Body mass index in a US national sample of Asian Americans: effects of nativity, years since immigration and socioeconomic status DS Lauderdale 1 * and PJ Rathouz 1 1 Department of Healt
Trang 1Body mass index in a US national sample of Asian Americans: effects of nativity, years since immigration and socioeconomic status
DS Lauderdale 1 * and PJ Rathouz 1
1 Department of Health Studies, University of Chicago, Chicago, IL 60637, USA
OBJECTIVE: To examine body mass index (BMI) and the proportion overweight and obese among adults age 18 ± 59 in the six largest Asian American ethnic groups (Chinese, Filipino, Asian Indian, Japanese, Korean, Vietnamese), and investigate whether BMI varies by nativity (foreign- vs native-born), years in US, or socioeconomic status.
DESIGN: Cross-sectional interview data were pooled from the 1992 ± 1995 National Health Interview Survey (NHIS) SUBJECTS: 254,153 persons aged 18 ± 59 included in the 1992 ± 1995 NHIS Sample sizes range from 816 to 1940 for each of six Asian American ethnic groups.
MEASUREMENTS: Self-reported height and weight used to calculate BMI and classify individuals as overweight (BMI 25 kg=m 2 ) or obese (BMI 30 kg=m 2 ), age, sex, years in the US, household income and household size RESULTS: For men, the percentage overweight ranges from 17% of Vietnamese to 42% of Japanese, while the total male population is 57% overweight For women, the percentage overweight ranges from 9% of Vietnamese and Chinese to 25% of Asian Indians, while the total female population is 38% overweight The percentage of Asian Americans classi®ed as obese is very low Adjusted for age and ethnicity, the odds ratio for obese is 3.5 for women and 4.0 for men for US - vs foreign-born Among the foreign-born, more years in the US is associated with higher risk
of being overweight or obese The association between household income for women is similar for US-born Asian Americans and Whites and Blacks, but is much weaker for foreign-born Asian Americans.
CONCLUSIONS: While these data ®nd low proportions of Asian Americans overweight at present, they also imply the proportion will increase with more US-born Asian Americans and longer duration in the US.
International Journal of Obesity (2000) 24, 1188±1194
Keywords: Asian Americans; body mass index; obesity=ethnology; acculturation; socioeconomic factors
Introduction
The public health signi®cance of obesity derives
largely from its well-documented associations with
chronic conditions such as diabetes mellitus,
osteoar-thritis and hypertension, conditions whose prevalence
varies by race and ethnicity The percentage of obese
adults has increased markedly in recent decades In
the Third National Health and Nutrition Examination
Survey (NHANES III), conducted from 1988 to 1994,
the proportion of the adult population in the United
States with body mass index (BMI, kg=m2) of 25 or
higher was approximately 54%.1 That proportion
varied substantially by the racial and ethnic categories
available in NHANES III (Hispanic White,
non-Hispanic Black and Mexican American), with 67% of
Mexican Americans, 63% of Blacks and 53% of
Whites overweight (BMI 25) Obesity (BMI 30)
similarly varied by ethnicity and race, with 21% of White, 30% of Black, and 28% of Mexican American adults found to be obese
There is a paucity of national data on the health status of Asian Americans, a numerous, rapidly increasing, and ethnically diverse minority group NHANES III, for example, over sampled Mexican Americans and Blacks, but not Asian Americans Nor can one distinguish Asian Americans in the publicly-released ®les, since they are classi®ed as `Other' Further, there are no other national data collections with physical examinations and suf®cient numbers to characterize BMI and obesity of Asian Americans Our knowledge of this and other cardiovascular risk factors among Asian Americans derives disproportio-nately from the study of one ethnic group in an environment atypical of the US as a whole, Japanese Americans in Hawaii Mortality data from the National Center for Health Statistics suggest that Asian Americans in general are a uniquely healthy group with the highest life expectancy in the country.2
However, since the majority of Asian American adults are foreign-born, the question arises of whether Asian American good health is owing to a `healthy immi-grant' effect The two most frequently presented
*Correspondence: DS Lauderdale, Department of Health
Studies, University of Chicago, 5841 S Maryland Avenue, MC
2007, Chicago, IL 60637, USA.
