The Encyclopedia Of Nutrition And Good Health - N pot

451 N-acetylcysteine NAC A derivative of the sulfur-containing amino acid CYSTEINE that occurs naturally in the body, N-acetylcysteine is used to produce the powerful cell ANTIOXIDANT GL

451

N-acetylcysteine (NAC) A derivative of the

sulfur-containing amino acid CYSTEINE that occurs

naturally in the body, N-acetylcysteine is used to

produce the powerful cell ANTIOXIDANT GLU

-TATHIONE This sulfur compound delivers cysteine

to cells It has been used medicinally to reduce

mucus buildup in people suffering from respiratory

ailments such as chronic bronchitis and asthma,

and adult respiratory distress syndrome NAC may

help reduce the risk of heart attack in patients with

unstable chest pain It is also helpful as an IV

treat-ment in ridding the body of toxic levels of the

nonsteroidal anti-inflammatory drug (NSAID)

acetaminophen

Now also available as a DIETARY SUPPLEMENT,

N-acetylcysteine is promoted as an aid in boosting the

body’s immune system, preventing heart disease

and cancer and slowing the progression of

Parkin-son’s disease and multiple sclerosis Initial studies

indicate N-acetylcysteine shows promise in treating

some or all of the conditions However, additional

research is needed to confirm these preliminary

studies Because N-acetylcysteine is sold as a

dietary supplement for treating these conditions

and not as a drug, its safety and efficacy has not

been tested by the U.S Food and Drug

Administra-tion (FDA) or any other government agency

Breast-feeding women should avoid NAC

Preg-nant women should use it only with a doctor’s

pre-scription

Grandjean, E M., P Berthet, R Ruffmann, et al “Efficacy

of Oral Long-Term N-acetylcysteine in Chronic

Bron-chopulmonary Disease: A Meta-Analysis of Published

Double-Blind, Placebo-Controlled Clinical Trials,”

Clinical Therapy 22 (2000): 209–221.

naphthoquinone See VITAMIN K

narcotics See ADDICTION

National Cholesterol Education Program See CHOLESTEROL

National Marine Fisheries Service Organization that is part of the National Oceanic and Atmos-pheric Association (NOAA), responsible for over-seeing fisheries management in the United States Under the federal Agricultural Marketing Act of

1946, NOAA administers a voluntary Seafood Inspection Program to ensure that seafood sold in the United States complies with applicable food regulations, including the Hazard Analysis and Critical Control Point (HACCP, pronounced “has-sip”) regulations implemented by the U.S Food and Drug Administration (FDA) in 1997 This pro-gram requires seafood processors, repackers, and warehouses that supply seafood for sale in the United States to have in place a food safety pro-gram to identify and eliminate sources of food-borne illnesses The aim of the HACCP system is to prevent problems before they start by conducting spot checks of manufacturing processes and ran-dom testing of seafood products Retailers are exempt from HACCP requirements

National Research Council (NRC) The branch

of the National Academy of Sciences promotes the effective use of scientific information and advises the federal government on scientific and technical matters The NRC provides services for following policies set by both the National Academy of Sci-ences and the National Academy of Engineering in meeting the needs of government, the public, and scientists It is administered by these academies, as well as by the Institute of Medicine

Among the many responsibilities of the NRC is

the development and evaluation of nutrient

stan-dards for good health In 1941, the Food and

Nutrition Board of the NRC published the

Recom-mended Dietary Allowances (RDAs), the

bench-mark of nutritional adequacy in the United States,

with revised editions generally appearing every

five years The 10th edition was published in 1989

Since then the Food and Nutrition Board, in

coop-eration with Health Canada, has been gradually

replacing the RDAs with DIETARY REFERENCE INTAKES

(DRIs), which incorporate more recent science that

has broadened our understanding of the role of

nutrients in human health (See also DIETARY

GUIDELINES FOR AMERICANS; NUTRIENT; VITAMIN.)

National School Lunch Program See SCHOOL

LUNCH PROGRAM

natural flavors See FLAVORS

natural food Foods that have been minimally

processed and grown without the use of synthetic

fertilizers and pesticides Other than for meat and

poultry, the term natural has not been defined

legally and therefore has many interpretations The

“natural” food label is a strong selling point, and

food producers have capitalized on consumer

inter-est Thus the word natural may be used to describe

PROCESSED FOODSand synthetic food and beverages,

which sometimes are highly processed and contain

preservatives and artificial coloring A beverage

labeled “natural” might be completely synthetic,

with only the lemon flavor as a nonsynthetic

ingre-dient; a product labeled “Natural Juice Apple Pie”

may indicate only the juice is natural Natural food

also has the connotation of being free from

addi-tives like PRESERVATIVES, EMULSIFIERS, and

thicken-ers Under current U.S FDAregulations, an entire

food cannot be labeled natural if it contains

syn-thetic ingredients, ARTIFICIAL FLAVORS, or ARTIFICIAL

FOOD COLORS

Natural Meat

According to the USDA, MEAT or poultry labeled

“natural” has not been processed more than in

usual kitchen practice This refers to procedures

that can be performed in the kitchen: washing,

shredding, chipping, grating, grinding, chopping, mixing, and cooking Natural meat must come from animals raised without hormones or at least not exposed to feedlot growth stimulants (such as antibiotics) to fatten livestock later than 60 days before slaughter The “natural beef” label doesn’t guarantee the meat is absolutely free of antibiotics and growth factors because some producers feed hormones and antibiotics up to 60 days before slaughter This time may be inadequate to com-pletely clear chemical residues from some animals (See also FOOD ADDITIVES; ORGANIC FOODS.)

