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Body Mass Index and Risk of Death in Asian Americans Yikyung Park, ScD, Sophia Wang, PhD, Cari M. Kitahara, PhD, Steven C. Moore, PhD, Amy Berrington de Gonzalez, DPhil, Leslie Bernstein, PhD, Ellen T. Chang, ScD, Alan J. Flint, DrPH, D. Michal Freedman, PhD, J. Michael Gaziano, MD, Robert N. Hoover, MD, Martha S. Linet, MD, Mark Purdue, PhD, Kim Robien, PhD, Catherine Schairer, PhD, Howard D. Sesso, ScD, Emily White, PhD, Bradley J. Willcox, MD, Michael J. Thun, MD, Patricia Hartge, ScD, and Walter C. Willett, MD
During the past 30 years the prevalence of obesity, defined as a body mass index (BMI, defined as weight in kilograms divided by the square of height in meters) of 30 or greater, increased in all racial or ethnic groups in the United States. Historically, obesity has been uncommon among Asian Americans compared with other racial or ethnic groups in the United States, but the prevalence of obesity in this population is rising. Between 1991 and 2008, the prevalence of obesity increased from 13.9% to 28.7% among US-born Asians and from 9.5% to 20.7% among Asian immigrants.1 Substantial epidemiological data have defined the dose---response association between BMI and mortality among Whites in the United States,2 and to a lesser degree among US Blacks.3 However, data for Asian Americans, a fast-growing racial group in the United States, are sparse.4 Recently, a pooled analysis among East Asians residing in Asia, including Chinese, Japanese, and Koreans, showed that the association between BMI and total mortality was nonlinear, in that BMIs greater than 25 and less than 22.6 were both associated with increased mortality.5 It was interesting that the increased mortality associated with a high BMI in East Asians was not observed among Indians and Bangladeshis in Asia.5 Asian Americans share an environment with US Whites and Blacks, which differs markedly from that of their counterparts in Asia. It is unclear whether this change in context results in different susceptibility to obesity and obesity-related diseases between Asian Americans and Asians residing in Asia, or whether a shared genetic background contributes to a similar association between obesity and disease. To investigate the association between BMI and risk of mortality among Asian Americans in the largest sample to date, we pooled data from 10 prospective cohort studies in the United States.6---15
Objectives. We investigated the association between body mass index (BMI) and mortality among Asian Americans. Methods. We pooled data from prospective cohort studies with 20 672 Asian American adults with no baseline cancer or heart disease history. We estimated hazard ratios and 95% confidence intervals (CIs) with Cox proportional hazards models. Results. A high, but not low, BMI was associated with increased risk of total mortality among individuals aged 35 to 69 years. The BMI was not related to total mortality among individuals aged 70 years and older. With a BMI 22.5 to < 25 as the reference category among never-smokers aged 35 to 69 years, the hazard ratios for total mortality were 0.83 (95% CI = 0.47, 1.47) for BMI 15 to < 18.5; 0.91 (95% CI = 0.62, 1.32) for BMI 18.5 to < 20; 1.08 (95% CI = 0.86, 1.36) for BMI 20 to < 22.5; 1.14 (95% CI = 0.90, 1.44) for BMI 25 to < 27.5; 1.13 (95% CI = 0.79, 1.62) for BMI 27.5 to < 30; 1.82 (95% CI = 1.25, 2.64) for BMI 30 to < 35; and 2.09 (95% CI = 1.06, 4.11) for BMI 35 to 50. Higher BMI was also related to increased cardiovascular disease and cancer mortality. Conclusions. High BMI is associated with increased mortality risk among Asian Americans. (Am J Public Health. 2014;104:520–525. doi:10.2105/AJPH. 2013.301573)
METHODS We included prospective cohort studies that participated in a previous report on BMI and total mortality among Whites2 in this study if they met 3 criteria: (1) collected information on height, body weight, smoking status, and prevalent cancer and heart disease at baseline; (2) accrued more than 5 years of follow-up; and (3) were conducted in the United States. We defined Asian Americans as individuals with self-reported Asian race (n = 26 809). We did not include Pacific Islanders in our definition of Asian Americans. However, 3 cohorts ascertained Asian and Pacific Islander race as a single group10,14,15; thus, an unspecified but likely small number of Pacific Islanders were included from these studies. We excluded individuals who were younger than 35 years or older than 84 years at baseline, who had missing information on height or body weight, whose BMI was less than 15 or greater than 50, or who had less than 1 year of follow-up (n = 2073). As shown
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in previous studies,2,16,17 inclusion of people with preexisting diseases at baseline attenuates the association between BMI and mortality, and adjustment for preexisting diseases in a multivariate analysis is not adequate to eliminate the effect of this powerful confounder. Therefore, we further excluded individuals with a history of cancer or heart disease at baseline (n = 4064). The analytic population consisted of 20 672 Asian Americans.
Exposure and Outcome Ascertainment Each study ascertained self-reported height and body weight at baseline, which were used to calculate BMI. Studies also contributed data on demographics, smoking status, physical activity, and alcohol consumption at baseline, which were standardized between studies, as described previously.2 Studies also provided vital status of each participant and causes of death, which were ascertained from death certificates, medical records, or linkage to the National Death Index
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file. Deaths from cardiovascular disease, cancer, and other causes were defined according to the International Classification of Diseases, Ninth Revision.18
Statistical Analysis After we combined all studies into a single data set, we used Cox proportional hazards models, with age as the underlying time metric and stratified by study, to estimate hazard ratios (HRs) and 2-sided 95% confidence intervals (CIs). We followed participants from baseline to the date of death, loss to follow-up, or the end of follow-up, whichever occurred first. When we conducted study-specific analyses and subsequently pooled the individual study results with random-effects models, we obtained similar pooled estimates to that of the combined data set. We tested heterogeneity between studies with the Q statistic and I statistic. We categorized BMI into predefined groups (15 to < 18.5, 18.5 to < 20, 20 to < 22.5, 22.5 to < 25, 25 to < 27.5, 27.5 to < 30, 30 to < 35, and 35 to 50) that encompass the definitions of underweight (< 18.5), normal weight (18.5---24.9), overweight (25.0---29.9), and obesity (‡ 30.0) used by the World Health Organization.19 We used BMI from 22.5 to less than 25 as a reference category, as in previous studies.2,3,5 We also evaluated whether the relation of BMI to total mortality was linear by comparing a nonparametric regression curve that used a restricted cubic spline with the linear model using the likelihood ratio test, and by visual inspection of the restricted cubic spline graphs.20 Because we observed a linear association within BMI range of 22.5 to 50, we estimated mortality risk per 5-unit increase in BMI among individuals with a BMI of at least 22.5. We adjusted the multivariate models for gender, education, marital status, smoking status, physical activity, and alcohol consumption. We included categories for missing responses for all these variables. We also performed stratified analyses by age at baseline (35---59, 60---69, or ‡ 70 years) and by smoking status (never, former, or current). Because we observed a significant interaction between BMI (range of 22.5 to 50) and age at baseline (P for interaction = .02), and the association of BMI with total mortality among individuals aged 70 years or older differed
from that found in those who were younger than 70 years, we restricted the primary analyses to individuals who were younger than 70 years. We conducted additional prespecified subgroup analyses within strata of gender, physical activity (low, medium, or high), education (high school or less, some college, or college graduate or postcollege), and alcohol consumption at baseline (nondrinkers or drinkers). We performed all analyses with SAS version 9.1 (SAS Institute, Cary, NC).
