South Asian men and women appear to have a high fat/low lean mass phenotype that may put them at greater risk for insulin resistance and diabetes, based on a study of individuals in four ethnic groups.
“Ethnic differences in lean mass do occur, such that South Asian men and women have significantly less lean mass than Aboriginal, Chinese, and European men and women of the same body size,” noted lead author Scott A. Lear, Ph.D. “South Asians have a distinct phenotype of high fat mass and low lean mass,” which may account for a substantial portion of their higher insulin levels compared with other ethnic groups.
A total of 828 participants were recruited as part of the Multicultural Community Health Assessment Trial, involving individuals of exclusive Aboriginal, Chinese, European, and South Asian origin. Body mass index ranges were less than 24.9 kg/m
Total body fat was moderately correlated with total body lean mass in both sexes of all four ethnic groups. Aboriginal, Chinese, and European men had 3.42 kg, 3.01 kg, and 3.57 kg more lean mass, respectively, than did South Asian men at a given total fat mass. Likewise, Aboriginal, Chinese, and European women had 1.98 kg, 2.24 kg, and 2.97 kg more lean mass than did South Asian women at a given total fat mass. “These models accounted for 66% and 63% of the variation in lean mass for men and women, respectively,” the researchers wrote (J. Clin. Endocrinol. Metab. 2009, Oct. 9 [doi: 10.1210/jc.2009-1030
The fat mass to lean mass (F:LM) ratio was significantly greater for South Asian men compared with Chinese and European men and there was a trend toward a greater F:LM ratio compared with Aboriginal men. Likewise, the F:LM ratio was also significantly greater for South Asian women compared with Chinese, European, and Aboriginal women. These relationships persisted after adjustment for age, height, humerus breadth, smoking status, physical activity, and diet (and menopausal status in women).
“South Asian men and women have a lower lean mass at a given fat mass even when taking into account possible confounders such as lifestyle habits known to affect body composition,” noted Dr. Lear, professor of biomedical physiology and kinesiology at Simon Fraser University, Vancouver, B.C., and his coinvestigators.
In a post hoc analysis, insulin and homeostasis model assessment (HOMA) levels were significantly greater for South Asian men than for Chinese and European men, even after adjustment for total body fat mass, age, height, humerus breadth, diet, and physical activity. When the models were adjusted for differences in the F:LM ratios instead of total body fat mass, there were no longer any differences in insulin or HOMA levels among the groups.
South Asian women had significantly greater insulin levels than did Chinese women in a similar post hoc analysis. There were no differences in HOMA between the groups. After adjustment for total body fat mass, age, menopausal status, height, humerus breadth, diet, and physical activity, South Asian women had significantly greater insulin and HOMA values than did Chinese and European women. As with men, when the models were adjusted for differences in the F:LM ratios instead of total body fat mass, there were no longer any differences in insulin or HOMA levels.
The study was supported by the Canadian Institutes of Health Research, Institute of Nutrition, Metabolism, and Diabetes. The authors reported that they had no conflicts of interest.