natural sweeteners Naturally occurring CARBO -HYDRATESor compounds related to SUGARSthat are used as sweeteners Carbohydrate sweeteners con-tribute four CALORIES per gram (less than half as much as fat) The most common are the simple sugars or materials that are primarily sugars: FRUC -TOSE and high fructose corn syrup; GLUCOSE (known in the food industry as DEXTROSE); CORN SYRUP; various forms of SUCROSE, including MOLASSES, brown sugar, table sugar, turbinado sugar, and cane sugar; BARLEY MALT; HONEY; fruit concentrates; and ground dates (date sugar) Amasake is a sweetener prepared from fermented rice Each provides carbohydrate with only a few other nutrients; therefore the content of VITAMINS, minerals, and FIBER, as compared to that found in whole foods, is low (EMPTY CALORIES)

Sugar alcohols taste sweet and are not as easily metabolized as sugars MANNITOL, related to the sugar mannose, is poorly utilized XYLITOL, derived from the sugar xylose, while degraded, contributes fewer calories than sugar SORBITOL, related to sor-bose, is the most widely used sugar alcohol and is only slowly absorbed by the intestine Relative to table sugar, fructose is 1.7 times as sweet; glucose, 0.7; mannitol, 0.7; and sorbitol, 0.6

Potential Problems with High Sugar Consumption Undernutrition Sugars supply empty calories because they provide low or negligible amounts of key nutrients CARBOHYDRATE METABOLISMrequires

B vitamins like THIAMIN and minerals like ZINC, CHROMIUM, and MAGNESIUM Excessive concen-trated sweeteners displace more nutritious food

452 National School Lunch Program

and increase the body’s burden for these nutrients.

Few people relying on a junk food diet increase

their consumption of high-quality foods to make

up the difference

Overnutrition Excessive consumption of sugar

and refined carbohydrate represents excessive

calo-ries Surplus calories are converted to FAT,

regard-less of their source It should be noted that starches

and sugars are not in themselves inherently

fatten-ing unless they represent excess calories

Blood Sugar Imbalances The excessive

con-sumption of simple sugars and sucrose can cause

BLOOD SUGAR levels to fluctuate In contrast,

STARCHES in vegetables and legumes are often

slowly digested and glucose is absorbed slowly,

per-mitting a more balanced control of blood sugar

Cavities Excessive sugar consumption leads to

more cavities and contributes to soaring dental costs

U.S DIETARY GUIDELINES FOR AMERICANS call for

eating less sugar and more complex carbohydrate,

starch, and fiber (whole foods) as vegetables, fruits,

and legumes The FOOD GUIDE PYRAMID, the most

recent meal planning guideline, specifies eating

sweets sparingly (See also ADDICTION AND SUGAR;

INVERT SUGAR; MAPLE SYRUP; NUTRIENT DENSITY; TEETH.)

Anderson, G H “Sugars, Sweetness and Food Intake,”

American Journal of Clinical Nutrition 62, supp 1

(1995): 195S–202S.

Wolever, T M., and J B Miller “Sugars and Blood

Glu-cose Control,” American Journal of Clinical Nutrition 62,

supp 1 (1995): 212S–221S.

natural vitamins See VITAMIN

nectar A FRUIT drink made with fruit juice and

fruit pulp Typical nectars are made from APRICOT,

GUAVA, MANGO, PEACH, and other fruit Ingredients

are specified by the STANDARDS OF IDENTITY

estab-lished by the U.S FDA These are standard recipes

long used by the food industry Nectars include

var-ious combinations of fruit juice, SUGAR, and other

natural sweeteners, together with fruit puree, fruit

pulp, VITAMIN C, and CITRIC ACID The term nectar

also refers to a sweet, syrupy fluid secreted by

flow-ers and collected by bees to produce HONEY

nectarine (Prunus persica) A subspecies of PEACH

with a smooth skin and a distinctive flavor The

close similarity between nectarines and peaches is demonstrated by the fact that nectarines occasion-ally mutate to peaches and vice versa Nectarines and peaches are believed to have originated in China There are more than 150 varieties of nec-tarine, differing slightly in taste, shape, size, and skin coloring Fantasia, Summer Grand, Royal Giant and May Grand are popular varieties The United States is a major producer, and California produces most of the domestic crop Hard nec-tarines can be softened by storing them in a paper bag at room temperature for several days The mature fruit does not get very much sweeter, but it can become softer and juicier

Nutrient contents of one raw nectarine, without pit, is 67 calories; protein, 1.3 g; carbohydrate, 16 g; fiber, 3.13 g; fat, 0.6 g; potassium, 288 mg; vitamin

