)

Symptoms of Deficiency

Severe thiamine deficiency leads to the condition known as beriberi In Asia, this is due to

consumption of diets of polished rice, which are deficient in the vitamin In Europe and North

America, thiamine deficiency is seen most commonly in alcoholics, although patients with chronic renal failure on dialysis and patients receiving total parenteral nutrition also may be at risk A severe form of acute thiamine deficiency also can occur in infants.

The major symptoms of thiamine deficiency are related to the nervous system (dry beriberi) and to the cardiovascular system (wet beriberi) Many of the neurological signs and symptoms are

characteristic of peripheral neuritis, with sensory disturbances in the extremities, including localized areas of hyperesthesia or anesthesia Muscle strength is lost gradually and may result in wrist-drop

or complete paralysis of a limb Personality disturbances, depression, lack of initiative, and poor memory also may result from lack of the vitamin, as may syndromes as extreme as Wernicke's

encephalopathy and Korsakoff's psychosis (see below).

Cardiovascular symptoms can be prominent and include dyspnea on exertion, palpitation,

tachycardia, and other cardiac abnormalities characterized by an abnormal electrocardiogram

(ECG) (chiefly low R-wave voltage, T-wave inversion, and prolongation of the Q-T interval) and

cardiac failure of the high-output type Such failure has been termed wet beriberi; there is extensive

edema, largely as a result of hypoproteinemia from an inadequate intake of protein or concomitant liver disease together with failing ventricular function

Absorption, Fate, and Excretion

Absorption of the usual dietary amounts of thiamine from the gastrointestinal tract occurs by

carrier-mediated active transport (Said et al. , 1999 ); at higher concentrations, passive diffusion also

is significant (Rindi and Ventura, 1972) Absorption usually is limited to a maximal daily amount of

8 to 15 mg, but this amount can be exceeded by oral administration in divided doses with food Cellular thiamine uptake is mediated by a specific plasma membrane transporter, which recently has been cloned (Diaz et al. , 1999 ; Dutta et al. , 1999 )

In adults, approximately 1 mg of thiamine per day is completely degraded by the tissues, and this is roughly the minimal daily requirement When intake is at this low level, little or no thiamine is excreted in the urine When intake exceeds the minimal requirement, tissue stores are first saturated Thereafter, the excess appears quantitatively in the urine as intact thiamine or as pyrimidine, which arises from degradation of the thiamine molecule As the intake of thiamine is increased further, more of the excess is excreted unchanged

Therapeutic Uses

The only established therapeutic use of thiamine is in the treatment or the prophylaxis of thiamine deficiency To correct the disorder as rapidly as possible, intravenous doses as large as 100 mg per liter of parenteral fluid commonly are used Once thiamine deficiency has been corrected, there is

no need for parenteral injection or the administration of amounts in excess of daily requirements except in instances when gastrointestinal disturbances preclude the ingestion or absorption of

adequate amounts of vitamin

The syndromes of thiamine deficiency seen clinically can range from beriberi through Wernicke's encephalopathy and Korsakoff's syndrome to alcoholic polyneuropathy Because normal

metabolism of carbohydrate results in consumption of thiamine, it has been observed repeatedly that administration of glucose may precipitate acute symptoms of thiamine deficiency in marginally nourished subjects This also has been noted during the correction of endogenous hyperglycemia

Thus, in any individual whose thiamine status may be suspect, the vitamin should be given before or

along with dextrose-containing fluids; all alcoholic patients seen in an emergency room should routinely receive 50 to 100 mg of thiamine The clinical findings depend on the amount of

deprivation Encephalopathy and Korsakoff's syndrome result from severe deprivation, whereas beriberi heart disease occurs in less-deficient subjects; polyneuritis is observed in milder

deprivation The following discussion describes briefly the varieties of thiamine deficiency and their treatment

Alcoholic Neuritis

Alcoholism is the most common cause of thiamine deficiency in the United States Alcoholic

neuritis is caused by an inadequate intake of thiamine Two factors contribute to such inadequate intake in the chronic alcoholic: (1) Appetite usually is poor, so food consumption drops; and (2) a large portion of the caloric intake is in the form of alcohol The symptoms of neurological

involvement in alcoholics are those of a polyneuritis with motor and sensory defects Wernicke's syndrome is an additional serious consequence of alcoholism and thiamine deficiency Certain characteristic signs of this disease, notably ophthalmoplegia, nystagmus, and ataxia, respond rapidly

to the administration of thiamine but to no other vitamin Wernicke's syndrome may be

accompanied by an acute global confusional state that also may respond to thiamine Left untreated, Wernicke's encephalopathy frequently leads to a chronic disorder in which learning and memory are impaired out of proportion to other cognitive functions in the otherwise alert and responsive patient This disorder (Korsakoff's psychosis) is characterized by confabulation, and it is less likely to be reversible once established (Victor et al. , 1971 ) Although the thiamine stores of some patients with Wernicke's encephalopathy are similar to those in patients without neurological findings, it has been found that patients with Wernicke's encephalopathy have an abnormality in the thiamine-dependent

enzyme transketolase (see Haas, 1988) In such instances, marginal concentrations of thiamine

might produce serious neurological damage The prevalence of Wernicke's encephalopathy in Australia decreased following the introduction of thiamine-enriched flour (Harper et al. , 1998 )

Chronic alcoholics with polyneuritis and motor or sensory defects should receive up to 40 mg of oral thiamine daily The Wernicke-Korsakoff syndrome represents an acute emergency that should

be treated with daily doses of at least 100 mg of the vitamin, intravenously

Infantile Beriberi

Thiamine deficiency also occurs as an acute disease in infancy and may run a rapid and fulminating course Although rare in modern societies, infantile beriberi has been a common cause of infant death throughout this century in regions where rice consumption is high It still is of significance in Third World countries and is related to the low content of thiamine in breast milk of thiamine-deficient women The onset consists of loss of appetite, vomiting, and greenish stools, followed by paroxysmal attacks of muscular rigidity Aphonia due to loss of laryngeal nerve function is a

diagnostic feature Signs of cardiac involvement are prominent, and death may occur within 12 to

24 hours unless vigorous treatment is instituted Infants with mild forms of this condition respond to oral therapy with 10 mg of thiamine daily If acute collapse occurs, doses of 25 mg intravenously can be given cautiously, but the prognosis remains poor

Subacute Necrotizing Encephalomyelopathy

This is a fatal inherited disease of children Neuropathological features resemble those of the

Wernicke-Korsakoff syndrome, and clinical features include difficulties with feeding and

swallowing, vomiting, hypotonia, external ophthalmoplegia, peripheral neuropathy, and seizures

Although the syndrome may have multiple causes, the distribution of lesions and the elevated plasma concentrations of pyruvate and lactate suggest a pathogenetic relationship to thiamine;

however, this remains unproven (see Haas, 1988) Some cases appear to be caused by a circulating

inhibitor of the enzyme that synthesizes thiamine triphosphate from thiamine pyrophosphate in the nervous system Metabolic abnormalities also have been found in tissue samples from affected

infants, including defects in pyruvate dehydrogenase and cytochrome c-oxidase (Medina et al. , 1990) Other inborn errors of metabolism that are sensitive to the administration of thiamine also

have been described (see Scriver, 1973).

