Jan. 3 (HealthDay News) -- Testosterone supplements increase lean body mass and decrease body fat in men over age 60 who have unusually low testosterone but do not improve strength, mobility or mental abilities, researchers report.

Testosterone levels normally decrease as men age, although the amount of decrease varies among men. Losing testosterone is associated with a loss of muscle mass, strength, mental facility and bone mass as well as an increase in body fat, the team noted in the Jan. 2 issue of the Journal of the American Medical Association.

Clinical trials of the effectiveness of testosterone supplementation have not been conclusive, added the team from the University Medical Center Utrecht, the Netherlands.

They analyzed quality of life and health data from 207 men between the ages of 60 and 80 who had lower than average testosterone levels. Participants took 80 milligrams of testosterone or a placebo twice daily for six months. The men did not know whether they were taking testosterone or the placebo.

The researchers found that the men who took testosterone had more lean body mass and less fat than their peers but no increase in mobility of strength. Although not statistically significant, the men taking testosterone were slightly more likely to have metabolic syndrome by the end of the study. Metabolic syndrome is a predictor of type 2 diabetes and is characterized by obesity and unhealthy cholesterol levels.

Men taking testosterone also had improved insulin sensitivity but lower levels of "good" high-density lipoprotein (HDL) cholesterol. Testosterone supplementation was associated with an increase of creatinine in the blood, as well as hemoglobin and hematocrit, measures of red blood cell health. There were no negative effects of testosterone on prostate health, reported the researchers.

"This study is, as far as we know, the largest study of testosterone supplementation with the most end points and a randomized, double-blind design. Adherence was high, and the dropout rate was low," the authors wrote in a prepared statement. "The findings in this study do not support a net benefit on several indicators of health and functional and cognitive performance with six months of modest testosterone supplementation in healthy men with circulating testosterone levels in the lower range."

SOURCE: Journal of the American Medical Association, news release, Jan. 1, 2008