Nature, and not nurture, is the chief reason why some children put on weight while others stay slim, according to research that suggests parents are not entirely to blame for the epidemic of childhood obesity.

British scientists have found that genes account for more than three quarters of the differences between children’s waistlines, with lifestyle factors such as diet and exercise playing a much smaller role.

The results, from a study of 5,092 pairs of twins aged between 8 and 11, indicate that different family environments explain only a little of the variation in obesity between today’s children. Those who get fat tend to do so because they have a genetic predisposition towards obesity.

It raises questions about the recent decision by Alan Johnson, the Health Secretary, to notify the parents of five-year-olds who are found to be obese. Such children are more likely to be putting on weight because of their genes than any influence their family and home environment has had on their lifestyle.

“This study shows that it is wrong to place all the blame for a child’s excessive weight gain on the parents,” Professor Jane Wardle, of University College London, who led the research, said. The findings, however, do not mean that environmental factors play no part in obesity. Despite the strong influence of susceptibility genes, their prevalence has not altered in recent decades and it is environmental change that has driven the rise in childhood obesity across the population.

What studies of lifestyle have failed to explain are the differences in individual children’s responses to their “obesogenic environment” — factors in the environment that encourage sedentary behaviour and overeating.

In the study, which is published in the American Journal of Clinical Nutrition, Professor Wardle and Susan Carnell looked at Body Mass Index (BMI) and waist circumference in pairs of twins.

As identical twins share all of their genes while fraternal twins share only half, and they share broadly similar environments, comparisons between twins who live together or grew up in the same families can tease out the relative contributions of nature and nurture.

The results, showed that identical twins were considerably more likely than fraternal twins to have similar patterns of obesity. For both BMI and waist circumference, genes accounted for about 77 per cent of the variation between individuals, and environmental factors for 23 per cent.

“We looked at identical pairs of twins who share all their genes and compared their measurements with non-identical pairs of twins who only share half their genes,” Professor Wardle said.

“Contrary to the widespread assumption that family environment is the key factor in determining weight gain, we found that this was not the case.

“These results do not mean that a child with a high complement of ‘susceptibility genes’ will inevitably become overweight, but that their genetic endowment gives them a stronger predisposition [to being overweight].

“In today’s environment — which provides unprecedented opportunities for all children to overeat and be sedentary — it is not surprising that these tendencies result in weight gain.”

BMI, which is the standard measure of obesity, is calculated by dividing a person’s weight in kilograms by the square of their height in metres. A score of 25 is the threshold for overweight, and 30 for obesity.

According to the latest government figures, 30 per cent of children are either obese or overweight by this measure. Obesity among children aged 2 to 10 has risen by 7 per cent in the past decade.

Though the results point to genetics as a key factor in childhood obesity, they also indicate the importance of public health policies that encourage children to eat less and to take more exercise, experts said.

Sarah Hiom, director of health information for Cancer Research UK, which partly funded the study, said: “This research highlights the importance of doing all we can to encourage children to eat healthily. If genetic influence is strong we must try to counter these inherited tendencies by providing the healthiest possible environment, and educating parents on the importance of a well-balanced diet and an active lifestyle.

“Our genes have not changed and yet childhood obesity is rising, and the major challenge is tackling the obesogenic environment that we all share.

“That includes looking at education, transport, town planning and so on, and not blaming parents for genetic susceptibilities that are not their fault.”

Times Recommends

• Iran arrests relatives of opposition leaders
• Banksy rival has final word in art feud
• Electrician travels 175 miles to change bulb