Cardiovascular Disease

Cardiovascular disease of nutritional origin is observed in chronic alcoholics, pregnant women, persons with gastrointestinal disorders, and those whose diet is deficient for other reasons When the diagnosis of cardiovascular disease due to thiamine deficiency has been made correctly, the response to the administration of thiamine is striking One of the pathognomonic features of the syndrome is an increased blood flow due to arteriolar dilation Within a few hours after the

administration of thiamine, the cardiac output is reduced and the utilization of oxygen begins to return to normal If edema is present and due to myocardial insufficiency, diuresis results after proper therapy However, individuals suffering from a chronic deficiency may require protracted treatment The usual dose of thiamine is 10 to 30 mg three times daily, given parenterally The dosage can be reduced and the patient maintained on oral medication or by dietary management after signs of the deficiency state have been reversed It is emphasized that administration of

glucose may precipitate heart failure in individuals with marginal thiamine status All patients potentially in this category should receive thiamine prophylactically; 100 mg is commonly given intramuscularly or added to the first few liters of intravenous fluid

Gastrointestinal Disorders

In experimental and clinical beriberi, certain symptoms are referable to the gastrointestinal tract On this basis, thiamine has been used uncritically as a therapeutic agent for such unrelated conditions as ulcerative colitis, gastrointestinal hypotonia, and chronic diarrhea Unless the disease being treated

is the direct result of a deficiency of thiamine, the vitamin is not efficacious

Neuritis of Pregnancy

Pregnancy increases the thiamine requirement slightly The neuritis of pregnancy takes the form of multiple peripheral nerve involvement, and the signs and symptoms in well-developed cases

resemble those described in patients with beriberi The problem may occur because of poor intake

of thiamine or in patients with hyperemesis gravidarum Proof that the neuritis is due to thiamine deficiency is gained in those cases in which dramatic clinical improvement follows thiamine

therapy The dose employed is from 5 to 10 mg daily, given parenterally if vomiting is severe.Megaloblastic Anemia

Thiamine-responsive megaloblastic anemia (TRMA) with diabetes mellitus and deafness is an autosomal recessive disease that responds to large doses of thiamine This disorder was shown to be caused by mutations in the plasma membrane–associated thiamine transporter (Diaz et al. , 1999 ; Fleming et al. , 1999 ) Defective thiamine transport in cultured fibroblasts from TRMA patients is associated with decreased cell survival, apparently by enhanced apoptosis (Stagg et al. , 1999 )

History

At various times from 1879 onward, series of yellow-pigmented compounds have been isolated

from a variety of sources and designated as flavins, prefixed to indicate the source (e.g., lacto-,

ovo-, and hepato-) Subsequently it has been demonstrated that these various flavins are identical in chemical composition

In the meantime, water-soluble vitamin B had been separated into a heat-labile antiberiberi factor (B1) and a heat-stable growth-promoting factor (B2), and it was eventually appreciated that

concentrates of so-called vitamin B2 had a yellow color In 1932, Warburg and Christian described a yellow respiratory enzyme in yeast, and in 1933 the yellow pigment portion of the enzyme was identified as vitamin B2 All doubt as to the identity of vitamin B2 and the naturally occurring

flavins was removed when lactoflavin was synthesized and the synthetic product was shown to possess full biological activity The vitamin was designated as riboflavin because of the presence of ribose in its structure

Chemistry

Riboflavin carries out its functions in the body in the form of one or the other of two coenzymes,

riboflavin phosphate, commonly called flavin mononucleotide (FMN), and flavin adenine

dinucleotide (FAD) Their structures are shown above.

Riboflavin is converted to FMN and FAD by two enzyme-catalyzed reactions, shown as Reactions (63–1) and (63–2):

Riboflavin + ATP FMN + ADP (63–1)

FMN + ATP FAD + PP (63–2)

Pharmacological Actions

No overt pharmacological effects follow the oral or parenteral administration of riboflavin

Physiological Functions

FMN and FAD, the physiologically active forms of riboflavin, serve a vital role in metabolism as

coenzymes for a wide variety of respiratory flavoproteins, some of which contain metals (e.g.,

xanthine oxidase)

Symptoms of Deficiency

The features of spontaneous or experimentally produced riboflavin deficiency have been reviewed

by McCormick (1989) Sore throat and angular stomatitis generally appear first Later, glossitis, cheilosis (red, denuded lips), seborrheic dermatitis of the face, and dermatitis over the trunk and extremities occur, followed by anemia and neuropathy In some subjects, corneal vascularization and cataract formation are prominent

The anemia that develops in riboflavin deficiency is normochromic and normocytic and is

associated with reticulocytopenia; leukocytes and platelets are generally normal Administration of riboflavin to deficient patients causes reticulocytosis, and the concentration of hemoglobin returns

to normal Anemia in patients with riboflavin deficiency may be related, at least in part, to

disturbances in folic acid metabolism

The problem in the clinical recognition of riboflavin deficiency is that certain features, such as glossitis and dermatitis, are common manifestations of other diseases, including deficiencies of other vitamins Recognition of riboflavin deficiency also is difficult because it rarely occurs in isolation In nutritional surveys of children in an urban area and of randomly selected hospitalized patients, deficiency of riboflavin was observed frequently, but almost invariably in conjunction with other vitamin deficiencies Riboflavin deficiency has been observed likewise in association with deficiencies of other vitamins in a large proportion of urban alcoholics of low economic status Biochemical evidence of riboflavin deficiency has been observed in newborn infants treated with ultraviolet light for hyperbilirubinemia Breast-fed infants are most susceptible to this problem because of the relatively low riboflavin content in breast milk Assessment of riboflavin status is made by correlating dietary history with clinical and laboratory findings Biochemical tests include evaluation of urinary excretion of the vitamin (excretion of less than 50 g of riboflavin daily is indicative of deficiency) Although concentrations of flavins in blood are not of diagnostic value, an enzyme activation assay that utilizes glutathione reductase from erythrocytes correlates well with riboflavin status (Prentice and Bates, 1981)

Human Requirements

The Recommended Dietary Allowance (RDA) of riboflavin is 1.3 mg/day for men and 1.1 mg/day

for women (see Table XIII–2) Turnover of riboflavin appears to be related to energy expenditure,

and periods of increased physical activity are associated with a modest increase in requirement.Food Sources

Riboflavin is abundant in milk, cheese, organ meats, eggs, green leafy vegetables, and whole-grain and enriched cereals and breads

Absorption, Fate, and Excretion

Riboflavin is absorbed readily from the upper gastrointestinal tract by a specific transport

mechanism involving phosphorylation of the vitamin to FMN [Reaction (63–1); Jusko and Levy, 1975] Here and in other tissues, riboflavin is converted to FMN by flavokinase, a reaction that is sensitive to thyroid-hormone status and inhibited by chlorpromazine and by tricyclic

antidepressants; the antimalarial quinacrine also interferes with the utilization of riboflavin

Riboflavin is distributed to all tissues, but concentrations are uniformly low, and little is stored When riboflavin is ingested in amounts that approximate the minimal daily requirement, only about 9% appears in the urine As the intake of riboflavin is increased above the minimal requirement, a larger proportion is excreted unchanged Boric acid, a common household chemical, forms a

complex with riboflavin and promotes its urinary excretion Boric acid poisoning, therefore, may induce riboflavin deficiency

