Obese men should go on a diet to boost their hopes of fathering a child, say researchers

Obese men should go on a diet to boost their hopes of fathering a child, researchers have said.

Excess body fat can damage sperm which could leave men infertile or at risk of passing on genetic defects, they said.

But the effect could be reversible and men can lose weight to get their partner pregnant.

Dr Ricardo Bertolla, from the urological research centre in Sao Paulo, Brazil, said obesity was thought to affect sperm's performance in several ways.

The first theory is that oxidative stress caused by obesity alters the genetic make-up of the sperm, while the second is that fat is laid down by the body around the scrotum which increases the body temperature there, damaging sperm. The third theory is that obesity plays a direct role in how the body's hormones function, decreasing sperm's ability to do its job.

More than 300 men were included in the latest study and split according to whether they were normal weight, overweight or obese. Their sperm was tested to look at how it was performing, including damage to DNA and the activity of the mitochondria - essential "powerhouses" within cells. The ability of the sperm to swim (motility) was also tested.

Dr Bertolla said: "Overweight and obese men presented a decrease in motility. When we looked at sperm and mitochondrial activity, both overweight and obese men had decreased mitochondrial activity. Only obese men had DNA fragmentation."

He explained that mitochondria was needed for fertilising a woman's egg. He added: "Sperm with fragmented DNA can lead to embryos with fragmented DNA that can lead to all sorts of diseases."

But he said he did not believe the general impact of obesity on sperm was irreversible. "I would recommend these men go on a diet," he urged.

The research was presented at the American Society for Reproductive Medicine (ASRM) conference in Atlanta.