You probably consider testosterone the “male hormone” and estrogen the “female hormone,” and for good reason. Men’s bodies do make and use more testosterone, while women’s bodies make and use more estrogen.

But it’s not quite that simple. Both genders need both hormones to varying degrees. And now new research shows that estrogen plays a very important role in keeping men fit and virile!

Testing Testosterone

When a male patient complains of low energy, loss of strength and a diminished sex drive, his doctor may have him tested for a condition called male hypogonadism, in which the body doesn’t produce enough testosterone. If results show that the man’s testosterone level is below a certain threshold, he’ll likely be given a prescription for testosterone—which, it’s hoped, will bring back his manly vigor.

Even though millions of men now are on testosterone therapy for just this reason, not a lot is known about the effects of different doses. So researchers at Massachusetts General Hospital in Boston designed a study to evaluate the effects of varying doses. And since men with low testosterone also tend to have low estradiol, a form of estrogen that in men is a by-product of testosterone, they also measured what happened when estradiol production was blocked.

The study included 400 healthy men between the ages of 20 and 50. All took a medication to suppress their natural production of testosterone. The men were divided into groups and received a daily dose of topical testosterone gel—1.25 grams (g), 2.5 g, 5.0 g or 10 g—or a placebo gel. Additionally, some men also were given a drug to prevent testosterone from being turned into estradiol, allowing the researchers to separately compare their effects.

The participants came in every four weeks for blood tests and an evaluation of their health status, including physical function and sexual function (as assessed by patients’ own reports on sexual desire and erectile function). At the beginning and end of the 16-week study, the researchers measured each man’s overall body fat, abdominal fat, lean muscle mass and strength.

The Big Surprise

As expected, the men who had been given the placebo gel—meaning nothing to replace their greatly diminished testosterone levels—gained fat, lost strength and muscle mass, and reported problems with libido and erections.

In the men who received varying levels of testosterone and whose production of estradiol was not blocked, results varied widely depending on the testosterone dosage…

  • Overall body fat increased in men receiving anything less than 5.0 grams of testosterone.
  • Abdominal fat didn’t change in men receiving any dose of testosterone.
  • Lean muscle mass decreased in men who received less than 2.5 grams of testosterone.
  • Strength did not change in men receiving any dose of testosterone.
  • Sexual function decreased in all the men—and the smaller the testosterone dosage, the worse the sex problems were.
  • Then came the group of men who had received varying doses of testosterone but whose production of estradiol had been blocked. Though lean muscle mass and strength were not affected by estradiol, there were some big surprises in other areas…

  • All of the men with blocked estradiol had significant increases in body fat.
  • All of the men had increases in abdominal fat—and the higher their testosterone dosage was, the more this fat increased.
  • Sexual desire and erectile function decreased significantly in all the men.
  • That these changes happened across the board suggested that it wasn’t only the lack of testosterone that caused problems—but also the lack of estradiol.

    What this means for patients: Going forward, in treating men who are found to have low testosterone, doctors may closely monitor their patients’ estrogen levels as well and prescribe medications to keep estrogen within the normal range. In the meantime: If you are being treated for low testosterone, it would be wise to talk with your doctor about how low estradiol could be contributing to your increased body fat and sexual problems…and to review your testosterone dosage, given that in this study the amount of testosterone required to maintain muscle mass, strength and sexual function varied widely.

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