Riboflavin is present in the feces This probably represents vitamin synthesized by intestinal

microorganisms, since, on low intakes of riboflavin, the amount excreted in the feces exceeds that ingested There is no evidence that riboflavin synthesized by the bacteria in the colon can be

absorbed

Therapeutic Uses

The only established therapeutic application of riboflavin is to treat or prevent disease caused by deficiency Ariboflavinosis seldom occurs in the United States as a discrete deficiency but may accompany other nutritional disorders Specific therapy with riboflavin, 5 to 10 mg daily, should thus be given in the context of treating multiple nutritional deficiencies A recent randomized, controlled trial of high-dose riboflavin (400 mg/day) in patients suffering migraine headaches showed significant reductions in attack frequency and illness days (Schoenen et al. , 1998 )

Nicotinic Acid

History

Pellagra (from the Italian pelleagra, "rough skin") has been known for centuries in countries where

maize is eaten in quantity, notably Italy and in North America In 1914, Funk postulated that the disease was due to dietary deficiency Over the next few years, Goldberger and his colleagues demonstrated conclusively that pellagra could be prevented by increasing the dietary intake of fresh meat, eggs, and milk Goldberger subsequently produced an excellent animal model of human pellagra, "black tongue," by feeding deficient diets to dogs Although initially thought to be a deficiency of essential amino acids, pellagra soon was found to be prevented by a distinct heat-resistant factor in "water-soluble B" vitamin preparations

In 1935, Warburg and associates obtained nicotinic acid amide (nicotinamide) from a coenzyme isolated from the red blood cells of the horse; this stimulated interest in the nutritional value of nicotinic acid Since liver extracts were known to be highly effective in curing human pellagra and canine black tongue, Elvehjem and associates prepared highly active concentrates of liver; in 1937, they identified nicotinamide as the substance that was effective in the treatment of black tongue Proof was established by the demonstration that synthetic nicotinic acid derivatives also were effective in alleviating the symptoms of black tongue and in curing human pellagra Goldberger and Tanner previously had shown that tryptophan could cure human pellagra; this effect later was determined to be due to the conversion of tryptophan to nicotinic acid Goldsmith (1958) produced pellagra experimentally in human beings by feeding a diet deficient in nicotinic acid and

tryptophan

Nicotinic acid also is known as niacin, a term introduced to avoid confusion between the vitamin and the alkaloid nicotine Pellagra now is quite uncommon in the United States, probably as a direct result of supplementation of flour with nicotinic acid since 1939.

Chemistry

Nicotinic acid functions in the body after conversion to either nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) It is to be noted that nicotinic acid occurs in these two nucleotides in the form of its amide, nicotinamide The structures of nicotinic

acid, nicotinamide, NAD, and NADP are shown below, where R= H in NAD and R= PO3H2 in NADP Synthetic analogs with antivitamin activity include pyridine-3-sulfonic acid and 3-acetyl pyridine

used in the treatment of hyperlipoproteinemia (see Chapter 36: Drug Therapy for

Hypercholesterolemia and Dyslipidemia) The important toxic effects of nicotinic acid are generally seen only with these doses

Physiological Functions

NAD and NADP, the physiologically active forms of nicotinic acid, serve a vital role in metabolism

as coenzymes for a wide variety of proteins that catalyze oxidation-reduction reactions essential for tissue respiration The coenzymes, bound to appropriate dehydrogenases, function as oxidants by accepting electrons and hydrogen from substrates and thus becoming reduced The reduced pyridine nucleotides, in turn, are reoxidized by flavoproteins NAD also participates as a substrate in the transfer of ADP-ribosyl moieties to proteins

The metabolic pathway for conversion of nicotinic acid to NAD has been elucidated for a variety of

tissues, including human erythrocytes [See Reactions (63–3), (63–4), and (63–5) below, where

PRPP is 5-phosphoribosyl-1-pyrophosphate NADP is synthesized from NAD according to

Reaction (63–6).] The biosynthesis of NAD from tryptophan is more complicated Tryptophan is converted to quinolinic acid by a series of enzymatic reactions; quinolinic acid is converted to nicotinic acid ribonucleotide, which enters the pathway at Reaction (63–4)

Nicotinic acid + PRPP Nicotinic Acid Ribonucleotide + PP (63–3)

Nicotinic Acid ribonucleotide + ATP Desamido-NAD + PP (63–4)

Desamido-NAD + Glutamine + ATP NAD + Glutamate + ADP + P (63–5)

NAD + ATP NADP + ADP (63–6)

Symptoms of Deficiency

A deficiency of nicotinic acid leads to the clinical condition known as pellagra Pellagra is

characterized by signs and symptoms referable especially to the skin, gastrointestinal tract, and central nervous system, a triad frequently referred to as dermatitis, diarrhea, and dementia, or the

"three D's." Pellagra now occurs most often in the setting of chronic alcoholism, protein-calorie malnutrition, and deficiencies of multiple vitamins An erythematous eruption resembling sunburn first appears on the back of the hands Other areas exposed to light (forehead, neck, and feet) are later involved, and eventually the lesions may be more widespread The cutaneous manifestations are characteristically symmetrical and may darken, desquamate, and scar

The chief symptoms referable to the digestive tract are stomatitis, enteritis, and diarrhea The

tongue becomes very red and swollen and may ulcerate Salivary secretion is excessive, and the salivary glands may be enlarged Nausea and vomiting are common Steatorrhea may be present, even in the absence of diarrhea When present, diarrhea is recurrent and stools may be watery and occasionally bloody

Symptoms referable to the central nervous system are headache, dizziness, insomnia, depression, and impairment of memory In severe cases, delusions, hallucinations, and dementia may appear Motor and sensory disturbances of the peripheral nerves also occur Common laboratory findings include macrocytic anemia, hypoalbuminemia, and hyperuricemia

Biochemical assessment of deficiency is attempted by the measurement of urinary excretion of

methylated metabolites of nicotinic acid (e.g., N-methylnicotinamide) These tests do not provide

unequivocal evidence of deficiency The measurement of nicotinamide in blood and urine has not been shown to be useful in evaluating niacin status In most cases, the diagnosis rests on a

correlation of clinical findings with the response to supplemental nicotinamide

nicotinic acid This conversion rate is reduced in women taking oral contraceptives The minimal requirement of nicotinic acid (including that formed from tryptophan) to prevent pellagra averages 4.4 mg/1000 kcal The RDA of niacin is 14 and 16 mg/day for women and men, respectively (see Table XIII–2)

The relationship between the nicotinic acid requirement and the intake of tryptophan has helped to explain the historical association between the incidence of pellagra and the presence of large

amounts of corn in the diet Corn protein is low in tryptophan, and the nicotinic acid in corn and other cereals is largely unavailable When cornmeal provides the major portion of dietary protein, pellagra will develop at levels of intake of nicotinic acid that would be adequate if the dietary protein contained more tryptophan Intake of animal protein is high among Americans; tryptophan thus helps significantly to meet the daily requirement for niacin

Food Sources

Nicotinic acid is obtained from liver, meat, fish, poultry, whole-grain and enriched breads and cereals, nuts, and legumes Tryptophan as a precursor is provided by animal protein, in particular.Absorption, Fate, and Excretion

Both nicotinic acid and nicotinamide are absorbed readily from all portions of the intestinal tract, and the vitamin is distributed to all tissues When therapeutic doses of nicotinic acid or its amide are administered, only small amounts of the unchanged vitamin appear in the urine When extremely high doses of these vitamins are given, the unchanged vitamin represents the major urinary

component The principal route of metabolism of nicotinic acid and nicotinamide is by the

formation of N-methylnicotinamide, which, in turn, is metabolized further.

