Women who have been forcing themselves to exercise regularly to get into shape and lose unwanted pounds may find that the benefits of their athletic pursuits extend beyond a trim look. Past research has determined that exercise may lead to a decreased chance of breast and reproductive tract cancer in women, and four Harvard researchers now hope to discover the biological basis for this reduced risk.
The group of experts last week began a three-year study, for which they received a $300,000 National Institutes of Health (NIH) grant, to closely examine the effects of exercise on "elite oarswomen," who are female athletes between 20 and 30 years of age and are competing for spots on the lightweight or heavyweight national rowing teams. The research, which should explain why only some exercising women experience the menstrual dysfunction that can lead to a reduced cancer risk, will include studies on the amount and location of fat present in the bodies of participants, as well as body fat's effect on the way estrogen is metabolized.
Athletics and Health
Rose E. Frisch, an associate professor at the School of Public Health (SPH) and principal investigator of the research team, has worked for more than a decade studying the age of menarche (the onset of menstruation) and the long-term reproductive health of former women athletes, and this work forms the basis for the current NIH study.
Frisch began the bulk of her work in the late 1970s, when she found that a group of ballet dancers she analyzed had a late menarche compared to their non-athletic peers. By 1981, Frisch broadened her study, analyzing the menstrual cycles of runners and swimmers on college varsity teams.
This later study confirmed her previous results, as she found that the selected athletes had a delayed menarche, a "lean" build and irregular menstrual cycles.
Frisch calculated that for each year of exercise, defined as two hours of energy-intensive activity twice a week, girls can delay menarche five months. A girl who begins exercising when she is nine, can delay menarche from the average age of twelve-and-one-half to the age of 15, she says.
Scientists have already associated a high risk of breast cancer with early menarche, Frisch says, so the later a girl's menarche, the better her future health will probably be. "It is a good thing for young girls to start exercise early," she says.
Frisch also concluded that some women athletes, who have lost a significant amount of body weight due to strenuous exercise, experience menstrual dysfunction and may become temporarily infertile.
"The hypothalamus normally sends out a [hormone] signal that puts the reproductive system on 'go,'" Frisch says. If body weight is too low, the brain will not send any hormonal signals to the reproductive organs, and menstruation will not occur, she says. Once exercise is cut back and weight is regained, the menstrual cycle and ovulation will be restored, and pregnancy will be possible. "The situation is not irreversible," she says.
Frisch has determined that if a woman weighs 10 to 15 percent below her normal weight, her reproductive cycle will be turned off. Doctors have even created a "fatness index," indicating the minimum amount a woman must weigh to be fertile. Women who become infertile can refer to this index to discover the weight they need to regain fertility, assuming nothing else is wrong, Frisch says.
Frisch, who works at the Center for Population Studies, conducted yet another analysis in 1985 of more than 5000 college alumnae, some of whom participated in athletics. "The alumni who had exercised in college had less cancer of the breast and reproductive system," she says.
The following year, Frisch determined further from the same data pool that the athletic women, who were between 20 and 80 years of age, had a lower lifetime occurrence of benign tumors and diabetes.
"The public health implications of those findings are a reason to find out" the biological basis of this phenomenon, says Robert Barbieri, an associate professor at the Medical School and another member of the research team.
Thesis research done by Rachel C. Snow, another member of the team who is doing post-doctoral work at the SPH, has also served as important background information for the current NIH analysis.
Snow, who also works at the Center for Population Studies, has researched estrogen metabolism in female athletes, basing most of her conclusions on a study of 10 "elite oarswomen" and the way their bodies process the female hormone involved in developing secondary sex characteristics.
She found that those athletes with menstrual dysfunction while in high-intensity athletic training metabolized a greater fraction of their estrogen to "non-potent" forms, to which the reproductive organs will not respond. The increase in non-potent estrogen production also means that there is less normal hormone circulating in the blood, and this may lead to a lower risk of estrogen-dependent tumor development, such as breast cancer, Snow says.
However, Snow, a former member of the national lightweight rowing team, found that not all women who exercise metabolize their estrogen in this way. Those women who do have the metabolic abnormality also have menstrual dysfunctions, are leaner and do not ovulate.
Snow says she hopes the current NIH-funded oarswoman study will help the team to "understand this individual variability."
The current research project centers around four sets of tests under the direction of a pair of physicians at the Brigham and Women's Hospital and the Massachusetts General Hospital (MGH). Three of the procedures measure body fat, and the other traces estrogen metabolism in the body.
One of the tests, which uses Magnetic Resonance Imaging (MRI) techniques, helps give doctors an accurate picture of where subjects' body fat is located and how it is stored, without any of the harmful radiation effects of X-rays, according to MRI experts.
The MRI procedure, supervised by Bruce R. Rosen, an assistant professor at the Medical School and the director of Clinical Nuclear Magnetic Resonance at MGH, serves as the main emphasis of the entire project, entitled, "MRI--of body fat, ovulation, and estrogen metabolism."
Another of the project's tests follows the breakdown of estrogen by allowing doctors to trace low levels of radioactive forms of the hormone molecules through the body. "Athletes destroy estrogen faster than non-athletes, so their bodies are exposed to less" of the hormone, and this may lead to decreased cancer risk, says Barbieri, who has worked with both Frisch and Snow in the past.
"Hopefully, we will be able to understand how estrogen metabolism and fatness work in individual women, and to explain the frequency of ovulation," Snow says of the project.