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Study: Estrogen makes men more hot to trot

NEW YORK - Estrogen, the main sex hormone in women, plays a bigger role in male libido than previously thought, according to research that may guide development of sex-drive treatments.

The study found that reducing estrogen in men undergoing treatment for low testosterone resulted in a drop in libido and an increase in body fat, even as testosterone levels improved. The research is published in the New England Journal of Medicine.

The finding surprised researchers, who have believed the testosterone caused men to have stronger sex drives than women, said Joel Finkelstein, a study author and endocrinologist at Massachusetts General Hospital. While it’s known that the body converts some testosterone into estrogen and that the hormone declines in men with low testosterone, researchers didn’t fully understand estrogen’s role in male sexual desire.

“What will surprise many people is that loss of sexual desire in men with low testosterone is due to lack of estrogen,” Finkelstein said. “People think estrogen in men makes them very effeminate; they think of it as a female hormone, they think it is testosterone that gives men their sexual desire.”

While there have been clues that estrogen as well as testosterone influence male sex drive there have been few definitive studies until now, Finkelstein said.

Researchers have found a drop in libido in mice by cutting their estrogen. There also have been anecdotal reports of men undergoing sex changes who see an increase in libido when they start estrogen therapy. In women, it may be the opposite, where testosterone plays a bigger role than estrogen in their sex drive.

“Both sexes require the opposite gender hormone to have adequate libido,” he said.

In the study, researchers looked at more than 300 men ages 20 to 50 that were given treatments to suppress production of all reproductive hormones. Half the men were then given a testosterone-boosting gel or a placebo. The other half received the testosterone gel along with a drug that lowered the production of estrogen.

The researchers found that testosterone was the main hormone involved in lean body mass and muscle strength. Estrogen played a bigger role in fat accumulation. While low testosterone did cause a decline in sexual desire, those getting the estrogen-suppressing drug saw a greater drop-off.

The finding shouldn’t mean a change in treatment for men with low testosterone. Since the body is able to convert most commercially available testosterone replacement therapies to estrogen, raising testosterone will in turn boost estrogen levels as well. It should, however, discourage drugmakers from trying to develop new forms of testosterone replacements that aren’t able to be converted to estrogen, Finkelstein said.

The U.S. market for testosterone replacement therapies, such as Eli Lilly ’s Axiron and AbbVie’s Angrogel, is almost $2 billion with as many as 13.8 million men older than 45 in the U.S. having low levels of testosterone, according to a 2006 study in the International Journal of Clinical Practice. Lower-than-normal levels can lead to a loss of libido, a decrease in bone and muscle mass, and depression.

Despite testosterone products being available for decades, Finkelstein said more research is needed into the health benefits and side effects of testosterone, especially in men over 50.

“It is amazing what we don’t know,” said Finkelstein. “Even the most basic things we don’t understand about this drug that has been around for almost 80 years.”

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