This could help explain recent findings that show older men are more likely to father children with birth defects, as well as being less fertile than younger men, the team at the University of Washington in Seattle said. 

Narendra Singh and his colleagues tested 66 men aged 20 to 57 and found the older men produced a higher percentage of sperm with highly damaged DNA.

Writing in the journal Fertility and Sterility, they said they could not tell any difference simply by looking at the sperm.

The total number of sperm and sperm shape were not affected by age, they said. 

But DNA tests showed that older men had more sperm with highly damaged DNA than younger men. And the older men's sperm were less likely to undergo a self-destruct mechanism called apoptosis, which is meant to get rid of damaged cells. 

"It takes a healthy sperm to make a healthy baby and with more research, we can perhaps determine how to best protect sperm from DNA damage."

Dr Anthony Thomas, Society for
Male Reproduction and Urology.

It may not be simply age that affects the sperm, but all the environmental damage that comes with age, the researchers said. While men produce fresh sperm daily, the organs that are
involved in sperm production can be affected by smoking, chemicals, sunlight and lifestyle. 

"Unfortunately, we can't stop age, but men who are putting off fatherhood might want to consider their lifestyle choices to minimise their risk of infertility, or perhaps revise their timetables," said Dr Anthony Thomas, president of the Society for Male Reproduction and Urology. 

"It takes healthy sperm to make a healthy baby and with more research, we can perhaps determine how to best protect sperm from DNA damage."

Australian promotion

Meanwhile, an Australian sperm bank has been flooded with enquiries after it offered a free trip "Down Under" to potential donors.

The Reproductive Medicine Centre in Albury, about 500 kilometres southwest of Sydney, advertised in a student newspaper in Calgary, Canada, because of a shortage of Australian donors. 

Programme director Ruth Keat said the reaction had been overwhelming. 

"We've had emails from Ukraine and Russia," she said. "We just
wanted a few donors but now it seems half the world wants to know.

"We never could have made this happen if we wanted to." 

Keat said the shortage of Australian donors was caused by a planned law change in New South Wales state allowing children, including those conceived using sperm banks, to know the identity of both parents when they become adults.

The sperm bank is offering a US$5,180 package, including free return trip, accommodation for a fortnight and daily allowance.

In return, they require a sperm donation every second day, and donors must be prepared to be identified to potential offspring, although they will have no legal obligation for them. 

"We've got so many patients waiting for donor sperm. The cost will be covered by the number of people that need to access the service," Keat said. 

Candidates must be fit and healthy, between 18 and 40, attend two counselling sessions and undergo blood and semen analysis.