The research, published in today's issue of Cell Metabolism, comes just a few weeks after a previous study revealed that obesity could be caused by certain viruses, and might therefore be tackled with a vaccine.
Both studies, which suggest that factors other than poor diet may be contributing to the growing incidence of obesity, could be good news for the food industry, which has been held largely responsible for the disease.
The new research found that a single change in a particular brain hormone could increase a person's risk of obesity.
According to scientists at the Cambridge Institute for Medical Research in the United Kingdom, and the Charité Universitätsmedizin Berlin in Germany, obese children are more likely to carry a rare variant of so-called b-melanocyte-stimulating hormone (b-MSH) than children of normal weight.
"The findings implicate the hormone in the maintenance of normal body weight and suggest that drugs that mimic the chemical might offer a new avenue for obesity treatment," said the scientists.
According to the report, the hormone's important role in maintaining body weight has so far been overlooked because mice and rats, the subjects of much obesity research, do not produce the chemical.
Earlier studies had mainly focused their attention on the related chemical a-MSH, a hormone derived from the same protein complex- proopiomelanocortin (POMC)- that is known to suppress appetite in humans, said the scientists.
And according to previous research, the loss of POMC leads to severe obesity, something that had been put down to the loss of a-MSH.
But these latest findings reveal that human body weight regulation is particularly dependent not only on a-MSH but also on b-MSH.
And although rats do not normally produce ß-MSH, previous studies have indicated that the animals respond to the administered hormone by eating less. The researchers now report that the mutant version of the hormone failed to reduce food intake in animals, consistent with its link to increased obesity risk.
"Our studies show that even subtle changes in POMC can lead to obesity, and that the potential role for ß-MSH had been unfairly neglected. The findings add weight to the notion that drugs that target the melanocortin system might be beneficial in the treatment of obesity," said lead author Stephen O'Rahilly.
However the abnormal hormone was only found to be present in a small number of obese children studied. Out of 538 obese children, only five were found to have the abnormal variant of ß-MSH.