Researchers have revealed men, who started smoking regularly before the age of 11 had sons who, on average, had 5-10kg more body fat than their others by the time they were in their teens.

The researchers at the University of Bristol say this could indicate that exposure to tobacco smoke before the start of puberty may lead to metabolic changes in the next generation.

The effect, although present, was not seen to the same degree in daughters. Many other factors, including genetic factors and the father's weight, were taken into account but none could explain the change. In fact, the fathers who started smoking before 11 tended to have lower BMIs (body mass index) on average.

The effect was not seen in the sons of men who started smoking after the age of 11, suggesting that the period before the start of puberty is a particularly sensitive period for environmental exposures.

Of the 9,886 fathers enrolled in the study, 5,376 (54 per cent) were smokers at some time and, of these, 166 (3 per cent) reported smoking regularly before the age of 11.

When measured at age 13, 15 and 17, the sons of the men in the latter category had the highest BMIs at each time point compared with the sons of men who had started smoking later or who had never smoked. More precisely, these boys had markedly higher levels of fat mass (recorded using whole-body scans), ranging from an extra 5kg to 10kg between ages 13 and 17.

The research has been published in the European Journal of Human Genetics.