Experts have suspected as much for years and now it has been confirmed: diabetes is a threat to male fertility, according to new research by Queen’s University Belfast.

The study found the impact was strongest in people with type 1 diabetes, which is usually present from childhood, but was also apparent in men who developed the condition later.

Fertility experts at the university have been studying sperm samples from 60 diabetic men for several years, comparing them with thousands of other men without the condition. The latest study, involving examination of the semen of eight men with diabetes, found that it had disrupted the DNA in their sperm.

The researchers concluded that high blood sugar levels meant men with the condition “have a significant decrease in their ability to repair sperm DNA, and once this is damaged it cannot be restored”.

Neil McClure, professor of obstetrics and gynaecology at the university, said the possibility of a link between the condition and reduced fertility had been mooted before but adequate research had never been carried out.

“Male fertility has always been looked at in diabetics in a haphazard way,” he said. “Under a light microscope there’s no difference, but in DNA analysis, you see a lot more damage. Diabetes affects virtually every part of the body. It sticks little sugar molecules onto everything, so when we looked at the sperm we found a lot of these molecules. The whole way the DNA functions in the diabetic man is upset.”

Corresponding research at the university’s In Vitro Fertilisation (IVF) unit found that where the father had diabetes, the treatment’s success rate decreased. McClure said they had also used research from Australia. It showed that the percentage of men with diabetes enrolling for IVF treatment was three times higher than average.

“Diabetes poses a threat to their fertility. It’s not an absolute barrier but it does make it more difficult,” said McClure. “We wouldn’t say diabetic men don’t need to use contraception but we would say if they and their partner are looking to get pregnant, it could take a lot longer.”

The team presented their findings at the European Society of Human Reproduction and Embryology conference in Barcelona this month. They said that the research was important because the age at which people are getting diabetes is decreasing and that the men affected are increasingly of reproductive age.

An Institute of Public Health report in 2006 estimated that there were 143,000 people living with diabetes in Ireland, of which 58,807 were men. One in 10 had type 1 diabetes, while the remaining 129,000 had late-onset type 2 diabetes. This often develops during adulthood as a result of obesity, high blood sugar, old age or genetics.

The Diabetes Federation of Ireland believes there are a further 200,000 people who are unaware that they have the condition and 250,000 people who have impaired glucose tolerance, or “pre-diabetes” — half of whom will go on to develop diabetes unless they make lifestyle changes.

Anna Clarke, spokeswoman for the Diabetes Federation, said the Queen’s University research needed more analysis. But McClure said that “meticulous control of blood sugar” was mandatory for men looking to have children.

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