A, 100 retinol equivalents; vitamin C, 7 mg; thiamin, 0.02 mg; riboflavin, 0.06 mg; niacin, 1.35 mg

nephron The functional unit of kidneys that ters the blood The nephron further modifies fil-tered fluid from the blood to produce urine Nephrons are composed of two structures: renal tubules and the glomerulus, a cluster of CAPILLAR -IES WATER, MINERALS, GLUCOSE, VITAMINS, AMINO ACIDS, very small proteins, nitrogenous wastes, and small molecules migrate from the capillaries of the glomerulus and enter the renal tubule, which draws urine from the kidneys

As the filtrate moves down the tubules, nearly

99 percent of the water is reabsorbed into the blood Only 1.5 liters (1 percent leaves as urine daily By reabsorbing most of the materials, the body can retain essential nutrients, while disposing

of wastes like UREA For example, normally all of the glucose filtered by the glomerulus is reab-sorbed, thus, maintaining blood glucose levels So much liquid is filtered by the kidney that if nutri-ents were not reabsorbed back into the blood-stream, the blood would be drained of these materials within half an hour

The region of the tubule adjacent to the capsule (proximal tubule) reabsorbs glucose, amino acids, SODIUM, CHLORIDE, BICARBONATE, POTASSIUM, and water Farther down the tubule, sodium, chloride, urea, and water are reabsorbed Sodium and water are controlled by the hormone ALDOSTERONE The

nephron 453

kidney releases potassium and AMMONIAto

main-tain ELECTROLYTEbalance and water balance of the

blood In order to maintain blood pH close to 7.4

(neutral pH) and to maintain acid-base balance,

sodium is reabsorbed while hydrogen ions are

released

The kidney plays a role in VITAMIN Dmetabolism

as well Tubules of the kidney activate vitamin D by

converting a partially modified form (called

hydroxycholecalciferol) to the fully active

hor-mone, called calciferol (See also GLOMERULAR FIL

-TRATION.)

nervous system A primary communication

sys-tem consisting of the brain, spinal cord, and nerve

fibers radiating to all parts of the body The brain

and spinal cord occupy a central position in the

body and are called the “central nervous system”

(CNS) The nervous system transmits information

via electrical nerve impulses conducted between

nerve cells Impulses are generated by changes in

the body’s internal or external environment

Specialized cells of the nervous system are called

neurons and neuroglia Neuroglia are connective

tissue cells supporting the neurons, while neurons

are specialized cells of the nervous system that

con-duct nerve impulses The nervous system is

com-posed of billions of neurons With age, neurons are

progressively and irreversibly lost

Sensory neurons transmit impulses to the spinal

cord and brain from all regions of the body, while

motoneurons transmit impulses away from the

brain and spinal cord to muscles and glands

Interneurons lie within the gray matter of either

the brain or spinal cord and conduct impulses from

sensory neurons to motoneurons A fatty layer

called MYELIN insulates nerve fibers, bundles of

neurons

Neurons contain a cell body with nucleus, CYTO

-PLASM and MITOCHONDRIA Each neuron possesses

many dendrites, highly branched extensions of the

cell body that are the “receiving” parts of neurons

and conduct a nerve impulse toward the cell body;

and an AXON, a single, long cytoplasmic extension

that conducts nerve impulses away from the cell

body to another cell Axons contain mitochondria

but no protein synthesizing machinery; they vary

in length from a few millimeters up to a meter or

more The ends of axons terminate in bulb-like structures that store NEUROTRANSMITTERS These chemicals determine whether an impulse will pass onto the next cell Nerve fiber refers to any process projecting from the cell body Typically, sensory neurons transmit impulses from the skin, sense organs, muscle joints and viscera to the spinal cord and the brain Motoneurons convey impulses from the brain and spinal cord to either muscles or glands Connecting neurons (intraneurons) carry impulses from sensory neurons to motor neurons and occur in the brain and spinal cord

Many nutrients support nerve function, and the following minerals are critical CALCIUMis required for nerve transmission; when blood calcium levels fall, nerves become hypersensitive, and the result

is frozen muscles (tetany) High calcium concen-trations depress nerve irritability The amount of neurotransmitters (serotonin, acetylcholine, nor-epinephrine) released is proportional to the cal-cium concentration in the nerve terminus POTASSIUM and SODIUM are important in nerve transmission Stimulation of a nerve causes potas-sium to migrate out of the neuron and sodium to migrate inward, altering the membrane potential (electrical current) and conducting a nerve impulse down the axon

Individual nerve cells communicate at special-ized gaps called synapses by means of chemical messages called neurotransmitters When a nerve impulse reaches the end of an axon it triggers the release of neurotransmitters that migrate across the gap and stimulate a nerve impulse or other response in the receiving cell Neurotransmitters are often derived from AMINO ACIDS Dopamine and norepinephrine are derived from the amino acid TYROSINE, while TRYPTOPHAN supplies SERO -TONIN Cholinergic nerve fibers release the neuro-transmitter ACETYLCHOLINE and are derived from the nitrogen-containing nutrient CHOLINE

