Obesity Among US Immigrant Subgroups by Duration of Residence FREE

Since the 1980s, the US population has become more obese, with similar trends documented worldwide.^1- 3 The World Health Organization recently described “globesity” as a global epidemic of obesity affecting at least 300 million people, with a 3-fold or greater increase since 1980 in parts of Eastern Europe, the Middle East, the Pacific Islands, and China.⁴ Nevertheless, the prevalence of obesity in most parts of the world is lower than in the United States.

The high prevalence of obesity has produced a major burden of obesity-related illnesses. In the year 2000, overweight and obesity accounted for nearly 17% of all deaths in the United States, a mortality rate surpassed only by that of tobacco use.⁵According to the Centers for Disease Control and Prevention, poor diet and physical inactivity, both modifiable behaviors, cause a large part of this mortality from overweight.⁵ Thus, many guidelines recommend obesity prevention through the promotion of exercise and healthy diet.^6- 8

Evidence suggests significant racial and ethnic differences in the prevalence of obesity and the susceptibility to obesity-related illnesses, particularly among individuals who are black, Latino, and Asian, relative to whites.^9- 10 However, few data are available about the epidemiology of obesity among immigrants, the fastest growing segment of the US population, currently comprising more than 11% of the total US population and an even larger proportion of many minority groups.¹¹ Immigrants generally originate from countries where the prevalence of obesity is lower than that of the United States, but acculturation to US norms over time may lead to an increasing prevalence of obesity among this population.^3,12 The magnitude of the change in body mass index (BMI) and the consistency of the effect of acculturation on BMI among various immigrant subgroups, however, remain unclear.

Additionally, immigrants face more barriers to quality health care¹³ and are less likely to receive preventive health care than persons born in the United States.¹⁴ Whether immigrants are less likely to discuss diet and exercise with clinicians is unknown.

We therefore examined the relationship between prevalence of obesity and years of US residence among immigrants nationally and explored whether counseling about diet and exercise may differ between immigrants and US-born adults.

We used data from the Sample Adult Module of the 2000 National Health Interview Survey (NHIS), an in-person health survey of the civilian, noninstitutionalized population, administered by the US Bureau of the Census for the National Center for Health Statistics.¹⁵ Latino and black populations are oversampled to allow for more precise estimation of these minority groups. A total of 100 618 respondents from 38 633 households provided information about basic measures of health status, utilization of health services, and sociodemographics including country of birth. In addition, 1 randomly selected adult per household, aged 18 years or older, was asked to complete the Sample Adult Module (n=32 374), which elicited detailed information on health care services, behavior, and health status including height and weight. The combined response rate to both components of the survey, based on the American Association for Public Opinion Research standards for Response Rate 5,¹⁶ was 72%. Sample weights provided by National Center for Health Statistics account for the complex sampling design of NHIS and also account for nonresponse. These weights are modified for poststratification adjustments for census sex, age, and race/ethnicity population controls. This weighting of data allows estimates that generalize to the civilian, non-institutionalized population. The survey is administered in only Spanish or English languages and does not allow proxy respondents for Sample Adult questions. Family members may translate for a non–English- or non–Spanish-speaking respondent who is present in the home.

We received institutional review board exemption from Beth Israel Deaconess Medical Center for this study.

We defined obesity as having a BMI of more than 30 (measured as weight in kilograms divided by the square of height in meters) among adults.¹⁷ The National Health Interview Survey calculates BMI from self-reported information on height and weight, measures previously established as largely valid for BMI when used in combination with adjustments for age.¹⁸ We used BMI as a continuous outcome for all linear regression analyses examining the relationship between years of residence in the United States and BMI. For all other analyses, we classified BMI as a categorical outcome based on the National Institutes of Health classification scheme¹⁷: underweight (BMI <18.5), normal weight (18.5-<25), overweight (25-<30), obese class I (30-<35), obese class II (35-<40), and obese class III (≥40).

We were also interested in exploring differences in receipt of dietary and exercise counseling, which were ascertained only in respondents who reported seeing or talking with a health care professional in the past year (n=26 402). We considered those responding to the following questions in the affirmative as having received that counseling. Dietary counseling was assessed with the question “During the past 12 months has a doctor or other health professional talked with you about your diet and eating habits?” Exercise counseling was assessed with the question “During the past 12 months did a doctor or other health professional recommend that you begin or continue to do any type of exercise or physical activity?”