Therapeutic Uses

Nicotinic acid, nicotinamide, and their derivatives are used for prophylaxis and treatment of

pellagra In the acute exacerbations of the disease, therapy must be intensive The recommended oral dose is 50 mg, given up to ten times daily If oral medication is impossible, intravenous

injection of 25 mg is given two or more times daily Pellagra may occur in the course of two

metabolic disorders In Hartnup's disease, intestinal and renal transport of tryptophan is defective

In some patients with carcinoid tumors, large amounts of tryptophan are utilized by the tumor for the synthesis of 5-hydroxytryptophan and 5-hydroxytryptamine (serotonin)

The response to nicotinic acid or its derivatives is dramatic Within 24 hours, the fiery redness and swelling of the tongue disappear and sialorrhea diminishes Associated oral infections heal rapidly Other infections of mucous membranes also disappear Nausea, vomiting, and diarrhea may stop within 24 hours, and at the same time the patient is relieved of epigastric distress, abdominal pain, and distention Appetite also improves Mental symptoms are quickly relieved, sometimes

overnight Confused patients become mentally clear, and those who are delirious become calm, adjusted to their environment, and remember with insight the events of their psychotic state So specific are nicotinic acid and its derivatives in this regard that they can be used as diagnostic agents in patients with frank psychoses but with questionable additional evidence of pellagra Large doses of niacin are recommended, especially when the psychosis is associated with encephalopathy The dermal lesions blanch and heal, but this occurs more slowly The vitamin has less effect on cutaneous lesions that are moist, ulcerated, or pigmented The porphyrinuria associated with

pellagra also disappears

Pellagra may be complicated by thiamine deficiency with associated peripheral neuritis This

complication does not respond to nicotinic acid or its congeners and must be treated with thiamine Many pellagrins also benefit from additional therapy with riboflavin and pyridoxine

In gram doses, nicotinic acid lowers circulating concentrations of low-density-lipoprotein

cholesterol and triglycerides, plasma fibrinogen, and lipoprotein(a) Nicotinic acid therefore is used

in the management of hyperlipoproteinemias (see Chapter 36: Drug Therapy for

Hypercholesterolemia and Dyslipidemia)

Nicotinamide has shown promise in the primary prevention of type I diabetes mellitus in high-risk individuals (Elliott et al. , 1996 ; Lampeter et al. , 1998 ) Large population-based intervention trials currently are in progress

Pyridoxine

History

In 1926, dermatitis was produced in rats by feeding a diet deficient in vitamin B2 However, in 1936 György distinguished from vitamin B2 the water-soluble factor whose deficiency was responsible for the dermatitis and named it vitamin B6 The structure of the vitamin was elucidated in 1939 Several related natural compounds (pyridoxine, pyridoxal, pyridoxamine) have been shown to possess the same biological properties, and therefore all should be called vitamin B6 However, the

Council on Pharmacy and Chemistry has assigned the name pyridoxine to the vitamin.

Chemistry

The structures of the three forms of vitamin B6—that is, pyridoxine, pyridoxal, and pyridoxamine—are shown below

The compounds differ in the nature of the substituent on the carbon atom in position 4 of the

pyridine nucleus: a primary alcohol (pyridoxine), the corresponding aldehyde (pyridoxal), an

aminoethyl group (pyridoxamine) Each of these compounds can be utilized readily by mammals after conversion in the liver to pyridoxal 5'-phosphate, the active form of the vitamin

Antimetabolites to pyridoxine have been synthesized and are capable of blocking the action of the

vitamin and producing signs and symptoms of deficiency The most active is 4-deoxypyridoxine, for which the antivitamin activity has been attributed to the formation in vivo of 4-deoxypyridoxine-5-

phosphate, a competitive inhibitor of several pyridoxal phosphate–dependent enzymes

Isonicotinic acid hydrazide (isoniazid ; see Chapter 48: Antimicrobial Agents: Drugs Used in the

Chemotherapy of Tuberculosis, Mycobacterium avium Complex Disease, and Leprosy ), as well as other carbonyl compounds, combines with pyridoxal or pyridoxal phosphate to form hydrazones; as

a result, it is a potent inhibitor of pyridoxal kinase Enzymatic reactions in which pyridoxal

phosphate participates as a coenzyme also are inhibited, but only by much greater concentrations than those required to inhibit the formation of pyridoxal phosphate Isoniazid thus appears to exert its antivitamin B6 effect primarily by inhibiting the formation of the coenzyme form of the vitamin.Pharmacological Actions

Pyridoxine has low acute toxicity and elicits no outstanding pharmacodynamic actions after either oral or intravenous administration However, neurotoxicity may develop after prolonged ingestion

of as little as 200 mg of pyridoxine per day (Schaumberg et al. , 1983 ; Parry and Bredesen, 1985).Physiological Functions

As a coenzyme, pyridoxal phosphate is involved in several metabolic transformations of amino acids—including decarboxylation, transamination, and racemization—as well as in enzymatic steps

in the metabolism of sulfur-containing and hydroxy-amino acids In the case of transamination, enzyme-bound pyridoxal phosphate is aminated to pyridoxamine phosphate by the donor amino acid, and the bound pyridoxamine phosphate is then deaminated to pyridoxal phosphate by the acceptor -keto acid Vitamin B6 also is involved in the metabolism of tryptophan A notable

reaction is the conversion of tryptophan to 5-hydroxytryptamine In vitamin B6–deficient human beings and in animals, a number of metabolites of tryptophan are excreted in abnormally large quantities The measurement of these urinary metabolites, particularly xanthurenic acid, following loading with tryptophan is used as a test of vitamin B6 status The conversion of methionine to

cysteine also is dependent on the vitamin.

Interactions with Drugs

Biochemical interactions occur between pyridoxal phosphate and certain drugs and toxins The relationship with isoniazid has been discussed above Prolonged use of penicillamine can cause deficiency of vitamin B6 The drugs cycloserine and hydralazine are also antagonists of the vitamin, and administration of vitamin B6 reduces the neurological side effects associated with the use of these compounds Vitamin B6 enhances the peripheral decarboxylation of levodopa and reduces its

effectiveness for the treatment of Parkinson's disease (see Chapter 22: Treatment of Central

Nervous System Degenerative Disorders)

Convulsive seizures may occur when human beings are maintained on a diet deficient in

pyridoxine, and these seizures can be prevented by the vitamin The induction of convulsive

seizures by pyridoxine deficiency may be the result of a lowered concentration of

gamma-aminobutyric acid; glutamate decarboxylase, a pyridoxal phosphate–requiring enzyme, synthesizes

this inhibitory central nervous system (CNS) neurotransmitter (see Chapter 12: Neurotransmission

and the Central Nervous System) In addition, pyridoxine deficiency leads to decreased

concentrations of the neurotransmitters norepinephrine and 5-hydroxytryptamine A peripheral

neuritis associated with carpal synovial swelling and tenderness (carpal tunnel syndrome) has been

attributed in some cases to deficiency of pyridoxine, although earlier claims that high doses of pyridoxine reverse carpal tunnel syndrome have not been confirmed (Smith et al. , 1984 )