The peripheral nervous system contains sensory nerves These carry information from organs like the skin to the brain Motor nerve fibers carry impulses from the brain to specific regions such as skeletal muscles

The automatic nervous system adapts the body

to change It consists of two functional parts The sympathetic nervous system responds to stress,

454 nervous system

and adaptations include dilation of bronchi and

increased heart rate The PARASYMPATHETIC NERVOUS

SYSTEMreturns the body to the normal, unstressed

state, counterbalancing the sympathetic nervous

system It restores energy during rest and

recupera-tion

The average human brain weighs about three

lbs (1.4 kg) and contains 11 billion cells Although

it represents only 2 percent of the total body

weight, it consumes 20 percent to 30 percent of the

GLUCOSEfrom digestion and CARBOHYDRATE METAB

-OLISM The thalamus of the forebrain is an

egg-shaped mass that relays sensory information to

other regions of the brain It seems to help with

concentration Lying beneath the thalamus is the

HYPOTHALAMUS, which regulates eating, drinking,

sexual behavior, heat production, and emotions It

also controls the ENDOCRINE SYSTEM, a system of

hormone-secreting glands located throughout the

body

Twelve pairs of nerves are attached at the under

surface of the brain Several are important in

regu-lating eating and digestion Their fibers conduct

pulses between the brain and the head, neck,

tho-rax, and abdominal cavity The olfactory nerve is

responsible for the sense of SMELL The trigeminal

nerve functions in the sensations involving the

face, teeth, and chewing The facial nerve is

responsible for TASTE The glossopharyngeal nerve

controls taste, swallowing, salivation, and

sensa-tions in the throat region The vagus nerve also

regulates swallowing, sensations of throat and

lar-ynx, and of the abdominal regions; for example, it

regulates PERISTALSIS The hypoglossal nerve

regu-lates tongue movements (swallowing)

It is now known that brain peptides, such as

ENDORPHINS, are synthesized in the intestine, while

hormones bind to targets in the brain and affect its

function The nervous system can activate the

immune system, while chemicals released by the

immune response can affect the central nervous

system The gastrointestinal tract is surrounded by

an elaborate network of nerves so that the stomach

and intestines are directly linked to the brain The

ability of the digestive system to monitor food has

important ramifications for WEIGHT MANAGEMENT,

SATIETY, and hunger With the discovery that the

hormonal system (endocrine system) as well as the

immune system can affect the brain and alter behavior and mood, and that the nervous system can alter the immune response and hormone bal-ance, has come a new appreciation of the interde-pendency of these three systems in maintaining a balance among all systems of the body (HOMEOSTA -SIS) This synthesis has spawned a new scientific discipline, psychoneuroimmunology

In addition to supplying energy, primarily as glucose, food can alter the nervous system in important ways One focus has been in the forma-tion of neurotransmitters, chemicals manufactured

by nerves to help transit nerve impulses Two amino acids that serve as the raw materials for the manufacture of neurotransmitters are tryptophan and tyrosine Tryptophan forms the neurotransmit-ter serotonin, used to regulate relaxation and sleepiness Low blood levels of tryptophan are linked to depression, and some clinical studies sug-gest that tryptophan supplementation can lessen depression Tryptophan tends to make normal peo-ple drowsy, and it has been sold as a mild sleep inducer However, the U.S FDAhas banned its sale because of deaths associated with contaminated tryptophan Foods low in protein and rich in car-bohydrate can raise brain levels of tryptophan, probably because they stimulate the release of insulin, which lowers the blood levels of most amino acids, except for tryptophan According to one hypothesis, tryptophan in the blood competes with more abundant, branched chain amino acids, which also possess lipid-like properties, for entry into the brain The amount of tryptophan reaching the brain increases after eating starchy or sweet foods On the other hand the more numerous amino acids can displace tryptophan for entry into the brain after eating a protein-rich meal As a result, brain tryptophan levels may drop The observation is that normal healthy adults can feel relaxed, sleepy, or more calm or lethargic within two hours following a high-carbohydrate meal Another possibility is that glucose from carbohy-drate digestion can affect the brain’s activity The amino acid tyrosine is converted to the neurotransmitter norepinephrine, which helps maintain mental alertness The brain level of norep-inephrine drops with stress Tyrosine supplements may help combat fatigue and improve the ability to

nervous system 455

perform mental tasks A light lunch or snack

increases mental functioning and decreases the

error rate in tasks, and seems to sharpen the mind

Certain B vitamins help in the manufacture of

serotonin and norepinephrine: VITAMIN B6, VITAMIN

B12, and FOLIC ACID Deficiencies of any of these

vit-amins can promote depression, senility, and

decreased ability to concentrate Correction of the

B vitamin deficiency can ease these symptoms

Even in healthy elderly people without obvious

deficiency symptoms, a lower level of folacin and

vitamin B12 has corresponded to lower scores on

reasoning tests Surveys have shown that between

20 percent and 66 percent of healthy people over

the age of 65 possess low levels of vitamin B6,

vit-amin B12, and folic acid (See also ADDICTION; DIGES

-TIVE TRACT; FOOD AND THE NERVOUS SYSTEM; MOUTH.)

