The scientists from Boston University Medical School, say about 10% of humans have a sequence variation close to a gene which controls insulin which might explain why some people are obese.

Professor Alan Herbert led the team of researchers who looked at 100,000 common genetic mutations in over 700 volunteers and they found that a mutation called rs7566605 increased the risk of obesity when it was present as a double copy.

In order to be sure that the mutation was common to all groups, the researchers also analyzed five other study populations, which included European and African-American participants.

This particular variant closely resembles a gene called INSIG2, which plays a key role in fat metabolism.

In four of the five studies the genetic variant was found to be present and the researchers say African Americans were at a much higher risk

Michael F. Christman, the chairman of genetics and genomics at the university and a member of the study team says the discovery does not however mean that there is presently a genetic treatment for obesity.

The gene INSIG2 which controls insulin is in fact induced by insulin and inhibits the manufacture in the body of fatty acid and cholesterol.

People with such a sequence variation near the gene tend to accumulate more fat in the body.

Theories abound as to why there are increased levels of obesity around the world - a change in eating habits, doing less exercise, too many carbohydrates and too much fat in the diet, and so on.

Many scientists believe that many obese people are so because of their genes but this is the first study to identify a genetic factor in obesity and will be a relief to the many obese people around the world who have been puzzled and distressed at their never-ending war against fat and wondering why they find it so hard.

What is clear is, whatever the theory absolutely no difference has been made to the speed at which obesity is spreading around the world.

Christman says they believe the other nine or so genes will be found very soon and there will be many such discoveries over the next few years which will ultimately allow researchers to find the molecular pathways of obesity, which can then be targeted by drugs.

The research is published in the current edition of the journal Science.