Men who start smoking before their teens are more likely to have overweight sons, according to new research.
Scientists claim men who were already smoking regularly before the age of 11, have sons with five to 10kg more body fat than those whose fathers never smoked or took up the habit later.
Scientists said results from the ‘Children of the 90s’ study at the University of Bristol show that toxic chemicals in tobacco smoke may have caused changes to inherited DNA that triggered a metabolic reaction in the boys.
One of the researchers said that the weight gain might be an adaptive response to counteract tobacco effects, since the fathers who started smoking young tended to be thin.
Daughters of early-smoking fathers also gained body fat, but to a much smaller degree that was not considered significant.
In total, 9,886 fathers were recruited for the study, 54% of whom smoked at some time in their lives.
Of these, 166 (3%) were smoking regularly before the age of 11.
Assessments of Body Mass Index (BMI), a measurement of weight in relation to height, were made of the men's sons and daughters at ages seven, nine, 11, 13, 15 and 17.
They showed that the average BMI of boys whose fathers smoked as children rose at each time point until it reached 25.9, just putting them in the "overweight" category.
Measurements of body fat made by whole body low-dose X-ray scan showed that teenage sons of young-smoking fathers had markedly higher levels of fat mass than their peers.
Lead scientist Professor Marcus Pembrey from the University of Bristol said, "It seems that we haven't got the whole story for why we have the rise in obesity. We have to entertain seriously the fact that there are effects coming from a previous generation."