Media Release: 12 February 2010
When it comes to obesity, it would seem that it’s very important to define terms, and equally important to ask questions about those terms.
So say Australian, Vietnamese and American scientists who challenge findings published in 1994 which concluded that for a given Body Mass Index (BMI), a measure of body fat based on weight and height, Asians have a higher percentage of body fat than Caucasians.
The 1994 study drew conclusions from measurements of percentage body fat and BMI of Chinese and Caucasian women living in New York. The study, though small, has been widely cited since and continues to influence the opinions of obesity experts worldwide.
Professor Tuan Nguyen, from Sydney’s Garvan Institute of Medical Research, took a close look at the original paper, saw that fat mass did not vary much between the two populations of women, and hypothesised that Asians and Caucasians have a similar percentage of body fat for a given body size.
To test the hypothesis, Professor Nguyen collaborated with Vietnamese and American colleagues to undertake a comparative study. In association with Dr Ho-Pham Thuc Lan from the Pham Ngoc Thach Medical University in Ho Chi Minh City, Nguyen had already measured the BMI and body fat percentage of 210 Vietnamese women for another study.
As it happened, Professor Elizabeth Barrett-Connor, from the University of California at San Diego, had collected similar data, using identical equipment, from over 400 American women of European descent.
The researchers agreed to share data, and to match the Vietnamese and American groups by age and BMI. When they compared the percentage of body fat in the women, they found it to be identical. Their findings are published in Obesity, now online.
“For years, it has been argued that Asian women with a BMI over 25 should be considered obese,” said Professor Nguyen.
“That would make roughly 40% of the women in Vietnam obese, which is patently absurd. If instead, you make the BMI threshold 30, as it is in the West, the obesity figure drops to less than 5%”
Obesity is defined as a condition where the amount of body fat is harmful to wellbeing and health - fat mass being the critical factor. The gold standard method for measuring fat mass involves using a relatively expensive densitometer, known as a DXA (dual-energy X-ray absorptionmetry) machine.
As DXA is not available in most healthcare settings, BMI is used instead. BMI is at best an approximate, and sometimes misleading, measurement because it does not distinguish between fat mass, muscle mass, bone and vital organs. Some athletes, for example, could be classified as obese because they have a high muscle mass that contributes to their high BMI.
Professor Nguyen believes that a re-evaluation of optimal BMI thresholds in Asia at least provides a useful new starting point from which to proceed.
“We still do not know what percent body fat should be used to define obesity in Asian populations because we still do not have a long term study,” he said.
“A few recent studies in Asian populations appear to suggest that the risk of mortality increases when BMI is greater than 30, so there seems to be no need for lower BMI cut-off points in Asians”.
“A few years ago, the World Health Organisation attempted to define an appropriate BMI cut-off level for Asians, but couldn’t because there wasn’t adequate data.”
“We believe our study draws a new line in the sand. We are saying that we don’t believe that 25 is a realistic threshold, and that 30 is probably closer to the mark. Now we must test that threshold in Asian populations over the coming decades.”