Other rare mutations that cause extreme obesity have been identified, but this is the first mutation that influences obesity in the general population.

People whose genetic makeup includes two copies of the mutation are on average 22 percent more likely to be obese. But the research suggests the double mutation increases obesity risk in African-Caribbean people by 136 percent.

Obesity is a risk factor for heart disease, diabetes and premature death. Last month the US surgeon general, Richard Carmona, warned that the health consequences of America's obesity epidemic would dwarf the threat from terrorism.

Risk Higher Among African Americans

In the study, Professor Alan Herbert at Boston University Medical School and his team looked at more than 100,000 common genetic mutations in nearly 700 US volunteers.

One mutation named rs7566605 made carriers more likely to be obese when present as a double copy.

To confirm the association was more than chance, they looked at five other study populations including European children and African American adults.

Four of the five gave the same result and the increased risk conferred by the gene was much higher in the African American group, the team reports in Science.

"When you do it in independent populations around the world using independent laboratories and come up with the same answer it is a very powerful reason to believe it is true," Herbert said.

Diet and Exercise Still Important

Other rare mutations that cause extreme obesity have been identified, but this is the first mutation that influences obesity in the general population. He stressed that there were probably several other common mutations involved in obesity and that diet and exercise were also important.

"It's not 100 percent determinative that if you have the gene you get obese. People often ask me what I would do if I had the mutation. If I could control my weight by diet and exercise, then I wouldn't worry."

Beckie Lang at the UK's Association for the Study of Obesity agrees. "Whatever underlying genes we have, what we do on top of those is more significant. Therefore it is crucial that we follow healthy eating patterns and an active lifestyle to prevent weight gain and other ill health."

Herbert's team is not sure yet what the mutation does, but it is just upstream on the DNA from a gene involved in making fat, so they believe it is probably involved in controlling that gene.

The team thought it changed the level of the gene or the way that it is made. "You could imagine that the whole fat pathway could be amplified and turned into hyperdrive."