Erythropoiesis

Although dietary deficiency of pyridoxine in human beings may cause anemia rarely, the usual pyridoxine-responsive anemia apparently is not due to inadequate supplies of this vitamin as judged

by normal standards This type of anemia is described in Chapter 54: Hematopoietic Agents:

Growth Factors, Minerals, and Vitamins

Human Requirements

The requirement for pyridoxine increases with the amount of protein in the diet The average adult minimal requirement for pyridoxine is about 1.6 mg per day in individuals ingesting 100 g of protein per day (Hansen et al. , 1997 ) The current RDA for pyridoxine has been set at 1.3 mg for

young adult men and women, with modest increases for individuals above 50 years of age (see

Table XIII–2)

Food Sources

Pyridoxine is supplied by meat, liver, whole-grain breads and cereals, soybeans, and vegetables Substantial losses occur during cooking, and pyridoxine is sensitive to both ultraviolet light and oxidation

Absorption, Fate, and Excretion

Pyridoxine, pyridoxal, and pyridoxamine are readily absorbed from the gastrointestinal tract

following hydrolysis of their phosphorylated derivatives Pyridoxal phosphate accounts for at least 60% of circulating vitamin B6 Pyridoxal is thought to be the primary form that crosses cell

membranes The principal excretory product when any of the three forms of the vitamin is fed to human beings is 4-pyridoxic acid, formed by the action of hepatic aldehyde oxidase on free

pyridoxal (see Leklem, 1988).

Therapeutic Uses

Although there is no doubt that pyridoxine is essential in human nutrition, the clinical syndrome of simple pyridoxine deficiency is rare Nevertheless, it may be presumed that an individual with a deficiency of other members of the B complex may also have a deficiency of pyridoxine Therefore, pyridoxine should be a component of therapy for individuals suffering from a deficiency of other members of the B complex On the basis that pyridoxine is essential in human nutrition, it is

incorporated into many multivitamin preparations for prophylactic use

As indicated above, vitamin B6 influences the metabolism of certain drugs and vice versa With considerable justification, vitamin B6 is given prophylactically to patients receiving isoniazid to prevent the development of peripheral neuritis In addition, pyridoxine is an antidote for the seizures and acidosis in patients who have ingested an overdose of isoniazid

The concentration of pyridoxal phosphate is reduced in the blood of women who are pregnant or who are taking oral contraceptives, although the recommended intakes of vitamin B6 appear to be sufficient to meet the requirements of such individuals

Pyridoxine-responsive anemia is a well-documented but uncommon condition The use of the vitamin in this disease is discussed in Chapter 54: Hematopoietic Agents: Growth Factors,

Minerals, and Vitamins A group of genetically determined clinical states of "pyridoxine

dependency," manifested by a requirement for large amounts of the vitamin, include responsive anemias in patients without apparent pyridoxine deficiency, a seizure disorder in infants that responds to the administration of pyridoxine, and those abnormalities characterized by

pyridoxine-xanthurenic aciduria, primary cystathioninuria, or homocystinuria (see Fowler, 1985).

"chick pellagra," it was not cured by nicotinic acid In 1939, Woolley and coworkers and also Jukes demonstrated that the chick antidermatitis factor was pantothenic acid Elucidation of the

biochemical function for the vitamin began in 1947 when Lipmann and coworkers showed that the acetylation of sulfanilamide required a cofactor that contained pantothenic acid

Chemistry

Pantothenate consists of pantoic acid complexed to -alanine This is transformed in the body to phosphopantetheine by phosphorylation and linkage to cysteamine; this derivative is incorporated into either coenzyme A or acyl carrier protein, the functional forms of the vitamin The chemical structures of pantothenic acid and coenzyme A are as follows:

4'-Many analogs of pantothenic acid have been studied in an attempt to find an antimetabolite

Although active antagonists have been synthesized (e.g., -methyl pantothenate) and are of value as

research tools, they are not therapeutic agents

Pharmacological Actions

Pantothenic acid has no outstanding pharmacological actions when it is administered to

experimental animals or normal human beings, even in large doses

Physiological Functions

Coenzyme A serves as a cofactor for a variety of enzyme-catalyzed reactions involving transfer of acetyl (two-carbon) groups; the precursor fragments of various lengths are bound to the sulfhydryl group of coenzyme A Such reactions are important in the oxidative metabolism of carbohydrates, gluconeogenesis, degradation of fatty acids, and the synthesis of sterols, steroid hormones, and porphyrins As a component of acyl carrier protein, pantothenate participates in fatty acid synthesis

Coenzyme A also participates in the posttranslational modification of proteins, including N-terminal

acetylation, acetylation of internal amino acids, and fatty acid acylation Such modifications can

influence the intracellular localization, stability, and activity of the proteins.

Symptoms of Deficiency

Deficiency of pantothenic acid is manifested by symptoms of neuromuscular degeneration and adrenocortical insufficiency By administering a diet devoid of pantothenic acid, a syndrome is produced that is characterized by fatigue, headache, sleep disturbances, nausea, abdominal cramps, vomiting, and flatulence, with complaints of paresthesias in the extremities, muscle cramps, and impaired coordination (Fry et al. , 1976 ) Pantothenic acid deficiency has not been recognized in human beings consuming a normal diet, presumably because of the ubiquitous occurrence of the vitamin in ordinary foods

Human Requirements

Pantothenic acid is a required nutrient, but the magnitude of need is not known precisely There is

no RDA for pantothenic acid The adequate intake is set at 5 mg/day for adults Intakes for other groups are proportional to caloric consumption In view of the widespread distribution of

pantothenic acid in foods, dietary deficiency is very unlikely

Food Sources

Pantothenic acid is ubiquitous It is particularly abundant in organ meats, beef, and egg yolk

However, pantothenic acid is easily destroyed by heat and alkali

Absorption, Fate, and Excretion

Pantothenic acid is absorbed readily from the gastrointestinal tract It is present in all tissues, in concentrations ranging from 2 to 45 g/g Pantothenic acid apparently is not degraded in the human body, since the intake and the excretion of the vitamin are approximately equal About 70% of the absorbed pantothenic acid is excreted in the urine

Therapeutic Uses

No clearly defined uses for pantothenic acid exist, although it commonly is included in

multivitamin preparations and in products for enteral and parenteral alimentation

Biotin

History

In 1916, Bateman observed that rats fed a diet containing raw egg white as the sole source of

protein developed a syndrome characterized by neuromuscular disorders, severe dermatitis, and loss

of hair The syndrome could be prevented by cooking the protein or by administering yeast, liver, or extracts of these In 1936, Kögl and Tönnis isolated from egg yolk a factor in crystalline form that was essential for growth of yeast, which they called biotin It was then demonstrated that biotin and the factor that protected against egg-white toxicity were the same (György, 1940) In 1942,

duVigneaud established the structural formula of biotin, and the vitamin was synthesized shortly thereafter

In the meantime, the nature of the antagonist to biotin in egg white received extensive study The

compound is a protein, first isolated by Eakin and associates in 1940 and called avidin Avidin is a

glycoprotein that binds biotin with great affinity and thus prevents its absorption

Chemistry

Biotin has the following structural formula:

Three forms of biotin, apart from free biotin itself, have been found in natural materials These derivatives are biocytin ( -biotinyl-L-lysine) and the D and L sulfoxides of biotin Although the derived forms of biotin are active in supporting growth of some microorganisms, their efficacy as substitutes for biotin in human nutrition is unknown Biocytin may represent a degradation product

of a biotin-protein complex, since, in its role as a coenzyme, the vitamin is covalently linked to an -amino group of a lysine residue of the apoenzyme involved

A number of compounds antagonize the actions of biotin Among them are biotin sulfone,

desthiobiotin, and certain imidazolidone carboxylic acids The antagonism between avidin and biotin is described above

Symptoms of Deficiency

In most species, presumably owing to synthesis of the vitamin by intestinal bacteria, it is necessary

to eliminate bacteria from the intestinal tract, feed raw egg white, or administer antimetabolites of biotin to produce biotin deficiency In human beings, signs and symptoms of deficiency include dermatitis, atrophic glossitis, hyperesthesia, muscle pain, lassitude, anorexia, slight anemia, and changes in the ECG Spontaneous deficiency has been observed in some individuals who have consumed raw eggs over long periods Inborn errors of biotin-dependent enzymes are known and

respond to the administration of massive doses of biotin (Baumgartner et al. , 1984 ).

