The old rules for making a healthy baby – she eats right, exercises, and quits drinking months in advance; you provide the sperm – are out-of-date. A healthy lifestyle, research has shown, is key to producing high-quality swimmers. But more than that, it seems that dads-to-be aren't just passing on their genetic material; they may be passing on traces of their lifestyle, too – just as we've long known moms do (hence all the prenatal vitamins). A new study found that obese fathers had altered gene expression in the pancreas and fat tissues, which suggested they might be at risk for a range of health problems later in life.

Researchers compared the female offspring of male rats who became obese from eating high-fat food with those of slim male rats eating a more balanced diet. Pups with obese dads, the team found, had altered gene expression in their fat and their pancreas – two tissues in the body that play a key role in metabolism. Many of the genes with altered expression are involved in chronic disease: glucose intolerance, a hallmark of type 2 diabetes; premature aging; and cancer.

A study earlier this year showed that fathers-to-be who are low on folate are more likely to have children with birth defects. The evidence, while not yet tested in humans, likely works in a similar way in humans, giving added motivation to men thinking of starting a family to do something we all know is good for us anyway: Eat well and stay at a healthy weight. "I think men have been told for some time to try to stop smoking and drinking before they conceive, but maybe now we know that healthy diet and healthy weight are important, too," says Margaret Morris, a neuropharmacologist at the University of New South Wales in Sydney, Australia, who led the study.