Obesity, as measured by BMI, is a well-known risk factor for metabolic syndrome. There are many studies that question the validity of BMI as a marker of obesity in Asian populations, given data suggesting higher adiposity at similar levels of BMI.17,26,27
While more precise measures of adiposity such as waist circumference, CT scans, and DEXA scans may be preferable to BMI to define obesity, BMI is without question the most cost-effective approach. Waist circumference commonly uses four body sites for measurement, and no consensus exists on the optimum site for measurement, especially since it may differ by sex.28–30
Another study in which both waist circumference and BMI are available for all participants shows that BMI and waist circumference are highly correlated in specific racial/ethnic populations (NHWs, African Americans, and Hispanics), although Asians or Asian subgroups were not examined.31
CT scans and DEXA scans have been used in research settings, but are too expensive to be routinely employed in clinical or public health settings. BMI has proved to be a reliable measure of adiposity, or total body fat, irrespective of height.26,32–34
Moreover, the National Institutes of Health35
and the World Health Organization36
have proposed using BMI as a method for defining overweight and obesity. Given the expense, difficulty, and debatable additional utility of more specialized measures of obesity, it may be preferable to customize BMI ranges to better define overweight/obesity for specific subpopulations than propose more accurate measurement approaches.
Racial/ethnic differences in the relationship between BMI and metabolic syndrome risk have previously been documented among African Americans and Hispanics in the US.8,9,37–39
Our study is the first to examine the relationship between BMI and metabolic syndrome in Asian Americans, and in Asian subgroups in the US. Given the heterogeneity of the Asian American population, future studies of Asians in the US should strive to disaggregate these diverse populations. International studies conducted among different Asian national populations in China, Korea, Philippines, Singapore, and Taiwan have shown increased risk of Type 2 diabetes and cardiovascular disease at lower BMI than European populations.11–20
Other international societies, such as the International Diabetes Federation,40
suggest population-specific cut points for obesity, recognizing increased metabolic risk for some populations (Japanese and South Asians) despite similar levels of obesity. Our findings in Asian populations in the US corroborate this increased disease risk for metabolic syndrome at lower BMI. While there is limited sample size to make robust comparisons among the Asian American subgroups, our data strongly suggest heterogeneity among Asian American subgroups. For example, Japanese patients in our population have higher HDL than the other Asian subgroups. Confirming the contrasts among Asian populations that we observe in other study groups would advance our understanding of this apparent heterogeneity.
A strength of our study is that the subgroups examined have similar healthcare access, socioeconomic status, and geographic location. The distribution of Asian subgroups in the PAMF population is approximately equal to the proportions observed in the US.41
Previous work in the PAMF population has shown sufficient validity of the electronic health records42
to provide accurate disease estimates. Limitations of this study include it being conducted a single geographic area with somewhat limited sample size in the smaller Asian subgroups (i.e. Korean and Vietnamese populations). The PAMF population is insured (30% PPO, 60% HMO, 10% Medicare), and thus under-represents the medically underserved. However, these limitations that affect the generalizability of the sample also minimize unmeasured confounding and therefore improve the internal validity of our comparisons; any racial/ethnic differences observed in the PAMF clinical population will likely only be magnified in the general population.
We found that Asian Americans generally have lower mean BMI values and lower prevalence of overweight and obesity than NHWs when using traditional BMI ranges in our population of insured northern Californians. However, despite lower BMI, Asian Americans have higher rates of metabolic syndrome for each BMI category (normal, overweight, class I obesity, class II obesity). Over the BMI continuum, Asians are more likely to manifest metabolic syndrome than NHWs. This is true for most of the components of the metabolic syndrome as well. These findings support the WHO recommendation for lower BMI ranges for defining overweight and obesity in Asian populations, including Asian Americans, than for NHWs. Using lower BMI ranges for Asian Americans would allow for earlier and more appropriate screening for the components of metabolic syndrome and may improve preventive efforts.
Asian Americans have higher prevalence of metabolic syndrome than NHWs at each level of BMI. Accurate and early diagnosis of overweight and obesity in Asian Americans is critical to stem the rising tide of metabolic syndrome and Type 2 diabetes in this rapidly growing population within the US. Our work adds to the growing body of evidence that population-specific BMI ranges are necessary for accurate, timely diagnosis of overweight and obesity to prevent metabolic diseases.