A gene that contributes to obesity has been identified for the first time, promising to explain why some people easily put on weight while others with similar lifestyles stay slim.

People who inherit one version of the FTO gene rather than another are 70 per cent more likely to be obese, British scientists have discovered.

One in six people have the most vulnerable genetic makeup and weigh an average of 3kg more than those with the lowest risk. They also have 15 per cent more body fat.

The findings provide the first robust link between any common gene and obesity, and could eventually lead to new ways of tackling one of the most significant causes of ill health in developed countries such as the UK.

One in four British adults is now classified as obese, and half of men and a third of women are overweight. Obesity is a major cause of heart disease, cancer and type-2 diabetes, and an adviser to the Government’s health spending watchdog described it recently as a bigger national hazard than smoking, alcohol or poverty.

If the biological function of the FTO gene can now be understood, it could become possible to design drugs that manipulate it to help people to control their weight.

“Even though we have yet to fully understand the role played by the FTO gene in obesity, our findings are a source of great excitement,” said Professor Mark McCarthy of the University of Oxford, who led the research.

“By identifying this genetic link, it should be possible to improve our understanding of why some people are more obese, with all the associated implications such as increased risk of diabetes and heart disease. New scientific insights will hopefully pave the way for us to explore novel ways of treating this condition.”

While it has long been understood from family studies that obesity is heavily influenced by genetics, scientists have struggled to pin down individual genes that are involved.

A handful of serious genetic mutations that cause rare obesity disorders such as Prader-Willi syndrome have been found, but the search for common genes that affect ordinary people’s risk of becoming obese or overweight has remained elusive.

The effect of FTO emerged from a major study of the genetic origins of disease funded by the Wellcome Trust known as the Case Control Consortium, in which 2,000 people with type-2 diabetes had their genomes compared to 3,000 healthy controls.

Scientists from Oxford and the University of Exeter first found that certain versions of the FTO gene were more common among people with type-2 diabetes, but that the effect disappeared when the data were adjusted for obesity. This led them to wonder whether FTO actually influenced obesity instead, and they followed up their theory in a further 37,000 people.

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