Obese teenage boys have up to 50 per cent less testosterone than their leaner pals - increasing their risk of infertility in later life, a study has found.

Researchers said the results were a 'grim message' for overweight young adults.

The study by scientists at the University at Buffalo in the U.S shows for the first time that obese young men aged 14 to 20 have around half the total testosterone than normal weight youths.

The same research team first reported in 2004 the presence of low testosterone levels - known as hypogonadism - in obese, type 2 diabetic adult males and confirmed it in 2010 in more than 2,000 obese men, both diabetic and non-diabetic.

The study's first author Dr Paresh Dandona, Distinguished Professor in the Department of Medicine, said: 'We were surprised to observe a 50 per cent reduction in testosterone in this paediatric study because these obese males were young and were not diabetic.

'The implications of our findings are, frankly, horrendous because these boys are potentially impotent and infertile. The message is a grim one with massive epidemiological implications.'

In 2009, 20 per cent of 11 to 15-year-olds boys in the UK were recorded as obese.

The study included 25 obese and 25 lean youths and was controlled for age and level of sexual maturity. Concentrations of total and free testosterone and estradiol, an oestrogen hormone, were measured in morning fasting blood samples.

Dr Dandona said the results need to be confirmed with a larger number of subjects.

However, he said: 'These findings demonstrate that the effect of obesity is powerful, even in the young, and that lifestyle and nutritional intake starting in childhood have major repercussions throughout all stages of life.'

Dr Dandona said in addition to the reproductive consequences, the absence or low levels of testosterone that were found also will increase the tendency toward abdominal fat and reduced muscle, leading to insulin resistance which contributes to diabetes.

He added: 'The good news is that we know that testosterone levels do return to normal in obese adult males who undergo gastric bypass surgery.

'It's possible that levels also will return to normal through weight loss as a result of lifestyle change, although this needs to be confirmed by larger studies.'

The researchers now intend to study whether or not weight loss accomplished either through lifestyle changes or through pharmacological intervention will restore testosterone levels in obese teenage boys.

The paper was published online in the journal Clinical Endocrinology.