EXERCISE DOES NOT ALTER REPRODUCTIVE HORMONES IN WOMEN

Low energy availability, not the stress of exercise, affects the release of reproductive hormones in women. Research findings concerning the pulsatility of luteinizing hormone (LH) in women will be discussed at a meeting of international scientists during Olympics

Portland, ME— Women who diet and exercise to extremes have been shown to suffer skeletal damage from reproductive disorders such as amenorrhea (lack of a menstrual period). It has traditionally been thought that low body weight or the stress of exercise can affect a woman's reproductive hormones. Recent research indicates that this is not the case, and that the cause of these disorders is diet, says Anne B. Loucks, Ph.D.

Dr. Loucks, Professor in the Department of Biological Sciences in the College of Arts and Sciences at Ohio University, will discuss these research findings at the 2000 Intersociety Meeting of the American Physiological Society. The meeting, "The Integrative Biology of Exercise," will be held September 20-23, in Portland, Maine.

Athletic women display a disproportionately high prevalence of amenorrhea. Previous endocrine and neuroendocrine experiments have found that menstrual and ovarian dysfunction in these women are due to the disruption of the pulsatile secretion (release) of luteinizing hormone (LH) by the pituitary, which is caused by the disruption of the release of gonadotropin-releasing hormone (GnRH) by the hypothalamus.

The so-called "exercise stress hypothesis" has held that exercise training is a chronic stressor that, like other chronic stressors, activates the hypothalamic-pituitary-adrenal axis which in turn disrupts the GnRH pulse generator. If this hypothesis were true, the appropriate intervention to prevent or reverse menstrual disorders in athletes would be to moderate the exercise regimen.

The so-called "energy availability hypothesis" holds that the GnRH pulse generator is disrupted by an as-yet unidentified signal that dietary energy intake is inadequate for the energy costs of both reproduction and locomotion. If this hypothesis were true, the appropriate intervention would be to eat more.  This hypothesis is supported by reports that athletic women consume less energy than would be expected for their activity level; by endocrine signs of chronic energy deficiency in amenorrheic athletes; by similarities between athletic amenorrhea, weight-loss amenorrhea, and anorexia nervosa; and by an extensive literature on the bioenergetics of reproduction.

To differentiate the independent effects of energy availability and exercise stress on LH pulsatility, Dr. Loucks and her colleagues studied 9 healthy, regularly menstruating, habitually sedentary, nonobese, nonsmoking women 18-29-years of age, with no recent history of dieting, weight loss, or aerobic training. Blood samples were drawn at 10-minute intervals over 24 hours on days 8, 9, or 10 of two menstrual cycles after 4 days of intense exercise. To test the hypothesis that LH pulsatility is disrupted by low energy availability, the subjects' dietary energy intakes were controlled. To test the hypothesis that LH pulsatility is disrupted by the stress of exercise, the researchers compared the resulting LH pulsatilities to those previously reported in women with similarly contrasting energy availabilities who had not exercised.

Study results indicated that exercise appears to have no effect on the release of LH beyond the impact of the energy cost of exercise on energy availability. As long as the exercising women took in as many calories as they used up while exercising, the pulsatility of LH was not affected.

In a following experiment, Dr. Loucks' team of investigators administered several different levels of dietary energy to men and women performing a similar standardized exercise regimen. They found LH pulsatility to be disrupted in women when they consumed a daily diet that provided less than ~25 kilocalories per kilogram of lean body mass in addition to the amount of energy they expended in exercise. For a woman with a body weight of 132 pounds, this amounts to ~1100 kilocalories in addition to the energy she expends in exercise.  Effects of low energy availability below this threshold were similar in men.

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The American Physiological Society is devoted to fostering scientific research, education, and the dissemination of scientific information. By providing a spectrum of physiological information, the Society strives to play a role in the progress of science, and the advancement of knowledge. Providing current, usable information to the physiological community is the Society's primary focus.

Editor's Note: For further information or to schedule an interview with Dr. Loucks, contact Donna Krupa at 703.527.7357; cell: 703.967.2751; or at djkrupa1@aol.com; or visit the APS website at www.the-aps.org.