A genetic cause of severe obesity has been uncovered.
Although affecting less than 1pc of the obese population, the defect raises new questions. The research highlights the role of a gene called Mrap2, which exists in mice and humans.
The gene helps to regulate metabolism. Mutations that deactivate Mrap2 can lead to massive weight gain.
Scientists conducting the research who "knocked out" the gene in mice found they rapidly put on weight while eating the same amount of food as their normal counterparts.
The findings are published today in the journal 'Science'.
Other mutations in the gene might be more widespread and interact with other factors to promote more usual forms of obesity, the researchers suspect.