Duthie, S J et al “Homocystine, Vitamin B Status, and

Cognitive Function in the Elderly,” American Journal of

Clinical Nutrition 75, no 5 (2002): 908–913.

net protein utilization (NPU) An index used to

estimate the ability of a food PROTEIN to supply

indispensable AMINO ACIDS Only retained nitrogen

is measured The NPU is based on the ratio of the

amount of dietary protein converted to body

pro-tein divided by the amount of propro-tein eaten Thus,

a poorly digested protein would have a low NPU

score In this respect, it differs from the BIOLOGICAL

VALUE, which does not account for digestibility of

food Frequently, proteins are measured for their

growth-promoting effect on young animals or for

their ability to maintain nitrogen equilibrium, a

balance between protein nitrogen consumed and

the amount lost daily (NITROGEN BALANCE) Since

only retained nitrogen is measured, NPU does not

specifically assess the inefficiency of digestion

Thus, a poorly digested protein would have a low

NPU score (See also CHEMICAL SCORE.)

neural tube defects (NTD) Congenital disorders

related to the incomplete development of the

neural tube Neural tube defects contribute to

infant mortality throughout the world

Anen-cephalus is the absence of brain and spinal cord In

spina bifida, the spinal cord is pushed through the

wall of the spinal canal between vertebrae

Encephalocele refers to the protrusion of the brain through a cranial opening Both genetic and envi-ronmental factors are implicated Several studies have confirmed that women who take the B vita-min FOLIC ACIDbefore becoming pregnant and dur-ing the first two months of pregnancy greatly reduce the risk of bearing a child with a neural tube defect Consequently, the Centers for Disease Con-trol and Prevention (CDC) recommends that all women of childbearing age take a multivitamin that contains at least 400 mcg of folic acid daily In

1998 the U.S FDA began requiring all enriched grain products, including cereals, breads, pasta, and rice, to be fortified with folic acid at the rate of 140 mcg per 100 g of grain This action was taken based

on studies that showed that only about 25 percent

of women of childbearing age in the United States regularly consumed enough folic acid In 2001 the CDC reported that the number of children born with these defects had dropped by 19 percent since the enrichment program began (See also BIRTH DEFECTS; HOMOCYSTEINE; VITAMIN.)

Honein, Margaret A “Impact of Folic Acid Fortification in the U.S Food Supply on the Occurrence of Neural

Tube Defects,” JAMA 285 (2001): 2,981–2,986.

neuron See NERVOUS SYSTEM

neuropathy, peripheral Disease of the peripheral nerves (the nerves feeding into the spinal column and brain) Nutritional deficiencies can cause peripheral nerve degeneration Severe deficiency

of the B vitamin THIAMIN, a disease whose chief characteristic is BERIBERI, also leads to neuropathy Certain toxic chemicals like plasticizers and several medications such as Isoniazid also cause neuropa-thy Diseases affecting the entire body like diabetes, certain AUTOIMMUNE DISEASES like Guillain-Barre syndrome, and a variety of rare hereditary diseases are associated with peripheral neuropathy Neu-ropathy can be caused by HEPATITIS, infectious mononucleosis, ALCOHOLISM, and lead poisoning (See also DEFICIENCY DISEASE; HEAVY METALS; NER -VOUS SYSTEM.)

McLeod, J G “Investigation of Peripheral Neuropathy,”

Journal of Neurological and Neurosurgical Psychiatry 58

(1995): 274–283.

456 net protein utilization

neurotransmitter A chemical required to transmit

nerve impulses between nerve cells Unlike an

elec-trical network, nerves are not continuous; the end

of a nerve cell does not touch its target Instead,

individual nerve cells are separated by a microscopic

gap called a synapse In response to a nerve impulse

traveling down the nerve cell, the nerve ending

releases neurotransmitter molecules, which then

diffuse across the gap and bind to the adjacent cell

Depending on the nature of the receiving cells, this

contact can stimulate or inhibit the target cell

At least three classes of neurotransmitters

respond to diet: Catecholamines (DOPAMINE and

norepinephrine) come from the AMINO ACID, TYRO

-SINE Eating a protein-rich meal can increase blood

tyrosine levels and increase levels of this amino

acid in the brain Elevated brain tyrosine increases

dopamine and norepinephrine production,

pro-moting wakefulness and alertness

On the other hand, SEROTONINtends to induce

relaxation and sleep Serotonin comes from the

essential amino acid-TRYPTOPHAN Eating a meal

high in carbohydrates is believed to increase

indi-rectly brain tryptophan levels Elevated blood sugar

stimulates insulin release; this hormone stimulates

the uptake by muscle of amino acids that compete

with tryptophan for uptake by the brain, allowing

more tryptophan into the brain This in turn leads

to higher serotonin formation in the brain,

pro-moting a state of relaxation Amino acid research

in humans is still considered preliminary

ACETYLCHOLINEis manufactured from CHOLINE, a

nitrogen-containing nutrient and raw material for

neurotransmitter synthesis The body can

synthe-size choline, and it comes also from dietary lipids,

particularly LECITHIN, a common phosphate-bearing

lipid found in most foods Acetylcholine

partici-pates in many brain functions, including memory

Victims of ALZHEIMER’S DISEASE have low levels of

acetylcholine in their brains (See also ENDORPHINS;