Race/ethnicity was ascertained with 2 questions. Respondents were asked, “Do you consider yourself to be Hispanic, or Latino?” and “What race do you consider yourself to be?” Respondents were asked to select 1 or more of 16 options for the latter question. We considered foreign birth as a proxy for immigrant status and defined foreign birth as birthplace either in a US territory or outside of the United States, based on responses to the question “Where were you born?” Choices for responses included specific states within the United States, the United States, other countries, or territories. The National Health Interview Survey categorized respondents born in US territories as foreign born; as these respondents were likely to be culturally more similar to other foreign-born respondents than to US-born respondents, we used this grouping also.

For our analyses, we considered race/ethnicity and birthplace together. We classified respondents into the following 8 nonoverlapping categories: US-born white, black, Latino, and Asian (including Asian American and Pacific Islander); and foreign-born white, black, Latino, and Asian (including Asian American and Pacific Islander).

Finally, we categorized respondents according to their years of residence in the United States (<1 year, 1-<5 years, 5-<10 years, 10-<15 years, or ≥15 years) based on their response to the question “About how long have you been in the United States?”

We also considered other sociodemographic characteristics, illness burden, measures of access to health care, and health behaviors. Sociodemographic characteristics included age (in decades), sex, marital status (married, not married), region of residence (Northeast, Midwest, South, West), level of education (<high school graduate, high school graduate, some post–high school education, ≥college graduate), and annual household income (<$20 000, ≥$20 000). We defined illness burden using several variables: self-reported health status (excellent/very good, good, fair/poor), which has previously been shown to be associated with mortality in a multiethnic cohort¹⁹; smoking status (never, current, past); alcohol use (rare [<1 drink/wk], moderate [between 1 drink/wk and no more than 2 drinks/d], heavy [>2 drinks/d]); presence of concurrent illnesses (diabetes, hypertension, coronary artery disease, stroke, ulcer, arthritis, cancer other than nonmelanoma skin cancer, or other); and hospitalizations in the past year (0, 1, ≥2). We measured access to care using the following proxies: type of health insurance (none, Medicare, Medicaid, private, other), and usual source of care (general practitioner, specialist including obstetrician/gynecologist, no usual clinician but usual source of care [such as a health center, physician’s office, hospital clinic, or some other place not including an emergency department], no usual source of care).

Because the level of current physical activity might influence rates of obesity and receipt of dietary and exercise counseling, we included level of leisure physical activity as a potential confounder. We categorized physical activity level as high (vigorous activity ≥2 times/wk or moderate activity ≥4 times/wk), moderate (vigorous activity 1 time/wk or moderate activity 1-3 times/wk), or sedentary (no vigorous or moderate activity/week) based on validated methods described previously.²⁰

Additionally, because respondent occupation may influence level of physical activity, in secondary subgroup analyses we also examined the association of occupation on change in BMI with duration of residence, and on reported receipt of dietary and exercise counseling in a subgroup of respondents with information on occupation. We qualitatively categorized NHIS-defined occupations into sedentary (eg, administrators, engineers, health care providers) or active (eg, police/firefighters, construction laborers, farm/agricultural workers) by adapting methods used by Wee et al.²¹We were able to classify occupational activity levels for 54% of those who reported having an occupation (n=19 707), and excluded those with occupations that were difficult to classify (eg, personal service, farm operators/managers).

We conducted bivariable analyses comparing baseline characteristics across our 8 groups of interest defined according to race/ethnicity and birthplace. We used χ² statistics for all categorical variables.

To determine differences in BMI among foreign-born adults according to duration of residence in the United States, we first described the age- and sex-adjusted prevalence of obesity among the foreign-born by years of US residence, and used the χ²test for trend in this relationship. We then fit a linear regression model with BMI as the continuous outcome and the categorical variable of years in the United States as the primary association of interest, collapsing the lowest 2 categories after ensuring no significant differences (0-<5 years, 5-<10 years, 10-<15 years, ≥15 years), and adjusting for age, sex, race/ethnicity, education, and income. To examine whether differences in BMI could be attributable to differences in lifestyle, we additionally adjusted for health behaviors such as alcohol use, smoking, and leisure physical activity. We then further adjusted for occupation (sedentary, active) among a subgroup with information on level of occupational activity. To examine the association between duration of US residence and BMI among various subgroups, we stratified our primary analysis by sex, then by race/ethnicity.