RESULTS During follow-up of the cohorts (range = 5--27 years), a total 1476 deaths occurred among 20 672 Asian Americans. Median age at baseline was 61 years in men and 59 years in women, and median BMI was 25.0 in men and 23.8 in women. The prevalence of overweight (BMI 25 to < 30) and obesity (BMI ‡ 30) was 39% and 7%, respectively, among men and 23% and 7%, respectively, among women. Seven percent of participants were current smokers and 32% were former smokers. The prevalence of former smoking increased across categories of BMI, but the prevalence of current smoking was relatively constant. Compared with individuals with BMI from 22.5 to less than 25, those who were overweight or obese tended to have fewer years of education, to be physically inactive, and to be former
smokers (Table A, available as a supplement to the online version of this article at http://www. ajph.org). Characteristics of the participating cohorts are presented in Table B (available as a supplement to the online version of this article at http://www.ajph.org). Thirty-nine percent of the deaths were attributed to cancer, followed by death from cardiovascular disease (29%) and other causes (27%). The association of BMI with total mortality differed by age at baseline (Table 1; P for interaction = .02): we found an increased risk of total mortality among obese individuals who were younger than 70 years, whereas we observed no association among those who were aged 70 years or older. In the analysis restricted to individuals who were aged 35 to 69 years, we observed a nonlinear, reverseL-shape association between BMI and total mortality (P for nonlinearity < .001): the risk of total mortality increased in the ranges of overweight and obese, but did not change in BMI less than 22.5 (Figure 1). However, further restriction to never-smokers showed a linear association over the entire range of BMI and total mortality (P for linearity < .001): the risk of total mortality monotonically increased in BMI 22.5 or more, but decreased in BMI less than 22.5. Among those aged 35 to 69 years, compared with never-smokers with BMI 22.5 to less than 25, those with BMI 30 to less than
TABLE 1—Hazard Ratios for Total Mortality by Category of BMI and Age at Baseline Among Asian American Adults: Pooled Analysis of Prospective Cohort Studies, United States Aged 35–59 Years at Baseline BMI, kg/m2
Deaths, No.
Aged 60–69 Years at Baseline
HRa (95% CI)
Deaths, No.
HRa (95% CI)
Aged ‡ 70 Years at Baseline Deaths, No.
HRa (95% CI) 0.50 (0.18, 1.38)
15 to < 18.5
7
0.81 (0.38, 1.74)
16
0.96 (0.58, 1.60)
4
18.5 to < 20
23
0.96 (0.61, 1.51)
44
1.01 (0.73, 1.40)
26
1.66 (1.05, 2.64)
20 to < 22.5
92
1.03 (0.78, 1.35)
173
1.11 (0.92, 1.36)
56
1.01 (0.71, 1.43)
22.5 to < 25
121
1.00 (Ref)
245
1.00 (Ref)
78
1.00 (Ref)
25 to < 27.5 27.5 to < 30
90 33
1.17 (0.88, 1.54) 1.08 (0.73, 1.60)
199 72
1.13 (0.93, 1.36) 1.07 (0.82, 1.40)
50 20
1.00 (0.70, 1.43) 1.19 (0.72, 1.96)
30 to < 35
29
1.66 (1.10, 2.53)
57
1.67 (1.25, 2.24)
7
1.05 (0.48, 2.30)
35 to 50
13
2.46 (1.37, 4.42)
21
3.15 (2.01, 4.95)
0
...
Note. BMI = body mass index; CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. a Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day).
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article at http://www.ajph.org). Among those aged 35 to 69 years, compared with BMI 22.5 to less than 25, the HRs for BMI 30 to less than 35 and BMI 35 or greater were 1.83 (95% CI = 1.19, 2.82) and 4.22 (95% CI = 2.43, 7.36), respectively, for cardiovascular mortality, and 1.73 (95% CI = 1.20, 2.49) and 1.58 (95% CI = 0.77, 3.24), respectively, for cancer mortality. The HRs for death from other causes were 1.41 (95% CI = 0.86, 2.32) for BMI 30 to less than 35 and 2.61 (95% CI = 1.26, 5.42) for BMI 35 or greater compared with BMI 22.5 to less than 25. The results were similar when we restricted the analyses to never-smokers (data not shown).
5.0 HR, healthy individuals 95% CI, healthy individuals
4.5
HR, healthy never smokers 95% CI, healthy never smokers
4.0
Hazard Ratio
3.5 3.0 2.5 2.0 1.5
DISCUSSION
1.0
In our pooled analysis of Asian Americans, having a high BMI was associated with increased risk of total mortality. This association was similar to that observed previously among Asians, especially East Asians in Asia5 and Koreans in Korea,21 and Whites2,22 and Blacks3 in the United States. A summary of the association between BMI and mortality in Asians in Asia5 and Whites2 and Blacks3 in the United States is provided in Table E (available as a supplement to the online version of this article at http://www.ajph.org). The strength of the association between obesity and total mortality in Asian Americans was stronger than that of East Asians in Asia and similar to those of Whites and Blacks in the United States. Overall, across racial or ethnic groups, the lowest risk of death was observed in the BMI range of 20 to 24.9. Taken together, the lack of increased risk of total mortality among people within the BMI range of 20 to less than 25 in this study of Asian Americans and in the previous study of East Asians residing in Asia5 does not support the use of Asian-specific BMI cutoff points to define overweight (BMI ‡ 23) and obesity (BMI ‡ 27.5) as proposed previously.23 However, additional examination of the relation between BMI and risk of mortality within this range with larger samples is needed. Our finding of an increased risk of total mortality with a higher BMI is consistent with the results reported in studies of US Whites2 and Blacks,3 and of East Asians in Asia,5 but differs from the results of a study of Indians and
0.5 0.0
18
20
22
24
26
28
30
32
36
38
40
45
Body Mass Index, kg/m2 Note. CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. The HRs were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day). Smoking was excluded in the analysis of nonsmokers. The reference hazard ratio was 1.0.
FIGURE 1—Association between body mass index and total mortality among healthy Asian Americans and healthy never-smokers, aged 35–69 years: pooled analysis of prospective cohort studies, United States.
