Obese teenage boys face being impotent and infertile because their testosterone levels are so low.

Researchers at the University of Buffalo have found that obese teen boys have 50 per cent less testosterone than their healthy weight counterparts.

Low testosterone levels significantly increases the potential of impotence and infertility in later life, the study said.

Obesity can also increase the risk of cancer, strokes, liver disease, diabetes and heart disease. In the UK obesity causes around 9,000 premature deaths every year and reduces life expectancy by nine years on average.

Professor Paresh Dandona, first author of the study, 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 2004, the same research team established a link between low testosterone levels in obese, type-2 diabetic male adults. They confirmed the link in a 2010 study.

The new research looked at 25 obese and 25 lean teenage boys, measuring their testosterone levels by taking morning fasting blood samples. Dandona says they now need to test a wider sample.

"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," he said.

"The good news is that we know that testosterone levels do return to normal in obese adult males who undergo gastric bypass surgery," says Dandona. "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."

NHS statistics show that in 2010, 26 per cent of adults in England were obese. A further 42 per cent of men and 32 per cent of women were overweight.

The same report showed that a third of children aged between two and 15 were considered either overweight or obese.

Tam Fry, spokesperson for the National Obesity Forum, said the research was a "real wake-up call". He told IBTimes UK: "Boys are not told about the prospective fertility problems they may face from being overweight."

He added that girls also face fertility problems from being overweight through having a higher risk of developing polycystic ovary syndrome.

This research follows news that the number of obese people in France has doubled in the last 15 years to reach seven million, according to pharmacy group Obepi-Roche.