E-mail: lauderdale@health.bsd.uchicago.edu
Received 27 September 1999; revised 4 April 2000; accepted
18 April 2000
www.nature.com/ijo
Trang 2explanations for a `healthy immigrant' effect are (1)
the self-selection of people able to and choosing to
immigrate, and (2) the maintenance of healthy
beha-viors associated with a traditional lifestyle, including
diet and physical activity
In this study, as one indicator of whether lifestyle
risk factors for chronic diseases are associated with
nativity (ie US-born vs foreign-born) for Asian
Amer-icans and whether risk factors change for the
foreign-born following immigration, we use national
inter-view data to examine variation in BMI, the proportion
overweight and the proportion obese for the six major
Asian American ethnic groups (Chinese, Filipino,
Japanese, Asian Indian, Korean and Vietnamese)
We ask whether BMI and the odds of being
over-weight or obese vary by nativity Among the
foreign-born, we further investigate whether BMI and the
odds of being overweight or obese vary with years
since immigration Finally we examine whether the
association between socioeconomic status and BMI is
the same for Asian Americans as it is for Whites and
Blacks
Methods
Data
The National Health Interview Survey (NHIS) is a
nationally representative annual cross-sectional
inter-view survey conducted by the National Center for
Health Statistics (NCHS) The target population is
civilian, non-institutionalized residents of the US The
sampling plan follows a multistage area probability
design that permits the representative sampling of
households Data are collected through a personal
household interview conducted by interviewers
employed and trained by the US Bureau of the
Census according to procedures speci®ed by NCHS
Translators are used to collect information from
persons with limited English pro®ciency
NHIS data are collected annually in 36,000 ± 47,000
households from 92,000 ± 125,000 persons.3 The
annual response rate is greater than 90% Beginning
in 1992, the NHIS added Asian ethnic group detail to
the `race' item for Japanese, Chinese, Filipino, Asian
Indian, Korean and Vietnamese These six groups
together comprise about 90% of the Asian American
population Then in 1996, the categories for the three
smaller groups, Asian Indian, Korean and
Vietna-mese, were combined into the `other Asian and Paci®c
Islander' group for data release Because the number
of Asian Americans surveyed in each ethnic group in
one single year is as few as 100, we combine data
from the 1992 ± 1995 NHIS to achieve adequate
sample sizes of subjects with maximum ethnic
detail.4 ± 7 This study includes persons aged 18 ± 59 y
of age Persons 60 and older are not included both
because health effects of BMI may differ for the
elderly and because BMI may increasingly re¯ect
the consequences of ill-health as age increases
Among the core questions asked of each member of sampled households are race=ethnicity, age, sex, height, weight, years in US for the foreign-born ( < 1 y, 1 ± 5, 5 ± 10, 10 ± 15, > 15 y), household income and household size We re-categorize years
in the US as less than 5 y, 5 ± 15 y and 15 y or more
We categorize income by $10,000 increments up to
$50,000 or greater, the highest category collected by NHIS Income is adjusted for household size in regression models We use household income rather than education as an indicator of socioeconomic status
in this study because equating the number of years of school for persons educated in the US and the dis-similar educational systems in each Asian country is clearly problematic We calculate the BMI by con-verting height and weight to the metric system and calculating kg=m2 for each subject Persons with missing data for height, weight, age, race or sex are not included in this study (approximately 2% of sample) Approximately 15% of records lack house-hold income data, and they are omitted only from the analyses using that variable
Analysis
Data collected in the NHIS are obtained through a multistage complex sample design involving both strati®cation and clustering Moreover, the sampling frame for the NHIS is redesigned every 10 y, and there were two major changes to the sampling design beginning in 1995: an increased number of primary sampling locations to permit estimation at the state level, and oversampling of the Hispanic population in addition to the prior oversampling of the Black population Since this study pools data from the
1992 ± 1995 NHIS, the change in sampling frame introduces an additional level of complexity to the estimation procedures Extrapolating from NCHS guidelines for combining 1994 and 1995 data, all four years were concatenated into a single data set, with each year read independently and treated as a stratum Stratum and primary sampling unit variables were created for 1995 to be consistent with previous years
Exploratory data analysis was carried out with STATA (Stata corporation, College Station, TX) Then, prevalence estimates and logistic and linear regression models were ®tted with the SUDAAN software package (Research Triangle Institute, Trian-gle Park, North Carolina), as recommended by the NCHS Using the method of generalized estimating equations, SUDAAN ®ts models and obtains standard errors that correctly account for multistage strati®ed sampling designs Regression parameter estimates are consistent and, if the covariance model is correctly speci®ed, ef®cient Even if the covariance model
