SCOTS are “hard-wired” to be fat, according to a new study into the growth rates of British babies.

The research, which challenges the view that overeating and lack of exercise during childhood are the principal causes of obesity, has shown that those born north of the border grow at a faster rate than their southern counterparts.

Scientists believe that the diet of mothers may be influencing the weight of their unborn children, and since the Scottish diet is notoriously poor, Scots babies are born with a genetic predisposition towards being overweight or obese.

Researchers at London’s Kings College and University College London measured the birth weights of 12,900 babies and compared them with their weights at nine months.

The average weight at birth of Scots babies was 7.7lb, compared with 7.6lb in Northern Ireland and England, and 7.5lb in Wales. By nine months, the average weight of Scots children had risen to 20lb, compared with 19.8lb in Wales, 19.7lb in England and 19.6lb in Northern Ireland.

According to the research, published in the International Journal of Obesity, the weight differences could not be explained by factors such as whether the babies were bottle or breast fed, whether their parents smoked, the social class of their mothers or the weight and height of their parents.

It suggests that Scots babies may be “programmed” to become fat while in the womb, and that the obesity epidemic in Scotland is the result of a complex interaction between their genes and the environment.

Currently, more than a third of Scots 12-year-olds are overweight, a fifth are obese and one in 10 is severely obese. One in five youngsters is already overweight by the age of three-and-a-half.

Among European countries, only Italy and Spain have more fat children.

Professor Tim Cole, of the Institute of Child Health at University College London and co-author of the study, said: “What is emerging is that the impact of the environment is drawing out a genetic feature which didn’t show itself before, and it may be showing itself more in Scotland than elsewhere.”

The new findings tally with a growing body of evidence suggesting that genes may explain rising obesity rates.

According to Professor Mark Hanson, director of the developmental origins of health and disease division at Southampton University, mothers may be programming their unborn babies to put on fat immediately after birth.

“What we may be seeing here is environmental factors altering the expression of genes which cause babies to lay down fat,” he said.

“Mothers who have a seriously unbalanced diet, which is high in calories and fat and low in fruit, are tricking the foetus to think that nutrition outside the womb is not good.

“As a result, the foetus adapts to what it expects the world outside to be like based on the information it gets from its mother and becomes fat as a child.

“It could be that Scottish children are affected to a greater extent. By the time they are a year or two it may be too late to reverse some of the trends established in the womb. In that sense, they are hard-wired.”

Hanson, however, added that Scots children were not doomed to a life of obesity.

“We can try and educate the women more about healthy diet and lifestyles and identify the babies at greater risk and help them over the first few years of their lives not to become obese,” he said.

Dr David Haslam, clinical director of the National Obesity Forum, agreed that environmental and genetic factors were probably involved. “Even before it is born the baby gets some metabolic imprinting that makes it more susceptible to obesity in later life,” he said.

Phil Hanlon, professor of public health at Glasgow University and a government advisor, added: “There has long been a debate about nature versus nurture and what is being suggested now is nature via nurture — that genes are turned on and off by a set of environmental factors.

“This interaction is a potentially fruitful one when looking at what is giving rise to this phenomenon.”

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