Becoming overweight as a child is more likely to be the result of your genes than your lifestyle, claims a study.
Genes may be key to a child's weight
University College London researchers examined more than 5,000 pairs of identical and non-identical twins.
Their American Journal of Clinical Nutrition study found that differences in body mass index and waist size were 77% governed by genes.
An anti-obesity group said regardless of genes, a balanced diet and exercise were vital to good health.
Children who are overweight are likely to be overweight or obese in adulthood, raising the risk of certain cancers, heart disease, stroke and diabetes later in life.
However, despite the emergence of some possible genes that contribute to obesity, there is still debate as to the extent to which we are pre-programmed to be overweight by our genetic makeup.
The study, from the Cancer Research UK Health Behaviour Research Centre at UCL, goes some way to answering that question.
Twin studies are a good way to test how far our genes or our environment influence our development.
Identical twins have exactly the same genes, while non-identical twins are genetically different, like brother and sister.
However, because they were born at the same time, and raised in the same household, they can be assumed to have roughly similar upbringing in terms of food.
This allows scientists to measure differences in weight and calculate how much of that difference can be blamed on environment, and how much on genes, even though it doesn't identify individual genes which might be linked to obesity.
They worked out that the effect of a bad environment was far less marked than the effect of a child's genes.
Professor Jane Wardle, who led the study, said: "It is wrong to place all the blame for a child's excessive weight gain on the parents - it is more likely to be due to the child's genetic susceptibility.
"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."
Tam Fry, from the Child Growth Foundation, said that it was important that parents did not give up on healthy lifestyles.
"The gene pool hasn't changed so dramatically in the last 30 years, at a time when obesity has grown out of sight.
"Even if someone has a gene which predisposes them to obesity, it doesn't mean they will become obese if they work hard to eat healthily, and take more exercise to burn off those calories."
Sara Hiom, from Cancer Research UK, said that parents of children showing early weight gain should have additional support to keep them healthy.
"We know that obesity is an important risk factor for a number of cancers so it is important for us all to do what we can to reduce our risk of the disease by eating healthily and maintaining an active lifestyle."
Dr Helen Wallace, of GeneWatch UK said making claims of strong genetic influence based on twin study results was "highly irresponsible".
She said: "Twin study findings depend on the assumptions that are made about how genes and lifestyle interact.
"It is impossible to draw reliable conclusions about the importance of genetic effects from twin data alone."
In a separate piece of research, US scientists said they had found clear signs that obesity was "hard-wired" into the brain at birth.
Differences in the brains of obesity-prone rats could be spotted just weeks after birth, the journal Cell Metabolism reported.