The genetic roots of obesity lie mainly in the brain, according to research that implicates inherited eating tendencies as one of the strongest influences over waistlines.

The discovery of seven genetic variants linked to obesity has suggested that DNA affects body shape mainly by changing eating behaviour, rather than by regulating fat storage.

Of the seven, five seem to be active in the brain, making it likely that work by fine-tuning appetite, the sense of fullness after eating, or even preferences for some foods over others.

The findings indicate that although genetic differences can help to explain why some people are overweight while others are slim, obesity cannot generally be blamed on genes that slow metabolism and allow fat to be laid down more easily.

Most of the genetic factors linked to obesity, which were found by two independent research teams, seem rather to work by altering the amount people eat. Some DNA profiles may simply make it easier or harder to control food consumption.

This insight demonstrates how nature and nurture are intertwined in the origins of common biological effects such as obesity. It is also encouraging for therapy, as it means that something people can control – their food intake — is ultimately responsible for weight gain even when genetic predisposition is also involved.

“In cases like this, the line between nature and nurture begins to blur,” said Kari Stefansson, of deCODE Genetics, an Icelandic company that conducted one of the studies. “Genetic factors seem to be influencing environmental risk factors.”

Joel Hirschhorn, of the Children’s Hospital, Boston, who led the other study, by the international Genetic Investigation of Anthropometric Traits (Giant) consortium, said:“The genes near these variants are all active in the central nervous system, suggesting that inherited variation in appetite regulation may have something to do with predisposition to obesity.”

Genetic factors are known to influence obesity risk, as twin studies have shown that 40 to 70 per cent of the variation in body mass index is inherited. But it was only 18 months ago that scientists identified a first genetic variant involved, in a gene called FTO. The discoveries, published in Nature Genetics, bring the total number of variants firmly linked to obesity to nine.

All seven variants were detected by the deCODE study and six were also identified by the Giant consortium. Each of the variants has a modest effect on obesity risk. The 1 per cent of people who have the riskier version of all of them would typically be 2kg (4lb) heavier than an average person, and 4.5kg (10lb) heavier than a person with the least risky genetic profile.

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