As men grow older, they experience changes in body composition, energy, strength and sexual function, which are usually attributed to the decrease in testosterone production that typically occurs in the middle years.

However, a study by Massachusetts General Hospital (MGH) in the US found that insufficient estrogen could be at least partially responsible for some of these symptoms.

"This study establishes testosterone levels at which various physiological functions start to become impaired, which may help provide a rationale for determining which men should be treated with testosterone supplements," said Joel Finkelstein, of the MGH Endocrine Unit, and corresponding author of the study.
"But the biggest surprise was that some of the symptoms routinely attributed to testosterone deficiency are actually partially or almost exclusively caused by the decline in estrogen that is an inseparable result of lower testosterone levels," he said.
A small portion of the testosterone in men is converted into estrogen by an enzyme called aromatase.
The higher the testosterone level in a normal man, the more is converted into estrogen, so men with low testosterone also have low estrogen levels, making it unclear which hormones support which functions.
The MGH team set out to determine the levels of hormone deficiency at which symptoms begin to occur in men and whether those changes are attributable to decreased levels of testosterone, estrogen or both.

The study enrolled two groups of men with normal reproductive function, ages 20 to 50 and all participants were first treated with a drug that suppresses normal production of all reproductive hormones.

Men in the first group were randomly assigned to receive daily doses of testosterone gel at one of four dosage levels or a placebo gel for 16 weeks.
Men in the second group received the same testosterone doses along with an aromatase inhibitor which markedly suppressed conversion of testosterone into estrogen.

Among participants in whom estrogen production was not blocked, increases in body fat were seen at what would be considered a mild level of testosterone deficiency.

The results showed that testosterone levels regulate lean body mass, muscle size and strength, while estrogen levels regulate fat accumulation.
Sexual function, both desire and erectile function is regulated by both hormones, researchers found.


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