35 had an 82% increased risk of total mortality, and persons with BMI 35 or greater doubled the risk of total mortality (Table 2). Overall, these associations were similar for never and former smokers; among current smokers, there was a stronger increased risk of total mortality associated with BMI 35 or greater. We observed a positive association between obesity and total mortality among both men and women (Table C, available as a supplement to the online version of this article at http://www.ajph.org). There was no evidence of significant heterogeneity between studies (Figure A, available as a supplement to the online version of this article at http:// www.ajph.org; P for heterogeneity = .37; I2 = 8.3%). Among individuals who were aged 35 to 69 years and had a BMI of at least 22.5, each
5-unit increase in BMI was associated with a 37% increase in total mortality (HR = 1.37; 95% CI = 1.24, 1.51; Figure 2). The results were similar regardless of the length of followup: the HR for a 5-unit increase in BMI was 1.43 (95% CI = 1.27, 1.61) for less than 10 years of follow-up versus 1.26 (95% CI = 1.06, 1.50) for 10 years or more of follow-up. We observed similar positive associations across subgroups defined by physical activity, education, and alcohol consumption (Figure 2). The hazard ratio for each 5-unit increase in the entire range of BMI (15 to 50) among nonsmokers aged 35 to 69 years was 1.21 (95% CI = 1.08, 1.36; data not shown). A greater BMI was associated with increased risk of mortality from cardiovascular disease, cancer, and other causes (Table D, available as a supplement to the online version of this
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TABLE 2—Hazard Ratios for Total Mortality by BMI Category and Smoking Among Asian Americans, Aged 35–69 Years: Pooled Analysis of Prospective Cohort Studies, United States All Participantsa BMI, kg/m2 15 to < 18.5
Deaths, No.
HR (95% CI)
23 0.89 (0.58, 1.35)
Never Smokersb Deaths, No.
Former Smokersb
HR (95% CI)
Deaths, No.
HR (95% CI)
Current Smokersb Deaths, No.
HR (95% CI)
13 0.83 (0.47, 1.47)
4 1.10 (0.40, 3.00)
6
0.84 (0.35, 1.99)
18.5 to < 20 67 0.99 (0.76, 1.28) 34 0.91 (0.62, 1.32) 20 to < 22.5 265 1.07 (0.91, 1.26) 144 1.08 (0.86, 1.36)
14 0.93 (0.53, 1.62) 76 1.06 (0.80, 1.41)
13 37
0.88 (0.48, 1.63) 0.88 (0.58, 1.34)
22.5 to < 25 366 1.00 (Ref)
61
1.00 (Ref)
25 to < 27.5 289 1.13 (0.96, 1.32) 120 1.14 (0.90, 1.44) 135 1.26 (0.99, 1.60)
160 1.00 (Ref)
136 1.00 (Ref)
29
0.78 (0.50, 1.22)
27.5 to < 30 105 1.07 (0.86, 1.33)
38 1.13 (0.79, 1.62)
52 1.18 (0.85, 1.64)
13
0.66 (0.36, 1.22)
30 to < 35
86 1.66 (1.31, 2.11)
34 1.82 (1.25, 2.64)
42 1.72 (1.21, 2.45)
10
1.12 (0.56, 2.24)
35 to 50
34 2.78 (1.95, 3.97)
9 2.09 (1.06, 4.11)
14 2.85 (1.62, 5.01)
9
4.15 (1.96, 8.78)
Note. BMI = body mass index; CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. a Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day). b Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day).
Bangladeshis in Asia.5 This may be in part explained by the similarities and differences in population characteristics among the studies. Asian Americans in our study showed similar population characteristics to those of US Whites and Blacks who share the same environment, and although our study did not have information on nationality, most participants in our analyses were likely to be East Asians, given that the large proportion of Asian Americans were East Asians in the 1990 US Census.24 East Asians residing in Asia in the pooled analysis5 included Chinese, Japanese, and Koreans, who had population characteristics comparable to those of all Americans. On the other hand, Indians and Bangladeshis in Asia had a notably high prevalence of underweight and a low prevalence of obesity and had lower socioeconomic status, implying poor nutritional status and hygienic conditions and little access to health care.25---27 These differences may explain the distinct relation observed between BMI and total mortality in these 2 South Asian subgroups. As reported in previous studies,2,7,28 we found that the association between obesity and total mortality varied with age, with little evidence of increased risk of mortality among individuals aged 70 years or older. The low prevalence of obesity and increased risk of
mortality with a low BMI (< 20) among those aged 70 years or older suggested the likelihood that undiagnosed preexisting illness and loss of lean mass because of inactivity caused weight loss in this age group (reverse causation). When we restricted the analysis to never-smokers who were younger than 70 years, which was less likely to have confounding by smoking and prevalent diseases, our study found no relation of underweight to total mortality among Asian Americans. Our study even suggested a lower risk of mortality among underweight individuals. This contrasts with findings from previous studies that showed an increased risk of total mortality among underweight individuals,2,3,5,20,21 but is similar to findings in the Nurses’ Health Study6 and 2 other studies of Asians.29,30 Although the relation of underweight to mortality and the underlying reasons for this relation are areas of ongoing research, studies have shown that the association is largely attributable to excess deaths from respiratory and infectious diseases and likely reflects at least in part weight loss caused by underlying chronic diseases.5,21 The Asian Americans in our study constitute a relatively well-educated group with low rates of smoking and thus reduced rates of chronic respiratory disease and complicating infections. We cannot, however, rule out the
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possibility that no association between underweight and total mortality was attributable to chance because of the small number of cases in the underweight category. The prevalence of obesity among Asian Americans has steadily increased during the past 30 years. Studies have reported that US-born Asians have a higher prevalence of overweight and obesity than immigrants, and that immigrants from Asia to the United States have higher BMI values than their counterparts in Asia.1,31 In addition, later generations, longer duration of US residence, and lower socioeconomic status have been positively associated with obesity among Asian Americans.1,31---34 Even among Asian Americans, the prevalence of obesity varies by Asian ethnicity or nationality: Japanese, Filipinos, and Asian Indians were more likely to have higher BMI than Chinese, Koreans, and individuals of other Asian nationalities.33 Furthermore, the percentage of body fat for a particular BMI may vary across Asian ethnic groups.35 Our study, however, lacked information on country of origin, generation, and years since immigration, which may modify the association. Nevertheless, our pooled analysis included participants living in geographically diverse areas and had a wide range of ages, suggesting that our study population was a mixture of various Asian ethnic groups with different backgrounds. Also, a similar association between BMI and mortality was observed across participating studies in our pooled analysis. Studies conducted among ethnic subgroups of Asian Americans, particularly those utilizing waist circumference or waist-to-hip ratio as a better measure of abdominal obesity, are needed to further understand the relationship between obesity and mortality in these groups. Other limitations of our study include the use of a single self-reported assessment of height and weight as opposed to measured or repeated assessments. However, a study based on a nationally representative sample of the US population showed that BMI estimated on the basis of self-reported height and weight was highly correlated with BMI using measured height and weight.34 In addition, a pooled analysis of Asians residing in Asia5 showed that the association between BMI and mortality
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BMI and mortality have been seen in such analyses.2,22 In conclusion, we found that obesity was associated with an increased risk of total mortality among Asian Americans as it was among East Asians residing in Asia and Whites and Blacks in the United States. Also, no increased risk of total mortality in the BMI range of 20 to less than 25 in our study argues against lower BMI cutoff points for Asians to define overweight and obesity. Furthermore, among nonsmoking Asian Americans who were younger than 70 years and had no history of cancer or heart disease, we found no evidence of an increase in mortality at low BMIs, as has been reported in some Asian populations living in Asia. More studies are needed to investigate the effects of BMI and central obesity on disease risks, and to understand the causes of obesity among Asian Americans. j
About the Authors
Note. CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline and had a body mass index of at least 22.5 kg/m2. Dots indicate the HRs and horizontal lines indicate 95% CIs. The HRs were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day), except for the stratification variable.
