FRIDAY, Dec. 18, 2009 (HealthDay News) -- Blacks tend to carry around less of a particularly unhealthy type of abdominal fat than whites, even though they suffer more from obesity-linked illness, researchers report.
The new finding suggests that body-mass index (BMI) guidelines may need to be tailored to specific racial groups to better reflect risk, experts say.
"The study clearly shows we have these racial differences in body fat, not just in the type of body fat but where the fat is stored, and these are important differences," said study author Peter Katzmarzyk, a professor of population science at Pennington Biomedical Research Center in Baton Rouge, La.
Adipose (fat) tissue is found throughout the body. Subcutaneous adipose tissue is found just under the skin, while visceral adipose tissue is found in the abdominal cavity around the organs.
Fat settling around the organs has been linked to development of obesity-related diseases such as cardiovascular disease and type 2 diabetes, according to background information in the article.
But you can't tell just by looking at someone how much visceral fat someone has. Even a pot belly won't tell you for sure because visceral fat is deep within the body cavity, Katzmarzyk said.
In the study, researchers used computer tomography (CT scans) and dual- energy X-ray absorptiometry (DXA) to measure visceral fat in about 1,400 white men and women and 570 black men and women aged 18 to 84. Participants' height, weight, BMI and total body fat composition were also measured.
At a given body fat percentage, black men and women had lower visceral fat than white men and women. Conversely, blacks also tended to have higher subcutaneous fat than whites. Researchers controlled for age and smoking status, among other variables.
The study appears in the January issue of The American Journal of Clinical Nutrition.
Despite the tendency to have less visceral fat, black Americans are still at higher risk of dying from obesity-related diseases such as diabetes and cardiovascular disease. One explanation for this is that blacks have higher overall obesity rates overall, Katzmarzyk said.
About 31 percent of white adults and 45 percent of black adults are obese, according to the study.
"It's a paradox," Katzmarzyk said. "The fact that white individuals have more visceral adipose tissue and also have lower rates of obesity-related disease is probably because African-Americans have higher rates of obesity overall."
The findings also bring up the issue of whether the "one-size-fits-all" BMI guidelines apply equally to all races, Katzmarzyk said.
BMI is a calculation based on height and weight. While a high BMI tends to mean you also have a lot of body fat, BMI is not a direct measurement of fat composition. A very muscular person, for example, may have a high BMI but low body fat composition.
A BMI of 30 or above is considered obese, while a BMI of 25 or above is overweight.
Yet previous research has suggested there may be racial variations in what constitutes a healthy BMI. Diabetes risk for Asians start to rise at a BMI of about 23, for example. An optimal BMI for blacks may also vary somewhat, Katzmarzyk said.
"BMI may mean different things for different people," he said. "The study suggests we may need to think about ethnic-specific thresholds to identify obesity-related health risks."
Eric Bailey, a professor of anthropology and public health at East Carolina University and author of Food Choice and Obesity in Black America: Creating a New Cultural Diet, said BMI recommendations were developed using too little data on black Americans or other racial and ethnic groups.
"We are using BMI as a 'gold standard,' but it needs to be reassessed," Bailey said. "It does not necessarily apply to each and every population in the same way. Our biology and genetic makeup is slightly different. African-Americans may be healthier at a different BMI compared to a European population."
Biology aside, Bailey added, there is no question that obesity remains a serious issue in the black community. Poverty can make affording fresh fruits and vegetables and the healthiest cuts of meat more difficult, while unsafe neighborhoods discourages getting adequate exercise.
"Many times, sociological, behavioral and cultural issues overrides the biological," Bailey said. "One-third of African Americans are still in poverty, and that influences the types of foods you are able to eat."
In another study from the same issue, researchers found that gene variants that have been implicated in a tendency toward obesity played only a small role in a person's BMI.
Previous research in twins has suggested as much as 40 percent to 85 percent of obesity can be blamed on the genes, but the new research shows the influence of genetic variants may be much smaller.
Researchers at the University of Cambridge looked for 12 genetic markers for obesity in more than 20,000 participants from the United Kingdom. Those with at least one gene marker were 3 percent to 14 percent more likely to be obese than those without the marker, while each additional genetic marker raised the risk of obesity by nearly 11 percent.
But taken together, the genetic variations accounted for only about a one percent variation in BMI -- meaning the currently known markers are poor predictors of who is at risk for obesity, according to the study.
Researchers said it's possible that other, more important obesity-related gene variants have yet to be identified.
There's a personal BMI calculator at the U.S. National Heart, Lung and Blood Institute.