Lower sex drive, hot flashes, and weight gain. Those are the symptoms of menopause, which result from a significant decrease in estrogen production in the female body. But are the effects exclusive to women?

A new study by researchers at Massachusetts General Hospital in the New England Journal of Medicine suggests that men go through their own menopause around the same time as women. The potbelly and reduced libido that men experience as they hit middle age may be tied to a decrease in estrogen levels, the female sex hormone responsible for women's curvier thighs and breasts.

"Some of the symptoms routinely attributed to testosterone deficiency are actually partially or almost exclusively caused by the decline in estrogens," Dr. Joel Finkelstein, the study's lead author, said in a news release.

For years, scientists had chalked these physical and sexual symptoms up to testosterone deficiency. It was the "obvious culprit," writes Gina Kolata at the New York Times, since it's been clearly demonstrated that men make less testosterone as they age. But this study shows that "estrogen turns out to play a much bigger role in men's bodies than previously thought, and falling levels contribute to their expanding waistlines just as they do in women's," says Kolata.

The study examined 400 men between the ages of 20 to 50 who agreed to have their testosterone production suppressed for 16 weeks. Half received a testosterone-boosting gel or a placebo, while the other half received a testosterone gel along with a drug that shut off estrogen synthesis. The results showed that while testosterone was the main hormone responsible for muscle strength and lean body mass, estrogen was associated with weight gain and flabbier physique. Men with lower estrogen levels also reported experiencing hot flashes.

Perhaps most surprising in the study is that estrogen is just as needed as testosterone to maintain sexual function in men. In fact, those receiving the estrogen suppression drug noticed a sharper decrease in sexual desire. "People think estrogen in men makes them very effeminate; they think it is testosterone that gives men their sexual desire," says Finkelstein. But both estrogen and testosterone can be responsible for lower libido and erectile dysfunction.

Ironically, because testosterone has been linked to masculinity so prominently, testosterone replacement therapy has become a nearly $2 billion market that aggressively targets middle-aged and older men anxious about aging.

However, the study's takeaway is not for middle-aged men to start estrogen replacement therapies in testosterone's stead. For one, high doses of estrogen causes men to develop feminine physical features, such as enlarged breasts. Also, since the body converts testosterone into estrogen, raising testosterone will also cause a rise in estrogen.