NERVOUS SYSTEM.)

neutral fats See FAT; VEGETABLE OIL

NHANES (National Health and Nutrition

Examina-tion Survey) In 1956 Congress passed the

National Health Survey Act, which authorized the

federal government to conduct a continuing survey

that would produce statistics on the amount, distri-bution, and effects of illness and disability in the United States During the next decades nutrition began to play a greater role in the understanding of sickness and disability

In 1970 a special task force reporting to the Department of Health, Education, and Welfare rec-ommended that future surveys include clinical observation and professional assessment as well as recording of dietary intake patterns Since then the U.S Center for Health Statistics has been conduct-ing regular National Health and Nutrition Exami-nation Surveys

NHANES I provided statistics between 1971 and 1975; NHANES II between 1976 and 1980; and NHANES III between 1988 and 1994 A Hispanic Health and Nutrition Examination survey was con-ducted from 1982 to 1984

Since 1999 NHANES has been conducted annu-ally Beginning in 2002 it merged with the U.S Department of Agriculture’s Continuing Survey of Food Intakes (CSFII), to produce the National Food and Nutrition Survey (NFNS) This integrated sur-vey provides comprehensive information on the health and nutrition intakes of the U.S population (See also EATING PATTERNS.)

Kleges, R C., L H Eck, and J W Ray “Who Underre-ports Dietary Intake in Dietary Recall? Evidence from the Second National Health and Nutrition

Examina-tion Survey,” Journal of Consultative and Clinical Psychol-ogy 63, no 3 (1995): 438–444.

niacin (nicotinic acid, vitamin B 3 ) A heat-stable member of the B VITAMIN complex needed by the body to extract ENERGY from FAT, CARBOHYDRATE, and PROTEIN Tissues convert niacin to two closely related COENZYMES(enzyme helpers): NICOTINAMIDE ADENINE DINUCLEOTIDE (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) NAD functions as an oxidizing agent used in processes that “burn” fuels to produce energy in the cell These include the oxidation of GLUCOSE(GLYCOLYSIS) and of FATTY ACIDS NAD transfers electrons to the mitochondrial ELECTRON TRANSPORT CHAIN, which ultimately reduces OXYGEN to water This process liberates vast amounts of chemical energy trapped

as ATPas well as releasing heat NADP is generated from the oxidation of glucose (pentose phosphate

niacin 457

pathway) The reduced form of NADP, NADPH, is

used as a reducing agent in biosynthetic reactions

rather than for energy production CHOLESTEROL,

fatty acids, and other important compounds require

NADPH for their synthesis NADPH is also the basis

of a powerful antioxidant system (GLUTATHIONE) to

protect most cells from the damaging effects of

highly reactive molecules called FREE RADICALS

NADPH also functions in the synthesis of DEOXYRI

-BOSE, the sugar building block of DNA

Possible Roles of Niacin in Disease

PELLAGRAis a severe niacin deficiency disease that

mimics schizophrenia Skin rashes, DIARRHEA, and

mouth sores are prevalent in pellagra At the turn

of the century, pellagra was epidemic in the

south-ern United States among people whose diet was

based on corn By 1937, research demonstrated

that niacin cured pellagra-like symptoms in dogs

and, shortly thereafter, that niacin treatment cured

pellagra Niacin has been used in food fortification

since 1941

Niacin is effective in the treatment of elevated

(hyperlipidemia) blood lipids that does not respond

to dietary intervention alone, and it has been

approved by the USFDA for this treatment It may

also reduce the risk of a second heart attack in men

Niacin is effective in lowering high blood

triglyc-erides and elevated LDL-cholesterol, the “bad”

form, and raising HDL-cholesterol, the beneficial

form Niacin is generally considered an adjunct

therapy, used in conjunction with

cholesterol-lowering drugs, such as statins and bile binding

resins As an example, niacin in combination with

bile-binding agents may effectively slow the

pro-gression of atherosclerosis in men with existing

car-diovascular disease

The use of niacin to treat schizophrenia and

DEPRESSION is controversial Generally negative

results have been reported for patients with

long-standing schizophrenia Some clinicians reported

improvement in patients with schizophrenia using

megavitamin treatment with niacin or

niaci-namide, VITAMIN B6 and VITAMIN C and in patients

with depression using niacinamide and

trypto-phan, together with taking steps to improve the

diet The American Psychological Association

dis-approved the use of niacin in the treatment of

mental disorders in 1979

Niacin when taken in combination with choles-terol-lowering “statin” drugs may prevent HEART ATTACKS

Niacin seems to increase the production of PROSTAGLANDINS These hormone-like chemicals are produced locally within tissues to help control many physiologic processes such as BLOOD CLOTTING and INFLAMMATION