Symptomatic biotin deficiency has been reported in children and adults who have received chronic parenteral nutrition lacking biotin; these patients suffered from chronic inflammatory bowel disease, and inadequate synthesis of biotin by gut flora was a probable contributory factor The lesions consist of severe exfoliative dermatitis and alopecia, and they are similar to those of zinc

deficiency; however, they respond to small doses of biotin Few reports have provided biochemical validation of biotin deficiency, but in one case the correction by biotin of an elevated rate of urinary excretion of -hydroxyisovaleric acid indicates defective function of the biotin-dependent -

methylcrotonyl CoA carboxylase (Gillis et al. , 1982 )

Human Requirements

The adequate intake of biotin for adults is 30 g/day (Table XIII–2) The average American diet provides 100 to 300 g of the vitamin Part of the biotin synthesized by the bacterial flora also is available for absorption

Food Sources

Organ meats, egg yolk, milk, fish, and nuts are rich sources of biotin Biotin is stable to cooking but less so in alkali

Absorption, Fate, and Excretion

Ingested biotin is rapidly absorbed from the gastrointestinal tract and appears in the urine

predominantly in the form of intact biotin and in lesser amounts as the metabolites bis-norbiotin and

biotin sulfoxide Mammals are unable to degrade the ring system of biotin

Therapeutic Uses

Large doses of biotin (5 to 10 mg daily) are administered to babies with infantile seborrhea and to individuals with genetic alterations of biotin-dependent enzymes Patients who receive long-term parenteral nutrition should be given vitamin formulations that contain biotin

Choline

Choline is not a vitamin as defined above, although historically it was identified as part of the vitamin B complex Sufficient ambiguity exists concerning a possible dietary requirement for this substance that it customarily is considered in discussions of water-soluble vitamins

metabolism

formation of the neurotransmitter acetylcholine (see Chapter 6: Neurotransmission: The Autonomic

and Somatic Motor Nervous Systems) and the autacoid platelet-activating factor (PAF) (see Chapter 26: Lipid-Derived Autacoids: Eicosanoids and Platelet-Activating Factor)

Phospholipid Constituent

Choline is a component of the major phospholipid lecithin and also is a constituent of

plasmalogens, which are abundant in mitochondria, and of sphingomyelin, which is particularly enriched in brain Choline thus provides an essential structural component of many biological membranes and also of the plasma lipoproteins

Lipotropic Action

As mentioned, the initial recognition of choline as a significant dietary factor depended on its

capacity to reduce the fat content of the liver of diabetic dogs Substances that stimulate removal of excess fat from the liver are known as lipotropic agents and include choline, inositol, methionine, vitamin B12, and folic acid Certain of these compounds appear to act by providing methyl groups for the synthesis of choline in the body Formation of the lipid components of plasma lipoproteins thus is permitted, and this facilitates transport of fat from the liver

Methyl Donor

Choline can donate methyl groups necessary for the synthesis of other compounds The first step in transfer is the formation of betaine, which is the immediate donor of the methyl group Thus,

choline can transfer a methyl group to homocysteine to form methionine The roles of

cyanocobalamin and folic acid in the metabolism of one-carbon compounds are discussed in

Chapter 54: Hematopoietic Agents: Growth Factors, Minerals, and Vitamins

Acetylcholine Formation

Acetylcholine is synthesized from choline and acetyl CoA by choline acetyltransferase and is

broken down by acetylcholinesterase (see Chapter 6: Neurotransmission: The Autonomic and

Somatic Motor Nervous Systems) Choline is transported between the brain and the plasma by a bidirectional system localized in the endothelium of brain capillaries This system operates by facilitated diffusion, and the amount of choline available to central neurons thus varies as a function

of the concentration of choline in the plasma When rats are given choline chloride, the

concentrations of plasma choline, brain choline, and brain acetylcholine increase sequentially These findings may be relevant to the treatment of diseases involving reduced capacity to

synthesize acetylcholine (see below).

Synthesis of PAF

This autacoid is formed from a subset of choline-containing membrane phospholipids in which the moiety in position 1 of the glycerol backbone is an alkyl ether rather than a fatty acid ester The phospholipid is acted upon by the hormonally regulated phospholipase A2 to form 1-O-alkyl-

lysophosphatidyl choline This intermediate is converted to PAF through acetylation at position 2

by acetyl CoA in a reaction catalyzed by lyso-PAF transacetylase PAF has many important

functions in inflammatory and other processes (see Chapter 26: Lipid-Derived Autacoids:

Eicosanoids and Platelet-Activating Factor)

The needs of the tissues for choline are met from both exogenous (dietary) and endogenous

(metabolic) sources Biosynthesis of choline occurs by transmethylation of ethanolamine with the methyl group of methionine or by a series of reactions requiring vitamin B12 and folate as cofactors

(see Chapter 54: Hematopoietic Agents: Growth Factors, Minerals, and Vitamins) Thus, an

adequate supply of methyl-group donors in the diet is desirable to protect against the hepatic

accumulation of lipid In addition, large amounts of choline appear to have a therapeutic effect on certain diseases of the nervous system, perhaps by stimulation of the synthesis of acetylcholine However, none of the functions of choline justifies its classification as a vitamin It has not been shown to act as a cofactor in any enzymatic reaction, and the doses needed to produce therapeutic effects (several grams) are much greater than those of any vitamin

The RDA for choline is 550 mg/day for men and 425 mg/day for women (Table XIII–2) The American diet provides 400 to 900 mg per day of choline as a constituent of lecithin; it is thus difficult to consume a diet that is low in choline However, when excess dietary methionine and folate are not available, choline deficiency may lead to biochemical signs of liver dysfunction, so under this circumstance choline may be considered a limiting nutrient (Jacob et al. , 1999 ) The Committee on Nutrition of the American Academy of Pediatrics (1993) recommends the

fortification of infant formula to at least 7 mg choline/100 kcal, which roughly corresponds to 9 ± 2 mg/dl of choline present in human breast milk

Food Sources

Choline is found in egg yolk, liver, and peanuts, mostly as lecithin.

