Testosterone helps to build muscle

A study of 800 men over 50 found that those with low levels had a 33% increased risk of death over an 18-year period than those with higher levels.

At a Toronto meeting of The Endocrine Society, researchers said they did not recommend taking supplements.

Experts warn there could be side effects and say men should keep active to help maintain testosterone levels.

The study participants, who were aged between 50 and 91, have been taking part in a chronic disease study in California since the 1970s.

Levels of testosterone were classified as low if they were at the lower limit of the normal range for young adult men.

Testosterone gives you a zing, if you have low testosterone it tends to make you less active

Professor Richard Sharpe

Testosterone levels decline with age but there is wide variation. In the study, 29% of the men had low levels of the hormone.

The higher risk of death in men with low testosterone levels could not be explained by smoking, drinking, physical activity level or pre-existing diseases such as diabetes or heart disease.

Metabolic syndrome

However, the researchers did note that men with lower testosterone levels were three times more likely to have a cluster of risk factors associated with cardiovascular disease and diabetes.

Collectively known as "metabolic syndrome", the risk factors include waist measurement over 40in, high levels of cholesterol, high blood pressure and high blood sugar.

Study author Dr Gail Laughlin, assistant professor in the Department of Family and Preventive Medicine at the University of California, San Diego, said: "Our study strongly suggests that the association between testosterone levels and death is not simply due to some acute illness".

She added that lifestyle may determine testosterone levels and that it may be possible to alter levels by lowering obesity.

Co-author Professor Elizabeth Barrett-Connor said it was not being recommended that men should go out and buy testosterone supplements.

"Maybe the decline in testosterone is healthy and comes with older age," she said.

"Maybe the decline is bad and associated with chronic diseases of ageing."

'Keep active'

Professor Richard Sharpe, from the MRC Human Reproductive Sciences Unit in Edinburgh, said the results were particularly important because studies had shown levels of testosterone in men of all ages were falling.

"The other important thing about this study is the association with metabolic syndrome. Being obese lowers the available testosterone and that makes you more obese so it's a vicious cycle.

"Testosterone gives you a zing, if you have low testosterone it tends to make you less active."

He said the use of testosterone supplements was a very contentious theory because of potential side effects.

"Instead you should adapt your lifestyle, to keep your body in shape and make the best of your testosterone.

"Men assume they're just getting older when they get a gut but keeping a good body shape will help maintain your testosterone levels."