To explore differences in reported dietary or exercise counseling by race/ethnicity and birthplace, we fit logistic regression models for each respective outcome, adjusting for potential confounders: sociodemographic characteristics, illness burden, and access to care, as previously defined. To examine the influence of physical activity, we further adjusted for level of leisure physical activity (high, moderate, sedentary) and performed subgroup analyses among those with classifiable occupational activity (active, sedentary).

For all multivariable analyses, we assessed confounding using a 10% change in the estimated β coefficients from the multivariable models as our criterion. For all analyses, data were weighted to reflect national population estimates and analyzed using SAS-callable SUDAAN software, version 7.5 (Research Triangle Institute, Research Triangle Park, NC) to adjust for the complex sampling design. We considered a 2-tailed P value of ≤.05 statistically significant for all analyses.

Because this is a secondary analysis of a survey database, we were limited to post hoc power analyses. Based on these, we estimated an 80% power to detect the following differences in BMI between immigrants living in the United States less than 5 years and those living in the United States for 10 to 15 years: 0.8 for all immigrants, 1.6 for whites, 2.7 for blacks, 1.0 for Latinos, and 1.6 for Asians. Comparing immigrants living in the United States less than 5 years and those living in the United States for 15 or more years, we could detect the following differences in BMI: 0.7 for all immigrants, 1.2 for whites, 2.5 for blacks, 1.0 for Latinos, and 1.3 for Asians.

For rates of reported dietary counseling, we had 80% power to detect the following absolute differences relative to our reference group of US-born whites: 2.3% for US-born blacks, 3.1% for US-born Latinos, 10.1% for US-born Asians, 4.3% for foreign-born whites, 6.5% for foreign-born blacks, 2.8% for foreign-born Latinos, and 5.3% for foreign-born Asians. This was similar for reported exercise counseling.

The 32 374 eligible respondents represent an estimated 201 million adults in the United States. Of these respondents, 14% were foreign born. Compared with US-born respondents, foreign-born respondents were generally older, had lower annual household incomes and education, had lower illness burden, and had poorer access to care (Table 1 and Table 2). The foreign-born respondents were less often obese than those born in the United States (16% vs 22%, P<.001), but they were also more often sedentary than US-born respondents.

In this cross-sectional analysis, adjusted for age and sex, the foreign-born were generally less likely than the US-born to be overweight and obese (Figure). However, the proportion of overweight and obese foreign-born individuals increased with longer duration of residence in the United States. The prevalence of obesity among foreign-born respondents living in the United States for less than 1 year was 8%. In contrast, the BMI distribution of foreign-born respondents living in the United States for at least 15 years approached that of US-born respondents, with 41% at normal weight, 38% overweight, and 19% obese, compared with 41%, 35%, and 22% of the US-born, respectively.

Examining the relationship between the magnitude of change in BMI and duration of US residence showed that living in the United States for 10 years or more was associated with a significantly higher BMI (Table 3). Results were similar after additional adjustment for health behaviors and in a subgroup with classifiable occupational activity. Years of US residence was associated with a significant increase in BMI for men, women, and all racial/ethnic groups, except for foreign-born blacks. We found no significant interaction between race/ethnicity and number of years in the United States.

Overall, 24% of respondents reported discussing their diet and eating habits with a clinician in the past year; foreign-born respondents were less likely to report counseling than were US-born respondents (18% vs 24%, P<.001). After adjustment, foreign-born blacks and Latinos were less likely to report dietary counseling than US-born whites (Table 4). Results were similar after additional adjustment for level of leisure physical activity and in a subgroup analysis incorporating occupational activity.

Overall, 23% of respondents reported that a clinician recommended beginning or continuing exercise within the last 12 months: 19% of foreign-born compared with 23% of US-born respondents (P<.001). After adjustment, foreign-born black, but not foreign-born Latino or Asian, respondents were less likely to report exercise counseling than were US-born whites; US-born black respondents were also significantly less likely to report exercise counseling (Table 4). Relationships were similar after adjustment for level of leisure physical activity and in a subgroup analysis adjusting for occupational activity.