Absorption, Fate, and Excretion

Choline is absorbed from the diet as such or as lecithin The latter is hydrolyzed by the intestinal mucosa to glycerophosphoryl choline, which passes either to the liver to liberate choline or to the

peripheral tissues via the intestinal lymphatics Free choline is not absorbed fully, especially after

large doses, and intestinal bacteria metabolize choline to trimethylamine Since this compound imparts a strong odor of decaying fish to the feces, lecithin is the preferred oral vehicle for the administration of choline

Therapeutic Uses

The use of choline to treat fatty liver and cirrhosis, usually alcoholic in etiology, has not proven to

be effective Because of the synthesis of choline from other methyl donors, provision of a balanced diet is as effective as choline treatment in alleviating the symptoms of hepatic damage Fatty infiltration of the liver frequently has been observed in patients receiving total parenteral nutrition (TPN) Since TPN solutions generally contain no added choline, a causal relationship between hepatotoxicity and choline deficiency may exist in such patients

well-The use of choline or its related compound, citicoline (cytidine 5'-diphosphate choline ester), in large doses for treatment of nervous system disorders that might be attributed to decreased

acetylcholine synthesis or cholinergic function had been advocated in the past, but in none of these circumstances (tardive dyskinesia, Huntington's chorea, Tourette's disease, Friedreich's ataxia, and

Alzheimer's disease) has a role for choline as a therapeutic agent been firmly established (see

Chapters 20: Drugs and the Treatment of Psychiatric Disorders: Psychosis and Mania and 22: Treatment of Central Nervous System Degenerative Disorders) Results of recent controlled clinical trials suggest favorable responses to citicoline in stroke (Clark et al. , 1997 ) and in conservation of verbal memory with age (Spiers et al. , 1996 ) Confirmatory studies are required, however, before these findings can be embraced

Inositol

History

Although inositol was identified more than one hundred years ago in the urine of diabetic patients, a role for this substance in animal nutrition was not suspected until 1941, when Gavin and McHenry found that inositol had a lipotropic action in rats Inositol subsequently was observed to cure

alopecia induced in rats and mice by dietary means A nutritional role for inositol was strengthened considerably when Eagle and colleagues showed in 1957 that this substance is essential for the growth of all human and other animal cells in tissue culture However, its status as a vitamin for human beings remains uncertain for reasons given below

Ca2+ from intracellular stores.

Human Requirements

There has been no demonstration of a dietary need for inositol in human beings, presumably due to its production by gut bacteria, variable tissue reserves following absorption from foodstuffs, and

possible de novo synthesis in some organs Although a human need for inositol has not been

demonstrated, a high concentration is present in human breast milk As with choline, it may be desirable to add inositol to infant formulas to mimic more closely the content of human milk

(Committee on Nutrition, The American Academy of Pediatrics, 1993)

Food Sources

The normal daily intake of inositol is about 1 g, mostly from fruits and plant sources Inositol is present in whole-grain cereals as the hexaphosphate, phytic acid Inositol in this form is partly available for absorption because of hydrolysis in the intestinal mucosa Inositol also occurs in vegetable and animal foods in other forms

Absorption, Fate, and Excretion

Inositol is absorbed easily from the gastrointestinal tract It is metabolized readily to glucose and is about one-third as effective as glucose in alleviating starvation ketosis The concentration of inositol

in normal human plasma is about 5 mg/liter (28 M) Within tissues, the concentration of inositol is particularly high in heart muscle, brain, and skeletal muscle (1.6, 0.9, and 0.4 g/100 g dry weight, respectively) Urine normally contains only small amounts of inositol, but in diabetic humans and animals the amount is markedly increased, probably because of competition between inositol and glucose for reabsorption by the renal tubule

Therapeutic Uses

Inositol has been given for the management of diseases associated with disturbances in the transport and metabolism of fat, but there is no persuasive evidence that it has therapeutic efficacy Peripheral nerves from diabetic animals and patients contain elevated quantities of free sugars and a decreased

level of myo-inositol; abnormal incorporation of myo-inositol into neural phospholipids also has been demonstrated, but the effects of administration of myo-inositol on diabetic neuropathies are unclear (see Chapter 61: Insulin, Oral Hypoglycemic Agents, and the Pharmacology of the

Endocrine Pancreas) Recent evidence implicates inositol deficiency in the pathogenesis of insulin resistance in the polycystic ovarian syndrome Administration of D-chiro-inositol to such patients improved insulin sensitivity and lowered levels of circulating triglycerides and androgens (Nestler

Carnitine ( -hydroxy- -trimethylammonium buty-rate) has the following structural formula:

Only L-carnitine is synthesized in tissues and possesses biological activity The pathway of

carnitine biosynthesis has been reviewed by Rebouche (1991)

Pharmacological Actions

The administration of L-carnitine to normal individuals has no appreciable effect, and oral doses of

up to 15 g per day are usually well tolerated By contrast, the administration of DL-carnitine can produce a syndrome that resembles myasthenia gravis, presumably because of the inhibitory effects

of the D isomer on the transport and function of L-carnitine

Physiological Functions

In general, carnitine is important for the oxidation of fatty acids; it also facilitates the aerobic

metabolism of carbohydrate, enhances the rate of oxidative phosphorylation, and promotes the excretion of certain organic acids (Rebouche, 1992) These functions result from the following circumstances: (1) There exist a number of carnitine acyltransferases (CATs) that catalyze the interconversion of fatty acid esters of coenzyme A (CoA) and carnitine; these are strategically located in the cytosol and in mitochondrial membranes (2) The esters of CoA and carnitine are thermodynamically equivalent, such that the net formation of either depends solely on the relative

concentrations of reactants (3) Specific translocases exist in mitochondrial and plasma membranes The translocase in mitochondrial membranes readily transports both free carnitine and its esters in either direction, while that in the luminal plasma membrane of renal tubular cells transports only

free carnitine from tubular urine almost exclusively The properties of translocases in the plasma

membranes of other cells are less well defined; nevertheless, free carnitine is actively transported into cells, and acylcarnitines (particularly short-chain esters) are transported out of cells (4) Fatty acid esters of CoA are formed almost exclusively in the cytosol and are not transported across membranes; they also inhibit enzymes of the Krebs cycle and those involved in oxidative

phosphorylation Hence, the oxidation of fatty acids requires the formation of acylcarnitines and their translocation into mitochondria, where the CoA esters are reformed and metabolized If O2

tension becomes limiting, carnitine serves to maintain a ratio of free to esterified CoA within

mitochondria that is optimal for oxidative phosphorylation and for the consumption of acetyl CoA;

in ischemic cardiac or skeletal muscle, this results in reduced formation of lactate and an increased

capacity to perform mechanical work (see Goa and Brogden, 1987).

