Testosterone – particularly the biologically active free testosterone – declines in men beginning at about age 30. Low levels of testosterone are linked to muscle and bone loss, coronary artery disease, diabetes, depression, erectile dysfunction and loss of self-esteem.

Researchers from the University of California, San Diego School of Medicine, led by Dr. Gail Laughlin found that it also leads to premature death from all causes. The study followed nearly 800 men aged 50 through 91 for 18 years.

One-third of subjects with the lowest testosterone levels had a 33 percent greater risk of death during the 18-year study than one-third of men with the highest levels.

Men with low testosterone had higher markers of inflammation and were more likely to have three or more symptoms associated with the Metabolic Syndrome – increased level of abdominal fat, cholesterol, blood pressure, blood sugar and triglycerides.

But what are the causes of low testosterone? This particular study didn’t address this question. It’s known that the causes of low testosterone range from poor diet habits and lack of exercise to genetics. However, testosterone also declines naturally, as we age. So, just because your testosterone levels are lower now than, say, 10 years ago, doesn’t mean you have a problem.

The authors recommended that men with low testosterone take supplemental testosterone, exercise regularly and maintain a normal weight. [sources: The Endocrine Society Journal, Fitness RX]