Lifestyle: Health & Fitness

By Terri Coles

TORONTO (Reuters) - The danger in putting on weight isn't just a matter of "how much" but also "where" -- and it seems some ethnic groups have a tendency to gain fat where it does the most damage.

People of Chinese and South Asian ancestry tend to have relatively more inner abdominal fat than people of European ancestry, which puts them at a higher risk of developing weight-related illnesses like heart disease, high blood pressure and diabetes, a Canadian study found.

The results, published this month in the American Journal of Clinical Nutrition, show that current methods for measuring body fat may not be accurate for people who are not of European descent because the targets generally used for waist circumference or body mass index (BMI) are based on studies whose participants were predominately of Caucasian European origin, said Dr. Scott Lear, the lead researcher on this study and an assistant professor at the School of Kinesiology at Simon Fraser University in Burnaby, British Columbia. 

This inaccuracy could make it harder to identify weight-related problems and treat people in certain ethnic populations, even when they are the same size as Europeans. "If we use targets based on Caucasians for the Asian population, we're not going to identify people until they're at a higher risk," Lear said.

The study, funded by the Canadian Institutes of Health Research's Institute for Nutrition, Metabolism and Diabetes, looked at 800 healthy Chinese, South Asian, Aboriginal and European participants, evenly distributed among the four ethnic groups.

The participants' amounts of abdominal fat were compared for the same amount of total body fat, Lear said, because in a random sample, the Chinese and South Asians would tend to be smaller than the Europeans and would have less overall body fat even at the same body mass index (BMI) or body fat percentage.

Inner abdominal fat, or visceral fat, is not the fat you grab when you pinch an inch on your stomach.  Instead, it sits behind the abdominal muscle wall around the organs, and tends to be more strongly associated with heart disease and diabetes.

The researchers found that Aboriginals didn't show any difference in their body fat distribution when compared to Europeans, but the Chinese and South Asian participants tended to have a greater proportion of their body fat in the inner abdominal region.

The research team recognized that other factors like diet and physical activity might differ across ethnic groups, so those were taken into account, along with smoking, education and income.  Even with those influences controlled for, they still saw the differences in body fat.
 
"What that lends us to believe," Lear said, "is that there's some physiological or genetic rule that is deciding that a higher proportion of fat goes into the abdominal area in Asians than it does in Europeans."

As for why visceral fat comes with stronger risks than other abdominal fat, there are two possible reasons, Lear explained. 

One could simply be its placement around the organs.  Scientists once believed that fat was benign, but now know that its cells actually produce a number of proteins and hormones.  Fat tissues exchange free fatty acids with the blood stream, which go through the veins running through the fat straight to the liver, telling the organ to make more cholesterol in order to produce more sugar.

Another theory, which is more strongly supported by evidence, is that the actual fat cells of visceral fat may be different from those elsewhere in the body, and therefore could produce different amounts of proteins and hormones.

Some believe that inner ab fat is more metabolically active than other fat, which is more neutral in terms of risk for heart disease and diabetes, Lear said.

In order to get a better idea of the risks a person faces from visceral fat, waist circumference should be used in place of BMI measurement, Lear said. 

BMI is calculated using weight and height, so a person with a high weight due to increased muscle mass could show up as overweight based on BMI, even with a low body fat percentage. Waist size usually goes up because of increased body fat, however, making it a better measurement. 

"If we wait until Asians reach the size of Europeans," Lear cautioned, "they're going to be at a higher risk than the Europeans and may already present with diabetes or high cholesterol.  Then it's more like catching up with a train that's already left the station."

The next step in this research is to find out what determines where the fat we gain ends up on our bodies. 

Lear is collaborating with other researchers to look at genes related to body fat distribution and see if they show differences across ethnic groups.  The researchers are also following up on this study's participants in three and five years' time, in order to do the comparisons again and see how the difference between groups shifts as weight shifts.

"It's a more complex question," Lear said, "but I think it's a more important one because if this difference increases with increasing body fat, that's even more remarkable and more of a concern."

People with Chinese or South Asian ancestry make up only a small portion of the population in North America.  In the United States, 1.2 percent of Americans have Chinese ancestry and 0.8 percent are Indian Americans, the largest group with South Asian heritage. 

In Canada, where the study was done, the portions are larger, with 3.5 percent Chinese and three percent Indo-Canadian.

There is no reason to believe their results wouldn't apply to Chinese and South Asians living around the world, Lear said, including those living in China or South Asia. Chinese populations in China and South Asians in South Asia currently have lower obesity rates than seen in North America, but those rates are rapidly increasing, and at a faster rate.

"If it reaches the same level as Canada, the United States and European countries," Lear concluded, "the Asian countries are going to experience a far greater burden of heart disease and diabetes than we ever saw."