FIGURE 2—Hazard ratios for total mortality per 5-unit increase in body mass index among Asian Americans, aged 35–69 years, according to selected characteristics: pooled analysis of prospective cohort studies, United States.
found in studies with self-reported height and weight was similar to that found in studies with measured height and weight. The number of underweight participants in our study was low, which limits our ability to draw conclusions about this group. The definition of Asian American in our analysis included some small number of Pacific Islanders, whose association between BMI and mortality may differ from that of other Asian groups. In addition, we cannot rule out potential residual confounding by unmeasured or unknown culture-specific risk factors. The strengths of our study include its relatively large size compared with previous
studies and its long length of follow-up, the wide range of BMI values, and the ability to exclude participants with prevalent cancer or heart disease at baseline or with less than 1 year of follow-up, which helps to reduce potential confounding by preexisting illness. We were also able to examine subgroups defined by smoking, physical activity, and education to assess potential effect modification. However, the size of the population precluded more-detailed analyses limited to never-smokers or analyses allowing lag periods of 5 or more years to reduce the influence of reverse causation; in earlier reports stronger associations between
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Yikyung Park, Cari M. Kitahara, Steven C. Moore, Amy Berrington de Gonzalez, D. Michal Freedman, Robert N. Hoover, Martha S. Linet, Mark Purdue, Catherine Schairer, and Patricia Hartge are with the Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD. Sophia Wang and Leslie Bernstein are with the Division of Cancer Etiology, Department of Population Sciences, Beckman Research Institute and the City of Hope, Duarte, CA. Ellen T. Chang is with Health Sciences Practice, Exponent Inc, Menlo Park, CA. Alan J. Flint and Walter C. Willett are with the Department of Nutrition, Harvard School of Public Health, Boston, MA. J. Michael Gaziano, Howard D. Sesso, and Walter C. Willett are with the Division of Preventive Medicine, Harvard Medical School, Brigham and Women’s Hospital, Boston, MA. Kim Robien is with Departments of Epidemiology and Biostatistics and Exercise Science, School of Public Health and Health Services, George Washington University, Washington, DC. Emily White is with the Cancer Prevention Program, Fred Hutchinson Cancer Research Center, Department of Epidemiology, University of Washington, Seattle. Bradley J. Willcox is with the Pacific Health Research and Education Institute and Queen’s Medical Center, Honolulu, HI. Michael J. Thun is with the Department of Epidemiology, Epidemiology and Surveillance Research, American Cancer Society, Atlanta, GA. Correspondence should be sent to Yikyung Park, ScD, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 9609 Medical Center Dr, Bethesda, MD 20892 (e-mail: [email protected]). Reprints can be ordered at http://www.ajph.org by clicking the “Reprints” link. This article was accepted on July 11, 2013.
Contributors Y. Park, C. M. Kitahara, S. C. Moore, A. Berrington de Gonzalez, P. Hartge, and W. C. Willett conceptualized the study concept and design. Y. Park analyzed and interpreted data, and drafted and revised the article. All
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authors were involved in data interpretation, provided critical revision of the article for intellectual content, and had final approval of the submitted article.
Acknowledgments This study was supported by the Intramural Research Program of the National Institutes of Health. The California Teachers Study was supported by the National Cancer Institute at the National Institutes of Health (grants R03 CA135687, R01 CA77398, and K05 CA136967), the California Breast Cancer Research fund (contract 97-10500), and the California Breast Cancer Act of 1993. The Health Professionals Follow-up Study is supported by the National Institutes of Health (grant UM1CA167752). The Physicians’ Health Study was supported by the National Institutes of Health (grants CA 097193, CA 34944, CA 40360, HL 26490, and HL 34595).
Human Participant Protection Each included study was approved by the institutional review boards of the participating institutions.
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3. Boggs DA, Rosenberg L, Cozier YC, et al. General and abdominal obesity and risk of death among Black women. N Engl J Med. 2011;365(10):901---908.
22. Prospective Studies Collaboration; Whitlock G, Lewington S, Sherliker P, et al. Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies. Lancet. 2009; 373(9669):1083---1096.
6. Adams KF, Schatzkin A, Harris TB, et al. Overweight, obesity, and mortality in a large prospective cohort of persons 50 to 71 years old. N Engl J Med. 2006;355(8):763---778. 7. Moore SC, Mayne ST, Graubard BI, et al. Past body mass index and risk of mortality among women. Int J Obes (Lond). 2008;32(5):730---739. 8. Bessonova L, Marshall SF, Ziogas A, et al. The association of body mass index with mortality in the California Teachers Study. Int J Cancer. 2011;129(10): 2492---2501. 9. Hu FB, Willett WC, Li T, Stampfer MJ, Colditz GA, Manson JE. Adiposity as compared with physical activity in predicting mortality among women. N Engl J Med. 2004;351(26):2694---2703. 10. Folsom AR, Kaye SA, Sellers TA, et al. Body fat distribution and 5-year risk of death in older women. JAMA. 1993;269(4):483---487. 11. Xiao Q, Hsing AW, Park Y, et al. Body mass index and mortality among Blacks and Whites adults in the prostate, lung, colorectal, and ovarian (PLCO) cancer screening trial. Obesity (Silver Spring). 2013;Epub ahead of print.
35. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403): 157---163.
20. Durrleman S, Simon R. Flexible regression models with cubic splines. Stat Med. 1989;8(5):551---561. 21. Jee SH, Sull JW, Park J, et al. Body-mass index and mortality in Korean men and women. N Engl J Med. 2006;355(8):779---787.
5. Zheng W, McLerran DF, Rolland B, et al. Association between body-mass index and risk of death in more than 1 million Asians. N Engl J Med. 2011;364(8): 719---729.
34. Oza-Frank R, Cunningham SA. The weight of US residence among immigrants: a systematic review. Obes Rev. 2010;11(4):271---280.
19. World Health Organization. Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee. World Health Organ Tech Rep Ser. 1995;854:312---344.
