Diabetes damages a man's sperm and may reduce fertility, say UK researchers.
Sperm in men with diabetes had more DNA damage
Tests showed DNA in sperm from diabetic men had more signs of damage than in men without the condition.
Defective sperm DNA is one cause of male infertility, pregnancy failure and miscarriage but the implications for sperm affected by diabetes is unknown.
Writing in Human Reproduction, the team said the findings were worrying given rapidly rising rates of diabetes and more research was needed.
The study of 56 sperm samples is the first to compare the quality of DNA in sperm from men with and without diabetes.
Around 52% of the DNA in the sperm cells was fragmented in diabetic men compared with 32% in men without the condition.
The study also found a higher rate of deletions of DNA in the mitochondria - tiny, energy-generating compartments found within cells.
Semen volume was significantly less in diabetic men, but there were no significant differences in sperm concentration, structure of the sperm or their ability to move.
Although the men in the study had type 1 diabetes - where the body does not produce insulin - the researchers have found the same DNA damage in sperm from men with type 2 diabetes - a more common form where the body does not produce enough insulin or does not respond properly to it.
Diet and obesity are known to be key factors in the increase of type 2 diabetes, which normally starts in adulthood.
But the incidence of type 1 diabetes, which is usually diagnosed in childhood or adolescence, is also increasing by 3% a year in European children.
Study author Dr Ishola Agbaje, a research fellow in the Reproductive Medicine Research Group at Queen's University, Belfast, said: "Diabetes will affect many more men prior to and during their reproductive years."
He added that up to one in six couples needs specialist help to conceive.
Co-author Professor Sheena Lewis stressed it was not possible to say whether the DNA damage caused by diabetes would have the same effect on fertility as DNA damage caused by other factors such as smoking.
"There are three things we need to look at - the number of men with diabetes and fertility problems, we need to look at children of diabetic fathers to see if there is an impact on their health and we need to find the exact nature of the DNA damage."
She said high levels of glucose in men with diabetes may be a cause.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said quality of sperm DNA was important.
"Although there is no significant evidence that men with diabetes are less fertile, or their children less healthy, it is of some concern that more of their sperm DNA may be damaged.
"It would be important to understand the mechanism by which this damage occurs so that if it can be avoided we can work out how to do this."
He added that men should visit their GP if they have concerns.
Matt Hunt, science information manager at Diabetes UK, said although the study was small the findings were somewhat alarming.
"This is the first research to suggest DNA damage maybe occurring at a cellular level and that is cause for great concern. We would welcome further investigation."