Scientists at the University of Copenhagen at the sperm of obese and thin men and noticed significant differences in parts of the gene associated with appetite.

Pregnant women, and those hoping to conceive, have long been warned to curb drinking and smoking and keep their weight down if they want to give birth to a healthy baby.

But now a new study suggests that the unhealthy lifestyles of men are written into their sperm and could be passed on to their children.

Scientists looked at the sperm of obese and thin men and noticed significant differences in parts of the gene associated with appetite.

Put simply, obese men could be passing on obesity to their children.

"Our research could lead to changing behavior, particularly pre-conception behavior of the father," says Dr Romain Barrès, an Associate Professor at the University of Copenhagen.

"It's common knowledge that when a woman is pregnant she should take care of herself--not drink alcohol, stay away from pollutants, but if the implication of our study holds true, then recommendations should be directed towards men, too."

Scientists already knew that certain life events which happen to the body can cause genes to acquire chemical – or epigenetic - marks which turn their activity up or down. For instance, studies have shown that children and even grandchildren of pregnant women exposed to famine are more likely to develop diabetes, cardiovascular disease and become obese.

But it is the first time that scientists have shown it could be passed through the male line. Researchers say it offers one explanation for why children of obese fathers are often obese themselves.

The team also looked to see if the sperm of obese men changed after they had gastric band surgery. They found an average of 5,000 structural changes to sperm cell DNA after surgery, when compared to sperm before the operation.

Dr Barrès believes that the process is an evolutionary remnant which allowed humans to take advantage of times of abundance.

"It's only recently that obesity is not an advantage," said Dr Barres. "Only decades ago, the ability to store energy was an advantage to resist infections and famines."

Researchers plan to follow up the findings by studying embryos generated from the sperm of men of different body weights.

British experts said the research marked the first indication that environment and nutrition of fathers could influence the future health of children.

Prof Wolf Reik, Head of Epigenetics Programme, Babraham Institute, said: “In humans, it seems that children of obese men have increased risk of developing obesity themselves as well as of autism spectrum disorder. Hence the finding reported here of epigenetic alterations in sperm of obese men is clearly provocative.”

Prof Allan Pacey, Professor of Andrology, University of Sheffield, said: "This is an interesting study which provides further evidence to support the theory that some characteristics can be passed by sperm from a father to his children, without altering the basic structure of the genetic code.

“Until we know more, would-be parents should just aim to be as healthy as possible at the time of conception and not be drawn to faddy diets or other activities in order to try and influence the health of their children in ways we don't properly understand."

The research was published in the journal Cell Metabolism.