2. Berrington de Gonzalez A, Hartge P, Cerhan JR, et al. Body-mass index and mortality among 1.46 million White adults. N Engl J Med. 2010;363(23):2211---2219.
4. Maskarinec G, Meng L, Kolonel LN. Alcohol intake, body weight, and mortality in a multiethnic prospective cohort. Epidemiology. 1998;9(6):654---661.
33. Lauderdale DS, Rathouz PJ. Body mass index in a US national sample of Asian Americans: effects of nativity, years since immigration and socioeconomic status. Int J Obes Relat Metab Disord. 2000;24(9): 1188---1194.
23. World Health Organization, International Association for the Study of Obesity, International Obesity Task Force. The Asia-Pacific Perspective: Redefining Obesity and Its Treatment. Sydney, Australia: Health Communications; 2000. 24. Paisano E. We the Americans: Asian. Washington, DC: Bureau of the Census; 1993. 25. Pednekar MS, Hakama M, Hebert JR, Gupta PC. Association of body mass index with all-cause and cause-specific mortality: findings from a prospective cohort study in Mumbai (Bombay), India. Int J Epidemiol. 2008;37(3):524---535. 26. Sauvaget C, Ramadas K, Thomas G, Vinoda J, Thara S, Sankaranarayanan R. Body mass index, weight change and mortality risk in a prospective study in India. Int J Epidemiol. 2008;37(5):990---1004. 27. Pierce BL, Kalra T, Argos M, et al. A prospective study of body mass index and mortality in Bangladesh. Int J Epidemiol. 2010;39(4):1037---1045. 28. Bender R, Jockel KH, Trautner C, Spraul M, Berger M. Effect of age on excess mortality in obesity. JAMA. 1999;281(16):1498---1504. 29. Zhang X, Shu XO, Chow WH, et al. Body mass index at various ages and mortality in Chinese women: impact of potential methodological biases. Int J Obes (Lond). 2008;32(7):1130---1136. 30. Wen CP, David Cheng TY, Tsai SP, et al. Are Asians at greater mortality risks for being overweight than
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Body Mass Index and Risk of Death in Asian Americans Yikyung Park, ScD, Sophia Wang, PhD, Cari M. Kitahara, PhD, Steven C. Moore, PhD, Amy Berrington de Gonzalez, DPhil, Leslie Bernstein, PhD, Ellen T. Chang, ScD, Alan J. Flint, DrPH, D. Michal Freedman, PhD, J. Michael Gaziano, MD, Robert N. Hoover, MD, Martha S. Linet, MD, Mark Purdue, PhD, Kim Robien, PhD, Catherine Schairer, PhD, Howard D. Sesso, ScD, Emily White, PhD, Bradley J. Willcox, MD, Michael J. Thun, MD, Patricia Hartge, ScD, and Walter C. Willett, MD
During the past 30 years the prevalence of obesity, defined as a body mass index (BMI, defined as weight in kilograms divided by the square of height in meters) of 30 or greater, increased in all racial or ethnic groups in the United States. Historically, obesity has been uncommon among Asian Americans compared with other racial or ethnic groups in the United States, but the prevalence of obesity in this population is rising. Between 1991 and 2008, the prevalence of obesity increased from 13.9% to 28.7% among US-born Asians and from 9.5% to 20.7% among Asian immigrants.1 Substantial epidemiological data have defined the dose---response association between BMI and mortality among Whites in the United States,2 and to a lesser degree among US Blacks.3 However, data for Asian Americans, a fast-growing racial group in the United States, are sparse.4 Recently, a pooled analysis among East Asians residing in Asia, including Chinese, Japanese, and Koreans, showed that the association between BMI and total mortality was nonlinear, in that BMIs greater than 25 and less than 22.6 were both associated with increased mortality.5 It was interesting that the increased mortality associated with a high BMI in East Asians was not observed among Indians and Bangladeshis in Asia.5 Asian Americans share an environment with US Whites and Blacks, which differs markedly from that of their counterparts in Asia. It is unclear whether this change in context results in different susceptibility to obesity and obesity-related diseases between Asian Americans and Asians residing in Asia, or whether a shared genetic background contributes to a similar association between obesity and disease. To investigate the association between BMI and risk of mortality among Asian Americans in the largest sample to date, we pooled data from 10 prospective cohort studies in the United States.6---15
Objectives. We investigated the association between body mass index (BMI) and mortality among Asian Americans. Methods. We pooled data from prospective cohort studies with 20 672 Asian American adults with no baseline cancer or heart disease history. We estimated hazard ratios and 95% confidence intervals (CIs) with Cox proportional hazards models. Results. A high, but not low, BMI was associated with increased risk of total mortality among individuals aged 35 to 69 years. The BMI was not related to total mortality among individuals aged 70 years and older. With a BMI 22.5 to < 25 as the reference category among never-smokers aged 35 to 69 years, the hazard ratios for total mortality were 0.83 (95% CI = 0.47, 1.47) for BMI 15 to < 18.5; 0.91 (95% CI = 0.62, 1.32) for BMI 18.5 to < 20; 1.08 (95% CI = 0.86, 1.36) for BMI 20 to < 22.5; 1.14 (95% CI = 0.90, 1.44) for BMI 25 to < 27.5; 1.13 (95% CI = 0.79, 1.62) for BMI 27.5 to < 30; 1.82 (95% CI = 1.25, 2.64) for BMI 30 to < 35; and 2.09 (95% CI = 1.06, 4.11) for BMI 35 to 50. Higher BMI was also related to increased cardiovascular disease and cancer mortality. Conclusions. High BMI is associated with increased mortality risk among Asian Americans. (Am J Public Health. 2014;104:520–525. doi:10.2105/AJPH. 2013.301573)
METHODS We included prospective cohort studies that participated in a previous report on BMI and total mortality among Whites2 in this study if they met 3 criteria: (1) collected information on height, body weight, smoking status, and prevalent cancer and heart disease at baseline; (2) accrued more than 5 years of follow-up; and (3) were conducted in the United States. We defined Asian Americans as individuals with self-reported Asian race (n = 26 809). We did not include Pacific Islanders in our definition of Asian Americans. However, 3 cohorts ascertained Asian and Pacific Islander race as a single group10,14,15; thus, an unspecified but likely small number of Pacific Islanders were included from these studies. We excluded individuals who were younger than 35 years or older than 84 years at baseline, who had missing information on height or body weight, whose BMI was less than 15 or greater than 50, or who had less than 1 year of follow-up (n = 2073). As shown
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in previous studies,2,16,17 inclusion of people with preexisting diseases at baseline attenuates the association between BMI and mortality, and adjustment for preexisting diseases in a multivariate analysis is not adequate to eliminate the effect of this powerful confounder. Therefore, we further excluded individuals with a history of cancer or heart disease at baseline (n = 4064). The analytic population consisted of 20 672 Asian Americans.
