Diet and lifestyle play a far smaller role than genetic factors in determining whether a child becomes overweight, according to a British study of twins published on Thursday.
Researchers looking at more than 5,000 pairs of twins wrote in the American Journal of Clinical Nutrition that genes account for about three-quarters of the differences in a child's waistline and weight.
"Contrary to the widespread assumption that family environment is the key factor in determining weight gain, we found this was not the case," said Jane Wardle, director of Cancer Research UK's Health Behavior Centre, who led the study.
Previous studies have pointed to environmental factors as the main cause of obesity, a major problem worldwide that increases the risk later in life of type-2 diabetes, cancer and heart problems.
The World Health Organization classifies around 400 million people worldwide as obese, including 200 million children under the age of five.
The British team looked at pairs of identical twins who share all their genes and compared their measurements with those of non-identical twins who share only half their genes.
A statistical analysis found that the differences in the children's body mass index and waist circumference were 77 percent attributable to genes and 23 percent due to the environment in which the children were growing up.
BMI is calculated by dividing weight by the square of height.
"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," the researchers said.
The results suggest that parents whose children are at the greatest genetic risk may need support to make sure they provide a healthy environment, the researchers said.
"This study shows that it is wrong to place all the blame for a child's excessive weight gain on the parents," the researchers said.