is misspeci®ed, however, the robust standard errors are correct, and hence con®dence intervals will have the correct coverage probabilities Although the
1189
Trang 3population of main interest in this study is Asian
Americans aged 18 ± 59, no NHIS observations were
deleted since analysis of subsetted data may result in
incorrectly computed standard errors.8 Instead, the
subpopulation option (SUBPOPN) in SUDAAN
pro-cedure was used to target a subdomain from the full
design database
Results
There were 7263 Asian Americans aged 18 ± 59 in the
six major ethnic groups interviewed by the NHIS from
1992 ± 1995 (Table 1) By ethnic group, the numbers
ranged from 816 Korean to 1940 Chinese As expected
from the immigration history,9a large majority of each
group except the Japanese is foreign-born For each of
the Korean, Vietnamese and Asian Indian groups, less
than 10% and fewer than 100 persons were US-born
Among the foreign-born, the subjects were distributed in
roughly equal numbers into the three categories of years
in the US (Table 2) Overall, 26% had been in the US
fewer than 5 y, 42% from 5 to 15 y, and 33% for more
than 15 y, and these percentages were generally similar
across ethnic groups
Figures 1 and 2 display box-and-whisker plots of
BMI for men and women by race category All of the
Asian American groups have lower median BMI than
the White or Black groups There is, however,
sig-ni®cant variation among Asian American groups For
men, the Japanese and Filipino have higher median
BMI than the other groups, while for women it is the
Filipino and Asian Indian groups with higher median
BMI For both men and women, the Vietnamese have
the lowest median BMI
BMI, proportion overweight and proportion obese are all higher for US-born Asian Americans than for the foreign-born, and the effect is similar for men and women For Asian American men, adjusting for age and ethnicity, mean BMI is 1.31 kg=m2 (95%
CI 0.90 ± 1.72) lower for the foreign-born than the native-born For women, the difference in mean BMI
is 1.14 kg=m2 (95% CI 0.77 ± 1.51) Adjusting for age and ethnicity, the odds ratio of being overweight for the US-born compared to the foreign-born is 1.85 for men (95% CI 1.52 ± 2.26) and 1.94 for women (95% CI 1.46 ± 2.58), and the odds ratio for obese is 4.03 (95% CI 2.40 ± 6.78) for men and 3.51 (95%
CI 1.74 ± 7.10) for women Among the foreign-born, the odds of being overweight or obese increases for Asian Americans with longer duration in the US (Table 3)
Finally we examine whether the association between socioeconomic status, measured as family income (adjusted for family size), and BMI is the same for Asian Americans as it is for Blacks and Whites Using the same 4 y of NHIS, we ®nd that family income is strongly inversely related to BMI for women The magnitude and direction of the associa-tion is very similar for White and Black women (Table 4) For White women, each $10,000 in income is associated with a BMI 0.55 kg=m2 lower,
or a total difference of 2.75 kg=m2between the highest and lowest income categories For Black women, BMI
is 0.59 kg=m2 lower per $10,000, a difference of 2.95 kg=m2 between the highest and lowest income categories For White men, there is a very small inverse association between family income and BMI, 0.03 lower per $10,000 For Black men, there
is a modest positive association, with BMI 0.07 higher for each $10,000 family income For US-born Asian American women, there is also a strong inverse association between BMI and income, although the magnitude of the association, 0.38 kg=m2 for each
$10,000 income, is somewhat smaller than for White and Black women There is no signi®cant association for US-born Asian American men, while the point estimate, 0.06, suggests a modest positive association and is similar to the point estimate for Black men The inverse association between income and BMI, how-ever, is very weak and of marginal statistical signi®-cance for foreign-born Asian American women, just 0.06 kg=m2 per $10,000 income For foreign-born Asian American men, the positive association is somewhat greater than for other men, 0.11 per
$10,000 income, and the trend is highly signi®cant When years in the US is also entered into the model, the positive association is attenuated
Discussion
We have found that BMI and the proportions over-weight and obese are lower among each of the Asian American ethnic groups than the US population in
Table 1 Distribution of six Asian American ethnic groups by
birthplace for persons aged 18 ± 59, from the National Health
Interview Survey, 1992 ± 1995, based on the unweighted sample
Table 2 Distribution of six Asian American ethnic groups by
years in the United States for foreign-born persons aged 18 ± 59,
from the National Health Interview Survey, 1992 ± 1995, based on
the unweighted sample
1190
Trang 4Figure 1 Body mass index for men by race categories from the National Health Interview Survey, 1992 ± 1995 Each box corresponds
to the interquartile range of the data, the 25th to 75th percentiles, and the line in the middle is the median The line above the box extends to the largest data point less than or equal to the 75th percentile plus 1.5 times the interquartile range The lower line is formed analogously Observations beyond this are individually plotted However, the ®gure does not display values greater than a BMI of 40.