Exposure and Outcome Ascertainment Each study ascertained self-reported height and body weight at baseline, which were used to calculate BMI. Studies also contributed data on demographics, smoking status, physical activity, and alcohol consumption at baseline, which were standardized between studies, as described previously.2 Studies also provided vital status of each participant and causes of death, which were ascertained from death certificates, medical records, or linkage to the National Death Index
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file. Deaths from cardiovascular disease, cancer, and other causes were defined according to the International Classification of Diseases, Ninth Revision.18
Statistical Analysis After we combined all studies into a single data set, we used Cox proportional hazards models, with age as the underlying time metric and stratified by study, to estimate hazard ratios (HRs) and 2-sided 95% confidence intervals (CIs). We followed participants from baseline to the date of death, loss to follow-up, or the end of follow-up, whichever occurred first. When we conducted study-specific analyses and subsequently pooled the individual study results with random-effects models, we obtained similar pooled estimates to that of the combined data set. We tested heterogeneity between studies with the Q statistic and I statistic. We categorized BMI into predefined groups (15 to < 18.5, 18.5 to < 20, 20 to < 22.5, 22.5 to < 25, 25 to < 27.5, 27.5 to < 30, 30 to < 35, and 35 to 50) that encompass the definitions of underweight (< 18.5), normal weight (18.5---24.9), overweight (25.0---29.9), and obesity (‡ 30.0) used by the World Health Organization.19 We used BMI from 22.5 to less than 25 as a reference category, as in previous studies.2,3,5 We also evaluated whether the relation of BMI to total mortality was linear by comparing a nonparametric regression curve that used a restricted cubic spline with the linear model using the likelihood ratio test, and by visual inspection of the restricted cubic spline graphs.20 Because we observed a linear association within BMI range of 22.5 to 50, we estimated mortality risk per 5-unit increase in BMI among individuals with a BMI of at least 22.5. We adjusted the multivariate models for gender, education, marital status, smoking status, physical activity, and alcohol consumption. We included categories for missing responses for all these variables. We also performed stratified analyses by age at baseline (35---59, 60---69, or ‡ 70 years) and by smoking status (never, former, or current). Because we observed a significant interaction between BMI (range of 22.5 to 50) and age at baseline (P for interaction = .02), and the association of BMI with total mortality among individuals aged 70 years or older differed
from that found in those who were younger than 70 years, we restricted the primary analyses to individuals who were younger than 70 years. We conducted additional prespecified subgroup analyses within strata of gender, physical activity (low, medium, or high), education (high school or less, some college, or college graduate or postcollege), and alcohol consumption at baseline (nondrinkers or drinkers). We performed all analyses with SAS version 9.1 (SAS Institute, Cary, NC).
RESULTS During follow-up of the cohorts (range = 5--27 years), a total 1476 deaths occurred among 20 672 Asian Americans. Median age at baseline was 61 years in men and 59 years in women, and median BMI was 25.0 in men and 23.8 in women. The prevalence of overweight (BMI 25 to < 30) and obesity (BMI ‡ 30) was 39% and 7%, respectively, among men and 23% and 7%, respectively, among women. Seven percent of participants were current smokers and 32% were former smokers. The prevalence of former smoking increased across categories of BMI, but the prevalence of current smoking was relatively constant. Compared with individuals with BMI from 22.5 to less than 25, those who were overweight or obese tended to have fewer years of education, to be physically inactive, and to be former
smokers (Table A, available as a supplement to the online version of this article at http://www. ajph.org). Characteristics of the participating cohorts are presented in Table B (available as a supplement to the online version of this article at http://www.ajph.org). Thirty-nine percent of the deaths were attributed to cancer, followed by death from cardiovascular disease (29%) and other causes (27%). The association of BMI with total mortality differed by age at baseline (Table 1; P for interaction = .02): we found an increased risk of total mortality among obese individuals who were younger than 70 years, whereas we observed no association among those who were aged 70 years or older. In the analysis restricted to individuals who were aged 35 to 69 years, we observed a nonlinear, reverseL-shape association between BMI and total mortality (P for nonlinearity < .001): the risk of total mortality increased in the ranges of overweight and obese, but did not change in BMI less than 22.5 (Figure 1). However, further restriction to never-smokers showed a linear association over the entire range of BMI and total mortality (P for linearity < .001): the risk of total mortality monotonically increased in BMI 22.5 or more, but decreased in BMI less than 22.5. Among those aged 35 to 69 years, compared with never-smokers with BMI 22.5 to less than 25, those with BMI 30 to less than
TABLE 1—Hazard Ratios for Total Mortality by Category of BMI and Age at Baseline Among Asian American Adults: Pooled Analysis of Prospective Cohort Studies, United States Aged 35–59 Years at Baseline BMI, kg/m2
Deaths, No.
Aged 60–69 Years at Baseline
HRa (95% CI)
Deaths, No.
HRa (95% CI)
Aged ‡ 70 Years at Baseline Deaths, No.
HRa (95% CI) 0.50 (0.18, 1.38)
15 to < 18.5
7
0.81 (0.38, 1.74)
16
0.96 (0.58, 1.60)
4
18.5 to < 20
23
0.96 (0.61, 1.51)
44
1.01 (0.73, 1.40)
26
1.66 (1.05, 2.64)
20 to < 22.5
92
1.03 (0.78, 1.35)
173
1.11 (0.92, 1.36)
56
1.01 (0.71, 1.43)
22.5 to < 25
121
1.00 (Ref)
245
1.00 (Ref)
78
1.00 (Ref)
25 to < 27.5 27.5 to < 30
90 33
1.17 (0.88, 1.54) 1.08 (0.73, 1.60)
199 72
1.13 (0.93, 1.36) 1.07 (0.82, 1.40)
50 20
1.00 (0.70, 1.43) 1.19 (0.72, 1.96)
30 to < 35
29
1.66 (1.10, 2.53)
57
1.67 (1.25, 2.24)
7
1.05 (0.48, 2.30)
35 to 50
13
2.46 (1.37, 4.42)
21
3.15 (2.01, 4.95)
0
...
Note. BMI = body mass index; CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. a Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day).
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article at http://www.ajph.org). Among those aged 35 to 69 years, compared with BMI 22.5 to less than 25, the HRs for BMI 30 to less than 35 and BMI 35 or greater were 1.83 (95% CI = 1.19, 2.82) and 4.22 (95% CI = 2.43, 7.36), respectively, for cardiovascular mortality, and 1.73 (95% CI = 1.20, 2.49) and 1.58 (95% CI = 0.77, 3.24), respectively, for cancer mortality. The HRs for death from other causes were 1.41 (95% CI = 0.86, 2.32) for BMI 30 to less than 35 and 2.61 (95% CI = 1.26, 5.42) for BMI 35 or greater compared with BMI 22.5 to less than 25. The results were similar when we restricted the analyses to never-smokers (data not shown).
