Men of America, are you feeling less than maximally macho?
Don't blame testosterone.
Your real problem may be lack of estrogen.
That's right, estrogen - the so-called "female" hormone that produces women's secondary sexual characteristics, such as curvy hips and breasts.
Doctors have known for years that testosterone declines as men age - a process that diminishes everything from upper-body strength and muscle mass to sex drive. The pharmaceutical industry aggressively markets hormone supplements for men anxious about reduced testosterone levels, which drug companies call "low T syndrome."
Yet at least some of the bodily changes seen in aging men - flabbier physique, low libido and erectile dysfunction - come from falling estrogen levels, according to a new study in The New England Journal of Medicine.
In a study of 198 healthy young men, researchers found that both testosterone and estrogen loss contributed to sexual dysfunction. But only estrogen was related to changes in body fat.
While those findings will surprise many people, they're consistent with studies in animals, says study co-author Joel Finkelstein, an endocrinologist at Massachusetts General Hospital. It has been established that a man's declining estrogen levels leave him at greater risk for thinning bones, a problem that is often more pronounced in women, Finkelstein says. And he notes that testosterone plays a role in women's libido.
But the notion that a little estrogen actually helps male sexual function? "That's quite a bit more surprising," says endocrinologist Matthew Drake, an assistant professor of medicine at the Mayo Clinic in Rochester, Minn. Drake, who was not involved in the new study, says it is "fascinating" and "extremely well-done." Its findings, he says, will make researchers "think a lot more about estrogen."
All men have some estrogen, just as all women have some testosterone. A small portion of the testosterone in a man's body is routinely converted to estrogen via a hormone called aromatase, says Finkelstein, who is also an associate professor at Harvard Medical School.
As a man's testosterone declines, so does his estrogen.
Testosterone levels in men fall much more slowly than estrogen levels in women, who often suffer hot flashes and other disruptive menopausal symptoms when their ovaries stop making estrogen, sometime around age 51.
By midlife, in fact, a man may have far more estrogen than his postmenopausal wife, says Louann Brizendine, a neuropsychiatrist and author of The Male Brain. While some of a man's estrogen is converted from testosterone, some is converted from fat cells. That's part of the reason why heavy-set men sometimes develop swollen breast tissue, says Brizendine, who wasn't involved in the new study.
Brizendine says she'd be curious to learn how changing hormone levels might affect the brain and patients' moods.
There's no reason for men already taking testosterone supplements to change their regimen, says Allan Kennedy, chair of endocrinology, diabetes and metabolism at the Cleveland Clinic, who wasn't involved in the new study. That's because commercially available testosterone creams and gels, such as the kind used in this study, have a type of hormone that is convertible to estrogen. So men taking testosterone therapy are already getting a slight estrogen boost along with it.
And there's certainly no reason for men to begin taking estrogen, Finkelstein says. That would cause them to develop breasts.
He says his study could help pharmaceutical companies make better testosterone replacements. In the past, drug companies have tried to create types of testosterone that aren't converted to estrogen.
In light of these findings, Finkelstein says, the best testosterone supplement would allow for normal estrogen conversion.
His study also gives doctors a more precise idea of the changes that result from different levels of testosterone and estrogen loss, he says.
In his study, men ages 20 to 50 - when few men have noticeable testosterone loss - took drugs to halt their natural testosterone production for 16 weeks. One group of men was then given either placebo or one of four doses of testosterone.
A second group took estrogen-blocking drugs, in addition to the testosterone supplements.
All the men had monthly blood tests and measurements of leg strength and body composition. They also answered questionnaires about their physical and sexual health.
Drake says the men in the study made an impressive sacrifice, if only for 16 weeks. "That's not an easy thing to go through for four months."