In the presence of a genetic deficiency of one of the acyl CoA dehydrogenases, carnitine serves to promote the removal of the corresponding organic acid from cells and the blood, since the

acylcarnitine can be transported out of mitochondria and into the circulation but cannot be

reabsorbed from renal tubules Such removal of acylcarnitines from cells or blood carries the risk of producing a state of relative carnitine deficiency

Symptoms of Deficiency

Primary carnitine deficiency is most clearly observed in a group of uncommon inherited disorders Lipid metabolism is severely affected, resulting in storage of fat in muscle and functional

abnormalities of cardiac and skeletal muscle These conditions have been classified as either

systemic or myopathic Systemic disorders are manifested by low concentrations of carnitine in plasma, muscle, and liver Symptoms are variable, but include muscle weakness, cardiomyopathy, abnormal hepatic function, impaired ketogenesis, and hypoglycemia during fasting Myopathic disease is characterized primarily by muscle weakness Fatty infiltration of muscle fibers is

observed at biopsy, and the concentration of carnitine is low; however, plasma concentrations of carnitine are normal (20 to 70 M) Defective transport of carnitine into muscle cells coupled with faulty renal reabsorption may underlie many cases of primary carnitine deficiency (Treem et al. , 1988)

Secondary forms of carnitine deficiency also are recognized These include renal tubular disorders,

in which excretion of carnitine may be excessive, and chronic renal failure, in which hemodialysis may promote excessive losses Patients with inborn errors of metabolism associated with increased circulating concentrations of organic acids also may become deficient in carnitine This

consequence is not surprising in view of the role of carnitine in promoting the excretion of organic acids Occasional patients receiving total parenteral nutrition with solutions lacking carnitine also may show biochemical and symptomatic evidence of carnitine deficiency that is reversed by

supplementation

Human Requirements

The need for carnitine in adults is satisfied by dietary sources and by synthesis, primarily in the liver and kidney However, low-birth-weight and preterm infants are at greatest risk for carnitine deficiency These infants are subjected to high-fat intakes to encourage growth and may benefit

from exogenous carnitine (see Rebouche, 1992) Carnitine is synthesized from lysine residues in

various proteins, beginning with formation of 6-N-trimethyllysine by a sequence of reactions

involving S-adenosylmethionine (see Rebouche, 1991) Four micronutrients are required for the

various enzymatic steps, including ascorbic acid, niacin, pyridoxine, and iron Although carnitine deficiency can be induced by administration of diets that are restricted to cereal grains and other vegetable sources of protein, formal nutritional requirements have not been established

Food Sources

The primary sources of dietary carnitine are meat and dairy products Cereal grains lack carnitine and also may be relatively deficient in lysine and methionine, its amino acid precursors

Absorption, Fate, and Excretion

Dietary L-carnitine is absorbed almost completely from the intestine, largely by a saturable transport mechanism; hence, fractional absorption declines as the oral dose is increased Carnitine is

transported into most cells by an active mechanism; D-carnitine also is transported and can inhibit the uptake of L-carnitine There is little metabolism of carnitine, and most of it is excreted in the

urine as acylcarnitines; renal tubules usually reabsorb more than 90% of unesterified carnitine (see

Goa and Brogden, 1987)

Therapeutic Uses

Carnitine (levocarnitine; CARNITOR) was first approved by the Food and Drug Administration in

1986 as an orphan drug for treatment of primary carnitine deficiency Subsequently, it was

approved for treatment of primary and secondary carnitine deficiencies of genetic origin and for prevention and treatment of carnitine deficiency in patients with end-stage renal disease who are undergoing dialysis One to three grams per day in divided oral doses with meals is adequate for most therapeutic purposes Intravenous doses range from 40 to 100 mg/kg For children, oral L-carnitine is given at 50 to 100 mg/kg per day with meals, up to a maximum of 3 g/day

Primary Carnitine Deficiency

The mainstay of treatment of systemic carnitine deficiency is a high-carbohydrate, low-fat diet Carnitine supplementation of patients with both the myopathic and systemic disorders has been tried frequently, but results have been variable Some patients report dramatic symptomatic and functional benefits following administration of up to 4 g per day, whereas others are not improved The relationship of biochemical changes to symptomatic relief is not predictable All patients with primary carnitine deficiency deserve a trial of supplemental oral carnitine

Renal Disease

Patients receiving chronic hemodialysis can develop skeletal and possibly myocardial muscle

carnitine deficiency Treatment with oral L-carnitine may minimize the degree of deficiency and has been reported to improve symptoms such as muscle weakness and cramps (Bellinghieri et al. , 1983) Carnitine also may improve cardiac function in hemodialysis patients (Fagher et al. , 1985 ), but this use is more controversial

Cardiomyopathies and Ischemic Cardiovascular Disease

Most myocardial energy needs are satisfied by fatty acid oxidation In light of the critical role

played by carnitine in normal cardiac energy metabolism and the development of cardiomyopathy

in established carnitine deficiency states, the possibility that some individuals with primary

cardiomyopathy may suffer carnitine deficiency has provoked great interest Moreover, myocardial ischemia causes depletion of cardiac carnitine and accumulation of long-chain fatty acid esters of CoA and carnitine; the acylcarnitines may be important in the genesis of arrhythmias The

administration of carnitine appears to improve the exercise tolerance of patients with coronary artery disease and may benefit patients with congestive heart failure (Ghidini et al. , 1988 ) Ischemia

in skeletal muscle causes similar disturbances in lipid and carnitine metabolism, and the

administration of carnitine can increase the walking tolerance of patients who suffer from

intermittent claudication (Brevetti et al. , 1999 ) In addition, a number of biochemical and clinical outcomes were improved by L-carnitine supplementation of patients with acute myocardial

infarction (Singh et al. , 1996 ) Although these results are provocative, the therapeutic role of

carnitine in these conditions remains to be confirmed

Ascorbic Acid (Vitamin C)

History

Scurvy, the deficiency disease caused by lack of vitamin C, has been known since the time of the Crusades, especially among northern European populations who subsisted on diets lacking fresh fruits and vegetables over extensive periods of the year The incidence of scurvy was reduced by the introduction of the potato (a source of vitamin C) to Europe in the seventeenth century However, the long sea voyages of exploration in the sixteenth to eighteenth centuries, which were undertaken without a supply of fresh fruits and vegetables, resulted in large numbers of the crews dying from scurvy

A dietary cause for scurvy had long been suspected In 1535, Jacques Cartier learned from the Indians of Canada how to cure the scurvy in his crew by making a decoction from spruce leaves, and several subsequent ship captains prevented or cured scurvy by administration of lemon juice However, a systematic study of the relationship of diet to scurvy had to wait until 1747 when Lind,

a physician in the British Royal Navy, carried out a clinical trial on cases of frank scurvy who were given either cider, vitriol, vinegar, sea water, oranges and lemons, or garlic and mustard Those who received citrus fruits recovered rapidly The consequent introduction of lemon juice into the British Navy in 1800 resulted in a dramatic reduction in the incidence of scurvy; whereas the Royal Naval Hospital at Portsmouth admitted 1457 cases in 1780, only 2 cases were seen there in 1806

The next significant episode in the history of vitamin C was the identification in 1907 of a suitable experimental animal by Holst and Fröhlich, who found that guinea pigs develop scurvy on a diet of oats and bran that is not supplemented with fresh vegetables It was subsequently shown that most mammals synthesize ascorbic acid; human beings, nonhuman primates, the guinea pig, and Indian fruit bats are exceptions The demonstration of scurvy in the guinea pig allowed testing of fractions from citrus fruits for antiscorbutic potency In 1928, Szent-Györgyi isolated a reducing agent in pure form from cabbage and from adrenal glands; in 1932, Waugh and King identified Szent-

Györgyi's compound as the active antiscorbutic factor in lemon juice The chemical structure of this

substance was then soon established in several laboratories, and the trivial chemical name ascorbic acid was assigned to designate its function in preventing scurvy.

The manifestations of scurvy due to vitamin C deficiency also have been revealed following

experimental scurvy induced by intentional dietary restrictions For example, the surgeon Crandon submitted himself to a diet devoid of vitamin C for 161 days; the concentration of ascorbic acid in