Estrogen has long been equated with all things female and testosterone, with all things male, even though both genders make each hormone.
But the hormone story, like much of life, isn't as simple as it might appear.
Estrogen may help men stay fertile, scientists now say, by preventing a plumbing problem in the reproductive system.
At the root of this new thinking: studies on infertile mice.
Mice that lacked a certain estrogen receptor (a cellular structure to which the estrogen must bind) in their efferent ducts, which transport sperm from the testes, were infertile, says Rex A. Hess, associate professor of veterinary biosciences at the University of Illinois, Champaign-Urbana, and lead author of the study on the role of estrogens in male reproduction, published earlier this month in the journal Nature.
Infertility resulted, he says, because the tubes lacking the receptors did not absorb seminal fluid as they should. (It's believed the estrogen sends the signal to do so.) As a result, the seminal fluid is diluted, reducing sperm counts and interfering with the maturation of the sperm. Then, fluid backs up into the testes, and they shut down.
"They do produce what appear to be normal sperm," Hess says, emphasizing that further study in that area is needed.
Although the studies were done only on mice, Hess says, "There's pretty good evidence this will apply to men."
Potential practical applications of the research are many, say Hess and co-author Dennis Lubahn of the University of Missouri, Columbia. The discoveries could lead to a treatment for male infertility, to a male birth-control treatment and to the development of a test to determine the effects of pesticides and industrial chemicals on male fertility.
While Lubahn and Hess continue to focus on male fertility and estrogen, others are investigating other benefits of both male and female hormones.
Here, a partial scorecard incorporating the latest research, along with anecdotal observations:
Benefit or Action Estrogen Testosterone Helps ward off osteoporosis Yes Yes Reduces colon cancer risk Yes No data Reduces heart disease risk Yes No data Improves HDL ("good") cholesterol Yes May worsen Reduces LDL ("bad") cholesterol Yes No Improves short-term memory and delays onset of Alzheimer's Yes No data Helps build muscle No Yes Required for erection, ejaculation No Yes Improves libido No Yes Improves appearance of skin Yes No Helps stimulate dark facial hair growth No Yes, at high levels Reduces age-related tooth loss Yes No data Improves ability to perform fine-motor movements Yes No data Required for marathon football viewing No Yes A handy excuse for tearful outbursts, eating a pint of Chunky Monkey Yes No
Sources: Dr. Morrie Gelfand, professor of obstetrics and gynecology, McGill University School of Medicine, Montreal; Dr. William Andrews, past president, American College of Obstetricians and Gynecologists.