MEN who start smoking as children are likely to have fatter sons, research has suggested.
A large study found that sons whose fathers smoked regularly before the age of 11 grew into chunky teenagers.
Between the ages of 13 and 17, they carried five to 10 kilograms (11 to 22 pounds) more body fat on average than those whose fathers never smoked or took up the habit later.
Scientists said the result from the Children of the 90s study was clear evidence that the effects of lifestyle could cross generations.
They believe toxic chemicals in tobacco smoke may have caused changes to inherited DNA that triggered a metabolic reaction in the boys.
One of the researchers even speculated 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.
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.
"Diet and lack of exercise may not be the whole story from the environmental side of things.
"We have to entertain seriously the fact that there are effects coming from a previous generation."
In total, 9,886 fathers were recruited fro 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.
Between the ages of 13 and 17, they gained an extra five to 10 kilograms on average, or up to 1.57 stone in fat.
Genetic changes caused by the environment that are handed down to future generations are known as epigenetic effects.
They have been demonstrated in animals exposed to toxins and drugs, with their influence emerging over three or four generations.