Large doses of niacin may alleviate noninflam-matory ARTHRITIS, while low doses of niacin can relieve migraine headaches Niacin supplements may help normalize blood sugar in patients with hypoglycemia Large doses of niacin may prevent harmful effects of chemical pollutants, drugs, and alcohol, and may help during recovery and drug rehabilitation (the mechanism is not known) This

is a promising area of research, but niacin cannot

be claimed to be a broad-spectrum detoxification agent

Sources

Good sources of niacin are EGGS, MEAT (especially liver), fish, POULTRY, and unprocessed food, includ-ing peanuts and potatoes COFFEE provides about 3

mg per cup Niacin is one of the commonly fortified nutrients; consequently, enriched flour and cereals are good sources MILKand CHEESEare good sources because they contain large amounts of the essential AMINO ACID, TRYPTOPHAN, which is partially con-verted by the body to niacin Protein-deficient diets are often related to pellagra This explains why diets incorporating milk can prevent or cure pellagra The form available from animal foods is NIACI -NAMIDE, a derivative of niacin; plant foods provide niacin itself Niacin is one of the most stable of the

B vitamins: It resists most cooking procedures and can be stored in the dry state indefinitely without loss Canning, DEHYDRATION, and exposure to air or light cause little destruction

Up to 70 percent of the niacin in most cereal grains, including corn, is present as a bound form called niacytin It is not released during digestion and therefore is poorly absorbed Traditionally, corn tortillas are prepared from corn pretreated with lime water to improve dough consistency, a treatment that also frees niacin Pellagra is uncom-mon in Mexico, Central America, and South Amer-ica, where corn is soaked in lime Niacin deficiency and pellagra are common only in certain regions of

458 niacin

Africa and Asia, where corn is a major source of

protein Because niacin is so prevalent in

high-quality protein, only alcoholics and heavy drinkers

are likely to be deficient in the United States

Symptoms of mild deficiency include apathy,

headache, irritability, and memory loss

Requirements

The RECOMMENDED DIETARY ALLOWANCE for health

adult men is 19 mg; for nonpregnant, nonlactating

women it is 15 mg The daily niacin requirement

varies with the number of calories burned daily

(the energy expenditure) and the protein intake

One niacin equivalent equals 1 mg of preformed

niacin or 60 mg of dietary tryptophan

Niacin can be synthesized in the body from the

essential amino acid tryptophan Tissues can form

an average of 1 mg of niacin from 60 mg of

trypto-phan provided by dietary protein Consequently, a

high-quality protein diet supplies substantial

tryp-tophan for niacin synthesis On the other hand, a

protein-deficient diet or a diet relying on low

tryp-tophan protein increases the dietary requirement

for niacin Most animal protein contains 1.4

per-cent tryptophan; vegetable protein, 1 perper-cent; and

corn protein, only 0.6 percent

Safety

Niacin (but not closely related niacinamide)

expands CAPILLARIES and can lead to itching and

flushing at doses commonly used in multivitamin

supplements (100 mg or more) While nausea,

diar-rhea, and flushing may accompany niacin usage,

these side effects are not considered dangerous

Excessive niacin can cause irregular heartbeat,

cramps, headache, and liver inflammation

“Sus-tained release” niacin may cause less flushing, but

some preparations may be more toxic to the liver

Other forms, such as inositol hexaniacinate, may

improve niacin therapy Large doses of niacin can

increase BLOOD SUGARin diabetics, increase the risk

of GOUT, and aggravate ulcers Niacin supplements

are not recommended for those with PEPTIC ULCERS,

COLITIS, ASTHMA, liver disease, GOUT, or erratic

heart-beat Using niacin therapeutically (1,000 mg or

more per day) requires medical supervision; liver

function and blood sugar need to be monitored

(See also CARBOHYDRATE METABOLISM; CHOLESTEROL

-LOWERING DRUGS; FAT METABOLISM; FORTIFICATION.)

Brown, B G et al “Simvastatin and Niacin, Antioxidant Vitamins, or the Combination for the Prevention of

Coronary Disease,” New England Journal of Medicine

345, no 22 (2001): 1,583–1,592.

niacinamide (nicotinamide) A derivative of the water-soluble vitamin NIACIN Niacinamide is read-ily formed in the body from niacin and incorpo-rated into two important COENZYMES (enzyme helpers): NICOTINAMIDE ADENINE DINUCLEOTIDE (NAD) and its phosphate-containing analog, nicoti-namide adenine dinucleotide phosphate (NADP)

By assisting in oxidation-reduction reactions, NAD performs an essential role in energy production from nutrients NAD is required by all cells in many aspects of metabolism, including CARBOHYDRATE, FATTY ACID, and AMINO ACID degradation The re-duced form of NADP, NADPH, participates in reductions required for biosynthesis

Niacinamide is the most prevalent form of this vitamin in animal products and MEAT, although niacin is more prevalent in plant foods Niacina-mide lacks the capillary-expanding (skin-flushing) activity of niacin and is somewhat safer than niacin; hence it is more often used when large amounts of the vitamin are required Side effects of niacinamide include nausea, headache, fatigue, sore mouth Large amounts can injure the liver (jaundice) (See also ANABOLISM; B COMPLEX; DETOX -IFICATION; GLUTATHIONE.)