Figure 2 Body mass index for women by race categories from the National Health Interview Survey, 1992 ± 1995 Each box corresponds to the interquartile range of the data, the 25th to 75th percentiles, and the line in the middle is the median The line above the box extends to the largest data point less than or equal to the 75th percentile plus 1.5 times the interquartile range The lower line is formed analogously Observations beyond this are individually plotted However, the ®gure does not display values greater than a BMI of 40.
1191
Trang 5general However, the proportions do vary by
ethni-city and nativity US-born Asian Americans are
sig-ni®cantly more likely to be obese or overweight than
the foreign-born Among the foreign-born, the number
of years spent in the US is directly related to the risk
of being overweight or obese For White and Black
women in the US, there is a strong inverse association
between BMI and economic status This association is
also seen for US-born Asian American women,
although the effect is weaker However, there is
only weak evidence of an association between
eco-nomic status and BMI for foreign-born Asian
Amer-ican women There is a positive association between
BMI and economic status for foreign-born Asian
American men
This study has several methodologic limitations, the
most signi®cant of which is that height and weight are
self-reported Previous studies have assessed the
validity of self-reported height and weight, and
gen-erally the level of agreement between self-report and
measured height and weight has been found to be very
high.10 However, there is evidence of modest
sys-tematic bias towards under-reporting weight and
over-reporting height.11,12 Such systematic mis-reporting
would underestimate prevalence of overweight and
obesity, which are calculated relative to ®xed BMI
values Nonetheless, the prevalence of overweight
found in the examination-based NHANES III data
(collected 1988 ± 1994) for persons age 20 ± 741is in
close agreement with the prevalence calculated for
1992 ± 1995 NHIS for the same age range
Speci®-cally, Flegel et al reported from NHANES III a crude
prevalence overweight of 60% for men and 51% for
women From NHIS, the crude prevalence is 61% for
men and 51% for women Because the emphasis in this study is on comparisons and relative measures, systematic mis-reporting would be less of a limitation than mis-reporting which was related to variables under investigation, such as nativity or ethnicity While there is some evidence from the UK that quality
of self-report may vary by demographic factors,13we are unaware of studies which have investigated the validity of self-reported height and weight for the foreign-born or for Asian Americans Another limita-tion to this study is that conclusions regarding the effect of years since immigration are based on cross-sectional rather than longitudinal data, possibly con-founding acculturation with cohort effects related to year and age at immigration We cannot examine the effect of age at immigration in models which already include age and years in the US Finally, sample sizes for US-born Korean, Vietnamese and Asian Indian Americans are too small, even combining 4 y of NHIS, to permit ethnicity-speci®c evaluations of nativity Our study is limited by the use of current household income (adjusted for family size) This measure may not re¯ect lifetime socioeconomic status (SES) as well for the foreign-born as the US-born The highest income category in the NHIS is
$50,000 and higher, which fails to distinguish grada-tions among those with high incomes
Despite these limitations, this study addresses a remarkable lack of information concerning the health of Asian Americans The immigration history, both the proportion US-born and the years of peak immigration, is different for each Asian American group Since both ethnicity and immigration status (nativity and duration in the US) may be related to
Table 3 The effect of duration in US on odds of overweight and obese for foreign-born Asian American men and women, adjusted for age and ethnicity
< 5 y 0.62 (0.48 ± 0.80) 0.77 (0.55 ± 1.08) 0.63 (0.31 ± 1.28) 0.28 (0.11 ± 0.70)
5 ± 15 y 0.81 (0.67 ± 0.98) 0.98 (0.76 ± 1.26) 0.69 (0.40 ± 1.19) 0.50 (0.27 ± 0.92)
OR odds ratio CI con®dence interval.