5.0 HR, healthy individuals 95% CI, healthy individuals
4.5
HR, healthy never smokers 95% CI, healthy never smokers
4.0
Hazard Ratio
3.5 3.0 2.5 2.0 1.5
DISCUSSION
1.0
In our pooled analysis of Asian Americans, having a high BMI was associated with increased risk of total mortality. This association was similar to that observed previously among Asians, especially East Asians in Asia5 and Koreans in Korea,21 and Whites2,22 and Blacks3 in the United States. A summary of the association between BMI and mortality in Asians in Asia5 and Whites2 and Blacks3 in the United States is provided in Table E (available as a supplement to the online version of this article at http://www.ajph.org). The strength of the association between obesity and total mortality in Asian Americans was stronger than that of East Asians in Asia and similar to those of Whites and Blacks in the United States. Overall, across racial or ethnic groups, the lowest risk of death was observed in the BMI range of 20 to 24.9. Taken together, the lack of increased risk of total mortality among people within the BMI range of 20 to less than 25 in this study of Asian Americans and in the previous study of East Asians residing in Asia5 does not support the use of Asian-specific BMI cutoff points to define overweight (BMI ‡ 23) and obesity (BMI ‡ 27.5) as proposed previously.23 However, additional examination of the relation between BMI and risk of mortality within this range with larger samples is needed. Our finding of an increased risk of total mortality with a higher BMI is consistent with the results reported in studies of US Whites2 and Blacks,3 and of East Asians in Asia,5 but differs from the results of a study of Indians and
0.5 0.0
18
20
22
24
26
28
30
32
36
38
40
45
Body Mass Index, kg/m2 Note. CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. The HRs were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day). Smoking was excluded in the analysis of nonsmokers. The reference hazard ratio was 1.0.
FIGURE 1—Association between body mass index and total mortality among healthy Asian Americans and healthy never-smokers, aged 35–69 years: pooled analysis of prospective cohort studies, United States.
35 had an 82% increased risk of total mortality, and persons with BMI 35 or greater doubled the risk of total mortality (Table 2). Overall, these associations were similar for never and former smokers; among current smokers, there was a stronger increased risk of total mortality associated with BMI 35 or greater. We observed a positive association between obesity and total mortality among both men and women (Table C, available as a supplement to the online version of this article at http://www.ajph.org). There was no evidence of significant heterogeneity between studies (Figure A, available as a supplement to the online version of this article at http:// www.ajph.org; P for heterogeneity = .37; I2 = 8.3%). Among individuals who were aged 35 to 69 years and had a BMI of at least 22.5, each
5-unit increase in BMI was associated with a 37% increase in total mortality (HR = 1.37; 95% CI = 1.24, 1.51; Figure 2). The results were similar regardless of the length of followup: the HR for a 5-unit increase in BMI was 1.43 (95% CI = 1.27, 1.61) for less than 10 years of follow-up versus 1.26 (95% CI = 1.06, 1.50) for 10 years or more of follow-up. We observed similar positive associations across subgroups defined by physical activity, education, and alcohol consumption (Figure 2). The hazard ratio for each 5-unit increase in the entire range of BMI (15 to 50) among nonsmokers aged 35 to 69 years was 1.21 (95% CI = 1.08, 1.36; data not shown). A greater BMI was associated with increased risk of mortality from cardiovascular disease, cancer, and other causes (Table D, available as a supplement to the online version of this
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TABLE 2—Hazard Ratios for Total Mortality by BMI Category and Smoking Among Asian Americans, Aged 35–69 Years: Pooled Analysis of Prospective Cohort Studies, United States All Participantsa BMI, kg/m2 15 to < 18.5
Deaths, No.
HR (95% CI)
23 0.89 (0.58, 1.35)
Never Smokersb Deaths, No.
Former Smokersb
HR (95% CI)
Deaths, No.
HR (95% CI)
Current Smokersb Deaths, No.
HR (95% CI)
13 0.83 (0.47, 1.47)
4 1.10 (0.40, 3.00)
6
0.84 (0.35, 1.99)
18.5 to < 20 67 0.99 (0.76, 1.28) 34 0.91 (0.62, 1.32) 20 to < 22.5 265 1.07 (0.91, 1.26) 144 1.08 (0.86, 1.36)
14 0.93 (0.53, 1.62) 76 1.06 (0.80, 1.41)
13 37
0.88 (0.48, 1.63) 0.88 (0.58, 1.34)
22.5 to < 25 366 1.00 (Ref)
61
1.00 (Ref)
25 to < 27.5 289 1.13 (0.96, 1.32) 120 1.14 (0.90, 1.44) 135 1.26 (0.99, 1.60)
160 1.00 (Ref)
136 1.00 (Ref)
29
0.78 (0.50, 1.22)
27.5 to < 30 105 1.07 (0.86, 1.33)
38 1.13 (0.79, 1.62)
52 1.18 (0.85, 1.64)
13
0.66 (0.36, 1.22)
30 to < 35
86 1.66 (1.31, 2.11)
34 1.82 (1.25, 2.64)
42 1.72 (1.21, 2.45)
10
1.12 (0.56, 2.24)
35 to 50
34 2.78 (1.95, 3.97)
9 2.09 (1.06, 4.11)
14 2.85 (1.62, 5.01)
9
4.15 (1.96, 8.78)
Note. BMI = body mass index; CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline. a Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day). b Hazard ratios were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day).