nickel A possible TRACE MINERALnutrient Nickel

is now known to be an essential trace mineral nutrient of all higher plants, including cereal grains and legumes, which require nickel for seeds to grow Nickel helps plants liberate nitrogen from soil and absorb IRON Nickel also seems to be a trace mineral nutrient for some animal species, though its function is unknown Diets that exclude nickel slow growth in sheep, goats, cows, and rats Nickel deficiency decreases red blood cell production in these animals as well Nickel-deficient chickens develop abnormally Human requirements, if any, are unknown A typical American diet supplies an estimated 0.3 to 0.6 mg of nickel daily

Like many trace minerals, minute amounts may

be essential nutrients, though high-level exposure is hazardous Because nickel is an industrial waste, it

nickel 459

has emerged as an environmental pollutant The

toxicity of high doses of nickel is well documented;

for example, nickel carbonyl is a hazardous

indus-trial chemical, and exposure in the workplace is

reg-ulated Nickel allergies are linked to jewelry; once a

person is sensitized, nickel allergies are long-lasting

nicotinamide See NIACINAMIDE

nicotinamide adenine dinucleotide (NAD,

NADH) An enzyme helper (COENZYME) that

func-tions in oxidation/reduction reacfunc-tions of cells NAD

contains the B vitamin NIACIN NAD assists

dehy-drogenases, a family of enzymes that remove

hydrogen atoms from substances (oxidation)

Lac-tate dehydrogenase is a typical dehydrogenase:

This enzyme oxidizes LACTIC ACIDto PYRUVIC ACIDas

an intermediate step in the formation of GLUCOSE,

the dominant sugar used by the body Other

dehy-drogenases oxidize fatty acids and perform

oxida-tion steps in the KREB’S CYCLE, the central energy

yielding pathway of the cell

Nicotinamide Adenine

Dinucleotide Phosphate (NADP, NADPH)

NADP closely resembles NAD in structure and

function As the second coenzyme based on niacin

it contains an additional phosphate group The

reduced form, NADPH, is used in biosynthetic

pathways, including fatty acid and cholesterol

syn-thesis NADPH is produced by the oxidation of

glu-cose (the pentose phosphate pathway) NADPH

supports GLUTATHIONE PEROXIDASE, an important

ANTIOXIDANTsystem for neutralizing oxidative

dam-age to lipids and membranes (See also B COMPLEX;

CARBOHYDRATE METABOLISM; FATTY ACID; OXIDATIVE

PHOSPHORYLATION.)

nicotinic acid See NIACIN

night blindness (nyctalopia) A condition

result-ing from a chronic deficiency of VITAMIN A Vitamin

A is required to form “visual purple,” the pigment

required for vision in dim light With vitamin A

deficiency, inadequate pigment causes an

abnor-mally slow adaptation in going from vision in

strong light to dim light Hereditary factors may

contribute to night blindness as well

nightshade family A plant family that includes TOMATOES, POTATOES, EGGPLANT, PEPPERS, and tobacco Anecdotal reports have been interpreted

by some to suggest that eating vegetables from this family may promote joint inflammation in suscep-tible people However, no carefully controlled stud-ies have been conducted, and the hyptothesis remains unproven Arthritic symptoms may dimin-ish by avoiding contact with foods that cause reac-tions (See also ARTHRITIS; FOOD SENSITIVITY.)

nitrate An inorganic, nitrogen-containing ion used as a FOOD ADDITIVE for processed MEAT and meat products Meat can legally contain 91 mg added sodium or potassium nitrate per pound Chopped meat can contain 778 mg per pound, and dry, cured meat can contain 991 mg per pound Its role in meat is not clear, though nitrate seems to provide a reservoir for bacterial conversion to nitrite, which acts as an antimicrobial agent Nitrite can be converted to a potential cancer-causing agent called NITROSOAMINE

Nitrate occurs naturally in food Green VEGETA -BLES are a major source of nitrates and BEETS, SPINACH, and LETTUCEare likely sources The level depends on the plant species and variety, the part

of the plant consumed, the stage of maturity, levels

of fertilizers in the soil, and the rate of plant growth The estimated-typical daily intake of nitrate is 6 mg from vegetables; 9.4 mg from cured meats; 2 mg from bread; and 1.4 mg from FRUITand fruit juices Drinking water supplies an average of 0.7 mg daily High levels of nitrate can occur in well water in rural areas, due to contamination from feed lots and run-off from fertilizer-affected fields

A limit of 10 mg of nitrate per liter has been set by the U.S Public Health Service

Nitrate in well water used to prepare formula can be a hazard for infants Up to six months after birth, stomach acid production in infants is low, which permits bacteria in the intestine to convert nitrate to nitrite Nitrite can disable infants’ hemo-globin and induce a condition called methenoglo-binemia, in which nitrite-modified hemoglobin cannot transport adequate oxygen In older chil-dren, nitrate is absorbed in the stomach before it can reach intestinal bacteria that could reduce it to nitrite Infants drink proportionately much more water (in formula) than adults do, and their

hemo-460 nicotinamide