Men who start smoking at a young age put their future sons' health at risk, with sons of early smokers carrying an extra 5 to 10 kilograms of weight, on average.

A longitudinal study looked at 9,886 fathers, with data drawn from the Avon Longitudinal Study of Parents and Children's questionnaire. It looked specifically at smoking habits, age and the growing patterns of their sons between the ages of 7 and 17, taking into account the child's Body Mass Index (BMI) and the age of the father when he adopted smoking.

"Our results highlight the importance of the developmental timing of the paternal exposure as well as gender differences in offspring outcomes," the report reads. "Smoking by boys in mid-childhood may contribute to obesity in adolescent boys of the next generation."

5,376 of the surveyed fathers had been regular smokers at some point and of those, 166 had taken up the habit before turning 11. When their sons were assessed periodically at ages 13, 15 and 17, it was found that they were, on average, 5 to 10 kilos - or 11 to 22 pounds - heavier than their counterparts whose fathers hadn't smoked at a young age.

"It seems that we haven't got the whole story for why we have the rise in obesity," said the study's lead author Professor Marcus Pembrey of England's University of Bristol. "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."

The researchers have posited that the early exposure to toxic tobacco smoke might have triggered changes in the father's DNA, which would then have sparked an adapted metabolic reaction in the boys once inherited.

"This discovery of trans-generational effects has big implications for research into the current rise in obesity and the evaluation of preventative measures," said Pembrey. "It is no longer acceptable to just study lifestyle factors in one generation."

The study was published in the European Journal of Human Genetics.