Men who take up exercise to shed their middle aged spread could find they gain a few inches just where they want it instead, according to a medical study.

For the effect of losing spare tyres and beer bellies among men in their fifties is a boost to their sex drive, a higher sperm count and stronger erections, it found.

Losing weight reduces the chances of low testosterone levels by up to 50 per cent in more mature males.

The results come from a study by Dublin doctors of 900 men with an average age of 54 taking part in a US diabetes prevention programme.

Weight loss can delay or avoid the onset of diabetes among those men who are most susceptible to the disease.

But scientists said the boost to their sex lives could be the added incentive men need to exercise and diet.
The 900 midlife volunteers were split into three groups to receive a year of treatments designed to help ward off diabetes.

A third were told to modify their lifestyles by dieting and 150 minutes a week of exercise, a third were given the diabetes drug metformin and the rest a placebo.

The number of men with low testosterone levels remained almost identical among those taking metformin or the placebo.

But in the group making lifestyle changes, the proportion with low testosterone fell from 20 per cent at the beginning of the study to just 11 per cent a year later.

Researcher Dr Frances Hayes of St Vincent's University Hospital in Dublin said low testosterone levels were common among overweight men before they got diabetes.

She said: 'Doctors should first encourage overweight men with low testosterone levels to try to lose weight through diet and exercise before resorting to testosterone therapy to raise their hormone levels.'

The study found levels of testosterone went up in direct proportion to the number of pounds lost and the decrease in waist size.

Dr Hayes said: 'Losing weight not only reduces the risk of prediabetic men progressing to diabetes but also appears to increase their body's production of testosterone.'

The research appears to contradict a recent study from Manchester University published in the journal Human Reproduction, which found being overweight, as measured by body mass index, did not affect sperm quality.