The obesity epidemic could be contributing to falling sperm counts and increasing male fertility problems, according to research that shows that both excess weight and diabetes can reduce the quality of a man's sperm.

Men who are obese or overweight are significantly more likely to produce abnormal sperm and low volumes of semen than those of healthy weight, and those with diabetes — which is commonly triggered by obesity — are more likely to have sperm with genetic damage, two British studies have found.

The results provide strong new evidence that obesity and its complications, which are already known to affect female fertility adversely, have a parallel impact on male reproductive health.

The lifestyles and diets that cause obesity could be causing sperm damage, or excess fat in the groin region may overheat the testicles. Diabetes damages many kinds of tissues, which the new research indicates include sperm.

Scientists behind the work told the European Society of Human Reproduction and Embryology conference in Barcelona that obese men who wish to become fathers should be advised to lose weight, as women are already. Many NHS trusts refuse to fund IVF for obese women.

The findings also suggest that rising rates of obesity and type 2 diabetes, which are increasingly affecting men and women of reproductive age, could be having a strong negative effect on fertility.

Male factor infertility accounted for 33 per cent of infertility treatments performed in the UK in 2006, up from 28 per cent in 2000. While some of this increase reflects the advent of better therapies for infertile men, some scientists believe the trend may also be connected to sperm counts, which have been declining for several decades.

Con Mallidis, of Queen's University, Belfast, who led the diabetes study, said: “There is massive growth in diabetes and obesity, and concern about fertility rates, and we asked is there a connection. It is not a coincidence.”

In England, 24.9 per cent of males and 25.2 per cent of women over 16 are obese, and 37 per cent of adults have a waist circumference that is greater than healthy levels, according to the most recent official statistics, for 2006. Obesity is rising according to both indicators - in 1993, 15 per cent of adults were obese and 23 per cent had a raised waist circumference.

The prevalence of diabetes increased from 2 per cent in 1991 to 4.3 per cent in 2003 among men, and from 2 per cent to 3.4 per cent among women. The risk of type 2 diabetes is up to ten times greater among obese people.

In the first study, a team at the University of Aberdeen examined patient records from 2,037 men treated at its fertility clinic, and compared body mass index (BMI) with sperm quality. They found that obese men were 60 per cent more likely to have lower semen volume, and 40 per cent more likely to have abnormal sperm.

The research did not examine the genetic quality of sperm or whether these changes affected pregnancy rates, though the group is now beginning a study to investigate these.

Ahmad Shayeb, who led the study, said: “Our findings were quite independent of any other factors, and seem to suggest that men who are trying for a baby with their partners should first try to achieve an ideal body weight. The pressure to do this has always been on women, but we should also be putting pressure on men.”

The Queen's study examined the DNA in sperm from eight men with type 1 diabetes. While their sperm looked outwardly normal, they had high levels of genetic damage that could impair their ability to fertilise an egg. The findings are also applicable to type 2 diabetes.

Dr Mallidis said: “We have shown for the first time that diabetes adversely influences male fertility at a molecular level.”

Neil McClure, Professor of Obstetrics and Gynaecology at Queen's University Belfast, said: “These two abstracts from the Belfast and Aberdeen groups on the effects of diabetes and obesity, respectively, on male fertility demonstrate a clear picture that male fitness and health are clearly linked to a man's fertility.

“For too long the role of general health in male fertility has been ignored. Very few centres take a detailed history from the man, concentrating instead on the female. This basic mistake is understandable but, now, those working in this area must give greater consideration to the male and to ensuring that he is in peak physique and health to maximise the couple's chances of successful conception, be it spontaneous or with treatments such as ovulation induction or assisted reproduction.”

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