Bangladeshis in Asia.5 This may be in part explained by the similarities and differences in population characteristics among the studies. Asian Americans in our study showed similar population characteristics to those of US Whites and Blacks who share the same environment, and although our study did not have information on nationality, most participants in our analyses were likely to be East Asians, given that the large proportion of Asian Americans were East Asians in the 1990 US Census.24 East Asians residing in Asia in the pooled analysis5 included Chinese, Japanese, and Koreans, who had population characteristics comparable to those of all Americans. On the other hand, Indians and Bangladeshis in Asia had a notably high prevalence of underweight and a low prevalence of obesity and had lower socioeconomic status, implying poor nutritional status and hygienic conditions and little access to health care.25---27 These differences may explain the distinct relation observed between BMI and total mortality in these 2 South Asian subgroups. As reported in previous studies,2,7,28 we found that the association between obesity and total mortality varied with age, with little evidence of increased risk of mortality among individuals aged 70 years or older. The low prevalence of obesity and increased risk of
mortality with a low BMI (< 20) among those aged 70 years or older suggested the likelihood that undiagnosed preexisting illness and loss of lean mass because of inactivity caused weight loss in this age group (reverse causation). When we restricted the analysis to never-smokers who were younger than 70 years, which was less likely to have confounding by smoking and prevalent diseases, our study found no relation of underweight to total mortality among Asian Americans. Our study even suggested a lower risk of mortality among underweight individuals. This contrasts with findings from previous studies that showed an increased risk of total mortality among underweight individuals,2,3,5,20,21 but is similar to findings in the Nurses’ Health Study6 and 2 other studies of Asians.29,30 Although the relation of underweight to mortality and the underlying reasons for this relation are areas of ongoing research, studies have shown that the association is largely attributable to excess deaths from respiratory and infectious diseases and likely reflects at least in part weight loss caused by underlying chronic diseases.5,21 The Asian Americans in our study constitute a relatively well-educated group with low rates of smoking and thus reduced rates of chronic respiratory disease and complicating infections. We cannot, however, rule out the
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possibility that no association between underweight and total mortality was attributable to chance because of the small number of cases in the underweight category. The prevalence of obesity among Asian Americans has steadily increased during the past 30 years. Studies have reported that US-born Asians have a higher prevalence of overweight and obesity than immigrants, and that immigrants from Asia to the United States have higher BMI values than their counterparts in Asia.1,31 In addition, later generations, longer duration of US residence, and lower socioeconomic status have been positively associated with obesity among Asian Americans.1,31---34 Even among Asian Americans, the prevalence of obesity varies by Asian ethnicity or nationality: Japanese, Filipinos, and Asian Indians were more likely to have higher BMI than Chinese, Koreans, and individuals of other Asian nationalities.33 Furthermore, the percentage of body fat for a particular BMI may vary across Asian ethnic groups.35 Our study, however, lacked information on country of origin, generation, and years since immigration, which may modify the association. Nevertheless, our pooled analysis included participants living in geographically diverse areas and had a wide range of ages, suggesting that our study population was a mixture of various Asian ethnic groups with different backgrounds. Also, a similar association between BMI and mortality was observed across participating studies in our pooled analysis. Studies conducted among ethnic subgroups of Asian Americans, particularly those utilizing waist circumference or waist-to-hip ratio as a better measure of abdominal obesity, are needed to further understand the relationship between obesity and mortality in these groups. Other limitations of our study include the use of a single self-reported assessment of height and weight as opposed to measured or repeated assessments. However, a study based on a nationally representative sample of the US population showed that BMI estimated on the basis of self-reported height and weight was highly correlated with BMI using measured height and weight.34 In addition, a pooled analysis of Asians residing in Asia5 showed that the association between BMI and mortality
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BMI and mortality have been seen in such analyses.2,22 In conclusion, we found that obesity was associated with an increased risk of total mortality among Asian Americans as it was among East Asians residing in Asia and Whites and Blacks in the United States. Also, no increased risk of total mortality in the BMI range of 20 to less than 25 in our study argues against lower BMI cutoff points for Asians to define overweight and obesity. Furthermore, among nonsmoking Asian Americans who were younger than 70 years and had no history of cancer or heart disease, we found no evidence of an increase in mortality at low BMIs, as has been reported in some Asian populations living in Asia. More studies are needed to investigate the effects of BMI and central obesity on disease risks, and to understand the causes of obesity among Asian Americans. j
About the Authors
Note. CI = confidence interval; HR = hazard ratio. Participants had no history of cancer or heart disease at baseline and had a body mass index of at least 22.5 kg/m2. Dots indicate the HRs and horizontal lines indicate 95% CIs. The HRs were stratified by study and adjusted for age, gender, education (high school, some college, college graduate), marital status (married, not married), smoking status (never, former, current), physical activity (low, medium, high), and alcohol consumption (0, > 0 g/day), except for the stratification variable.
FIGURE 2—Hazard ratios for total mortality per 5-unit increase in body mass index among Asian Americans, aged 35–69 years, according to selected characteristics: pooled analysis of prospective cohort studies, United States.
found in studies with self-reported height and weight was similar to that found in studies with measured height and weight. The number of underweight participants in our study was low, which limits our ability to draw conclusions about this group. The definition of Asian American in our analysis included some small number of Pacific Islanders, whose association between BMI and mortality may differ from that of other Asian groups. In addition, we cannot rule out potential residual confounding by unmeasured or unknown culture-specific risk factors. The strengths of our study include its relatively large size compared with previous
studies and its long length of follow-up, the wide range of BMI values, and the ability to exclude participants with prevalent cancer or heart disease at baseline or with less than 1 year of follow-up, which helps to reduce potential confounding by preexisting illness. We were also able to examine subgroups defined by smoking, physical activity, and education to assess potential effect modification. However, the size of the population precluded more-detailed analyses limited to never-smokers or analyses allowing lag periods of 5 or more years to reduce the influence of reverse causation; in earlier reports stronger associations between
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Yikyung Park, Cari M. Kitahara, Steven C. Moore, Amy Berrington de Gonzalez, D. Michal Freedman, Robert N. Hoover, Martha S. Linet, Mark Purdue, Catherine Schairer, and Patricia Hartge are with the Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD. Sophia Wang and Leslie Bernstein are with the Division of Cancer Etiology, Department of Population Sciences, Beckman Research Institute and the City of Hope, Duarte, CA. Ellen T. Chang is with Health Sciences Practice, Exponent Inc, Menlo Park, CA. Alan J. Flint and Walter C. Willett are with the Department of Nutrition, Harvard School of Public Health, Boston, MA. J. Michael Gaziano, Howard D. Sesso, and Walter C. Willett are with the Division of Preventive Medicine, Harvard Medical School, Brigham and Women’s Hospital, Boston, MA. Kim Robien is with Departments of Epidemiology and Biostatistics and Exercise Science, School of Public Health and Health Services, George Washington University, Washington, DC. Emily White is with the Cancer Prevention Program, Fred Hutchinson Cancer Research Center, Department of Epidemiology, University of Washington, Seattle. Bradley J. Willcox is with the Pacific Health Research and Education Institute and Queen’s Medical Center, Honolulu, HI. Michael J. Thun is with the Department of Epidemiology, Epidemiology and Surveillance Research, American Cancer Society, Atlanta, GA. Correspondence should be sent to Yikyung Park, ScD, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 9609 Medical Center Dr, Bethesda, MD 20892 (e-mail: [email protected]). Reprints can be ordered at http://www.ajph.org by clicking the “Reprints” link. This article was accepted on July 11, 2013.
Contributors Y. Park, C. M. Kitahara, S. C. Moore, A. Berrington de Gonzalez, P. Hartge, and W. C. Willett conceptualized the study concept and design. Y. Park analyzed and interpreted data, and drafted and revised the article. All
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authors were involved in data interpretation, provided critical revision of the article for intellectual content, and had final approval of the submitted article.
Acknowledgments This study was supported by the Intramural Research Program of the National Institutes of Health. The California Teachers Study was supported by the National Cancer Institute at the National Institutes of Health (grants R03 CA135687, R01 CA77398, and K05 CA136967), the California Breast Cancer Research fund (contract 97-10500), and the California Breast Cancer Act of 1993. The Health Professionals Follow-up Study is supported by the National Institutes of Health (grant UM1CA167752). The Physicians’ Health Study was supported by the National Institutes of Health (grants CA 097193, CA 34944, CA 40360, HL 26490, and HL 34595).
Human Participant Protection Each included study was approved by the institutional review boards of the participating institutions.
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