What does a would-be father's waistline have to do with the health of his future children? More than you'd think – and it could be the incentive that helps men stick to their weight loss resolutions.
Not long ago, the reason children of overweight parents became overweight themselves seemed simple – if the parents' extra kilos were the result of too many kilojoules and too little activity, it would be easy to "transmit" those same habits to their children. But now there's growing evidence that some of the influences that shape children's weight and health can start before they're born or even before they were conceived – and that their father's health can play a part.
"We know that a woman's health and weight affect her unborn baby's health – if a woman is obese or has raised blood sugar or high blood pressure in pregnancy, she can pass on a risk of these same problems developing in her children," says Professor Rob McLachlan, director of Andrology Australia, the national centre for male reproductive health based at Monash University.
"But evidence is now emerging that a man's lifestyle habits and weight before conception can make a difference too – and if he's obese at the time of conception it can increase the chances of his children developing weight problems."
He points to a 2015 study at the University of Adelaide in which researchers took male mice from the same litter and overfed half of them so that they became overweight. When they compared the offspring of these two groups of mice, they found that the offspring fathered by the overweight mice were themselves overweight.
"This means that obesity can be imprinted on the next generation through the father, and while this and similar studies are in mice, not people, it's logical that the same pattern can occur in humans," he says.
Meanwhile, in new research reported in the journal Cell Metabolism in December, Danish researchers comparing the sperm cells of lean men with those of obese men found there were differences in these cells that could influence the appetite of the next generation.
So how can too much flab affect the DNA of sperm?
It's to do with epigenetics, explains McLachlan – the ability of lifestyle habits and environment to cause genes to switch on and off. This creates changes to the way the DNA code is read and the effects can flow on to our children.
Are these changes driven by obesity itself or the diet that underpins it?
"That's not clear – but studies in women suggest that both are important," says Professor Michelle Lane, a senior research fellow with the University of Adelaide's Gamete and Embryo Laboratory.
"But the good news is that you can turn the ship around by changing lifestyle habits – and that's a powerful message," McLachlan says.
"Sperm takes two months to develop which means that the most important time for men to improve their health and lifestyle is in the months before conception. This is a great reason to be quitting smoking, drinking in moderation, eating a healthy diet, exercising regularly and losing a few kilos if they are overweight," he says.
"You don't have to lose a ridiculous amount of weight to make a difference – when the process of weight loss starts, sperm quality begins to improve.
"It's a precious thing to create a child and while our parents might have thought about what school their future children would go to, parents today need to think about their health before they conceive, especially as we're much heavier than we were 15 years ago and we're having children when we're are older. It needs to be a much louder message."
As for alcohol and its effects on unborn children, the advice for women is that it's safest to avoid it altogether in pregnancy and when trying to conceive, but with men – who metabolise alcohol differently – alcohol in moderation is OK, McLachlan says.
This doesn't apply to cigarettes. Besides affecting fertility in both partners, smoking can increase the risk of health problems in children not just when women smoke in pregnancy but when fathers smoke before conception.
"Cigarettes put stress on the integrity of DNA and some studies have found that the children of fathers who smoke have a higher risk of health problems including cancer," he says.
"The bottom line for couples wanting to have children is that the best chance of conceiving and having a healthy pregnancy is for both partners to make healthy lifestyle choices," Michelle Lane says.