Table 4 The effect of household income on body mass index
b coef®cient per $10,000 P-trend b coef®cientper $10,000 P-trend
a Adjusted for age and family size.
b Adjusted for age, family size and ethnicity.
1192
Trang 6health, the ability to stratify Asian Americans by both
ethnicity and immigration status is key to avoiding
unmeasured confounding of one by the other The
NHIS provides the opportunity to study a few health
indicators for nationally-representative samples of the
six largest Asian American ethnic groups With no
oversampling of Asian Americans in the NHIS,
indi-vidual years do not provide adequate sample size for
most analyses, nor do the supplements, such as the
1992 Cancer Control Supplement, which are
adminis-tered to a subsample of respondents We overcome
this limitation by combining several years of data
To our knowledge, only one previous project has
investigated BMI and obesity in a multi-ethnic Asian
American population with ethnicity-level detail
Klatsky and Armstrong14 assessed cardiovascular
risk factors among Asian Americans who volunteered
for an examination at a northern California prepaid
heath plan between 1978 and 1985 Their data, which
include both mean BMI and proportion with BMI
greater than 24.4 (overweight), are presented for
Chinese, Japanese, Filipino and other Asian The
proportions overweight are very similar to the
propor-tions found in the NHIS data For Chinese, 27% of
men and 13% of women were overweight; for
Filipi-nos, 42% of men and 26% of women were
over-weight; and for Japanese, 38% of men and 18% of
women were overweight They also found evidence of
increased odds of being overweight for those born in
the US, although the effect was only statistically
signi®cant for men Further evidence of a nativity
effect for BMI comes from the National Longitudinal
Study of Adolescent Health Popkin and Udry found
that US-born Asian American adolescents (in
aggre-gate) were more than twice as likely to be overweight
as the foreign-born adolescents.15
The association between SES and obesity has been
studied in diverse cultures, with generally consistent
®ndings.16 In developed countries, there is a strong
inverse association between SES and obesity for
women and no consistent association for men In
developing countries, by contrast, there is a positive
association for both men and women; the higher
prevalence of obesity among those with greater
wealth likely re¯ects both greater access to food and
a related cultural preference for physical evidence of
such access.17The SES effects we have found for
US-born Asian Americans conform to expectations for
persons in a developed country The lack of
associa-tion for foreign-born women and the moderate
posi-tive association for foreign-born men suggest an effect
intermediate in direction between those of developed
and developing countries
The health signi®cance of BMI in part derives from
its correlation with adiposity Several previous studies
have investigated whether the ability of BMI to
predict percentage body fat varied by ethnicity
Gal-lagher and others found that, while the association
between BMI and adiposity did vary by age and sex, it
did not differ between Black and White adults in a
study conducted in New York City.18Comparing BMI and percentage body fat for Asians and Whites, Wang and others did ®nd some differences.19 Asians, although mean BMI was lower, had higher percentage body fat and more upper-body subcutaneous fat In a meta-analysis which included data for three Asian groups, Deurenberg and others found that the percen-tage body fat was higher than predicted at low BMI levels for Chinese Body fat was underestimated across all BMI levels for Thais and Indonesians.20In
a study of women in Hawaii, Novotny and others found that Asian women had a greater percentage of body fat than did White women with the same BMI.21
One implication of such data is that the accepted cut-off values for overweight and obese may be less appropriate for Asian American populations in terms
of their association with heart disease and proximal risk factors such as hypertension, glucose intolerance
or lipid pro®les
While the proportions overweight and obese are much lower for Asian Americans than other racial categories at present, the strong associations with birthplace and years since immigration suggest these proportions may increase signi®cantly as the demo-graphy of the population shifts, with increased dura-tion of residence in the US and a higher propordura-tion US-born The limited data concerning the correlation between BMI and adiposity suggest that health effects
of BMI may differ for Asian Americans Higher levels
of BMI could have a signi®cant impact on morbidity and mortality
Acknowledgements
We thank Ye Luo, PhD, for computer programming and Kate Pickett, PhD, for comments on the manu-script Data were presented in part at the annual meeting of the Society for Epidemiological Research (Baltimore MD, June 1999)
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