AFRICAN Americans have the highest rate of age-adjusted obesity (48 percent) of all ethnic groups, according to the Centers for Disease Control and Prevention. (By contrast, the rate for non-Hispanic whites is 34.5 percent.) A recently discovered genetic variant unique to African Americans may help explain why.

About 1 percent of African Americans, West Africans and others of African ancestry carry a variant of the semaphorin-4D (SEMA4D) gene, which increases their obesity risk.

"So far, we have not seen this variant in non-African Americans or non-Africans," says Charles N. Rotimi, chief of the National Human Genome Research Institute's metabolic, cardiovascular and inflammatory disease genomics branch, who led the study that found the variant. "Not everyone who carries it is obese, but if you carry it, your risk of obesity is very high."

To investigate the genomic basis of obesity in continental Africans, Rotimi and colleagues scanned the complete sets of DNA of thousands of individuals, looking for genetic changes linked to obesity. That's where they found the variant, which is absent in both Europeans and Asians.

"By studying people of West Africa, the ancestral home of most African Americans, and replicating our results in a large group of African Americans, we are providing new insights into biological pathways for obesity that have not been previously explored," says Ayo P. Doumatey, a staff scientist in Rotimi's lab and a co-author of the study.

Those with the variant were about six pounds heavier than those who did not have it, according to the study.

Six pounds may not seem excessive, but shedding that weight could have a significant effect on diabetes rates, Rotimi says.

African Americans disproportionately suffer from diabetes. For example, they are 1.7 times as likely to have diabetes as non-Hispanic whites, according to the American Diabetes Association.

"Six pounds population-wide is a huge amount," Rotimi says. "If you could get Americans to drop six pounds, you probably could reduce the rate of [Type 2] diabetes by 10 percent. Dropping six pounds could make a tremendous difference in health status."