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Obesity in Ethnic Minorities
Abstract & Commentary
By Michael H. Crawford, MD, Professor of Medicine, and Chief of Clinical Cardiology, at the University of California, San Francisco. Dr. Crawford is on the speaker's bureau for Pfizer.
Source: Razak F, et al. Defining Obesity Cut Points in a Multiethnic Population. Circulation. 2007;115:2111-2118.
Synopsis: Revisions of BMI cut-points to define obesity in ethnic minorities may be warranted.
Obesity is often measured as body mass index (BMI) and well developed cut-points for overweight and obesity have been defined largely in European populations. Evidence has emerged that these cut-points may not be appropriate for other ethnic groups. Thus, this group of investigators from Canada studied 301 Europeans, 289 South Asians, 281 Chinese and 207 Aboriginals, assessing their metabolic risk for cardiovascular (CV) disease in relation to BMI. Among 14 clinical and biochemical variables, 3 factors explained over half of the variation in the variables between ethnic groups: glucose metabolism, lipid metabolism and blood pressure. BMI cut-points were then determined for these 3 factors. Glucose and lipid factors were higher in non-Europeans and blood pressure was highest in the Chinese and lowest in the Aboriginals. When the European BMI cut-point of 30 kg/m2 was applied to the glucose factor, a similar abnormality was seen at a BMI of 21 in South Asians and Chinese, and 22 in Aboriginals. For the lipid factor it was 23 in South Asians, 26 in Chinese and Aboriginals. For blood pressure it was 29 for South Asians and 25 for Chinese. On average the cut-point to define obesity in relation to these 3 factors in non Europeans was about 24 kg/m2. The authors concluded that revisions of BMI cut-points to define obesity in ethnic minorities may be warranted and would increase the estimated burden of obesity-related disease in non-Europeans.
The links between obesity and increased CV disease are complex and not fully understood. In this analysis, glucose metabolism, lipid levels and blood pressure only explained 56% of the variability in 14 clinical and biomarkers. Clearly other factors such as lifestyle (exercise) and genetics play a role, but these are difficult to quantitate at this time. Given our current understanding, the authors' approach of defining obesity in relation to when these 3 factors become abnormal makes sense. The main message when this is done with ethnic minorities is that generally lower cut-points are found compared to Europeans. Thus, our current BMI definitions of obesity in ethnic minorities underestimate the burden of CV disease, so physicians need to take this into consideration when advising and treating patients.
The problem with this 3-factor approach is that the results are not uniform across ethnic groups. For example, the lipid metabolism factor becomes abnormal at BMI of 22 in South Asians and 26 in Chinese and Aboriginals. Thus, picking new cut-points is complex. A good rule of thumb may be that the European cut-off of 25 kg/m2 for defining overweight, be used in Asians to define obesity.
The results provide other interesting data. For example, at a BMI of 21-22 in South Asians is related to glucose and lipid metabolism levels found in Europeans at 30. This may explain their increased incidence of CAD when they live in industrialized nations. Also, Aboriginals have lower blood pressures than the other groups and it doesn't vary much with BMI.
There are some limitations to the study. It is small and only examines 3 ethnic groups in Canada. There are no outcome data, which would strengthen any conclusions about BMI levels. There are no waist or hip circumference measures or other data about abdominal fat. Finally, the study was too small to define an overweight group. Regardless, this is an important study which will impact our care of Asian minorities.