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But it might not be the testosterone itself. Rather, it could be that men undergoing the hormone therapy feel better and stronger so they exercise more and watch their diet, speculated experts not involved with the study.
"It [the study's conclusion] is overly optimistic," said Dr. Bradley Anawalt, a spokesman for The Endocrine Society and chief of medicine at the University of Washington Medical Center in Seattle. "There is very little evidence that testosterone should cause weight loss in men obese or otherwise; it usually results in weight gain."
"If you were a drug company, the best product in the world would be a drug that causes you to lose weight, increases muscle and strength and improves your sex drive," he said.
Testosterone shouldn't be looked at as a miracle weight-loss drug, Anawalt said. "Before anybody got excited about this, there [would] need to be lot more studies done to reproduce these findings," he added.
The findings were to be presented Tuesday at the European Congress on Obesity in Lyon, France.
The study was led by Dr. Farid Saad, from Bayer Pharma AG in Berlin.
For the study, the researchers followed 251 obese men aged 38 to 83 with low testosterone levels. Among these men, 214 were followed for two years and 115 were followed for five years.
The men were given 1,000 milligrams of testosterone by injection when the study started, again at six weeks and then every 12 weeks until the end of the trial.
The men who were followed for five years lost an average of 35 pounds. Their average body-mass index -- a measurement that takes height and weight into account -- dropped from 34 to 29, moving them from the obese category into the overweight category. In addition, they also saw improvements in their cholesterol and triglyceride levels, along with their blood pressure.
In addition, there was no increase in the risk of prostate cancer, researchers noted.
"Raising serum testosterone to normal reduced body weight, waist circumference and blood pressure, and improved metabolic profiles. These improvements were progressive over the full five years of the study," the study authors said.
Obesity has been linked with reduced testosterone, according to the researchers. Men older than 45 who have low testosterone levels are about twice as likely to be obese, have type 2 diabetes and have high blood pressure as men with normal testosterone levels, they added.
There may be a number of reasons why, the study authors said. Increased testosterone improves energy and motivation to do physical activity; testosterone also increases lean body mass, which increases the amount of energy used.
Another expert, Dr. Spyros Mezitis, an endocrinologist at Lenox Hill Hospital in New York City, added that "anecdotally we have been seeing improvement in elderly men with low testosterone taking testosterone, including improved cholesterol, more lean body mass and fat loss."
Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
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