Our study shows that among a number of immigrant subgroups, longer duration of residence in the United States is associated with higher BMI, such that the distribution of obesity among immigrants residing in the United States for 15 or more years approaches that of the US born. To illustrate the impact, for a typical 5′4″ (1.63-m) immigrant woman and a typical 5′9″ (1.75-m) immigrant man this amounts to an excess 9 lb (4.05 kg) and 11 lb (4.95 kg), respectively, in addition to any weight gained due to aging or other factors. Using a nationally representative sample of foreign-born individuals, we found that this trend is present among immigrant whites, Latinos, and Asians. Our study also suggests that some immigrant minority groups may be less likely to discuss diet or exercise with their clinicians.

Using data from the 1993-1994 NHIS, Singh and Siahpush²² reported that immigrants with longer duration of residence in the United States appear to be at higher risk of obesity than recent US immigrants. As part of a larger study examining causes of morbidity and mortality among immigrants, they described a cross-sectional relationship of increasing prevalence of obesity among immigrants and longer duration of residence in the United States, after adjusting for sociodemographic factors. Kaplan et al²³ confirmed this among Hispanics using 1998 NHIS data. Our study, using more recent data from 2000, in addition to being consistent with these previous results, estimated the magnitude of the increase in BMI associated with longer duration of residence in the United States and described these relationships across racial/ethnic subgroups. We found that BMI did not increase substantially until after living in the United States for at least 10 years, suggesting a threshold effect.

Trends in obesity among immigrants may reflect acculturation and adoption of the US lifestyle, such as increased sedentary behavior and poor dietary patterns. They may also be a response to the physical environment of the United States, with increased availability of calorically dense foods and higher reliance on labor-saving technologies. After 10 years, BMI appears to increase more substantially, a pattern that was consistent across sex and race/ethnicity, except among immigrant blacks for whom there was no change in BMI associated with years of US residence. Because of the cross-sectional nature of our data, we cannot establish a causal relationship between duration of residence and BMI. Our findings could reflect a cohort effect in which immigrants residing in the United States for a longer duration were more obese at the time of immigration than more recent immigrants; however, this is unlikely given the global trend toward obesity and the consistency between our findings and earlier studies.

Increasing obesity with longer duration of US residence is concerning given the rapid growth of the immigrant population¹¹ and the adverse health care consequences associated with obesity.⁵ Unfortunately, our findings also suggest that clinicians may be paying less attention to diet and exercise among some immigrant groups. We found that foreign-born minorities were generally less likely to report discussing diet and exercise with their clinicians, and US-born blacks were also less likely to report discussing exercise.

Reasons for these differences are unclear. Variation in sociodemographics, illness burden, and access to care may have contributed. However, the measured differences may be an artifact of how questions about counseling in the NHIS were interpreted by different cultural groups.²⁴ Research is needed to replicate our findings using more culturally sensitive methods and should also explore potential mechanisms for any observed differences such as language barriers, cultural discordance between patient and clinician, and clinician perceptions about lifestyle behaviors and obesity risk among foreign-born individuals.

Our study has some important limitations. Our results were based on self-reported information, which may lead to recall bias. Whether foreign-born and US-born adults have different biases in reporting weight, height, receipt of counseling, illness burden, or health behaviors is unclear, given that these concepts may be understood differently in different cultural groups.²⁴ The NHIS attempts to account for the nonresponse rate of 28% in the weighting procedure; however, the possibility of residual bias remains, which could limit the generalizability of our findings. It is unclear what proportion of non–English- and non–Spanish-speaking adults were excluded and what proportion used translators. In addition, we had no direct measures of acculturation, except for years of US residence, thereby limiting our ability to examine mechanisms of weight gain among immigrants. Finally, our study may have missed some important associations due to inadequate power to detect potentially clinically important differences in reported counseling rates, up to 5.3% for foreign-born Asians.

In summary, immigrants appear to assume a similarly high prevalence of obesity as US-born adults with longer duration of residence. With the growing immigrant population in the United States, early clinician intervention on diet and physical activity may represent an important opportunity to prevent weight gain, obesity, and obesity-related chronic illnesses.