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Volume CXXXII, Number 11 |
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Can you exercise too much?
JEFF BENSON, M.D.
Dear Dr. Jeff: "Is there really such a thing as too much exercise?" B.D.
Dear G.M.: The Surgeon General, the National Institutes of Health, and the American College of Sports Medicine have all long advocated regular, moderate-intensity exercise. These recommendations are based on a very long series of epidemiologic studies that show that moderate exercise is protective against heart disease, high blood pressure, diabetes, osteoporosis, colon cancer, and anxiety and depression, among many other problems. Thirty minutes or so per day, most days of the week, is considered adequate.
Now, if thirty minutes per day is good, is 60 minutes better? Or 90 minutes? Or more? Well -- it depends! And probably most importantly, on your diet.
Muscles preferably burn carbohydrates for fuel, primarily in the form of glycogen, which is stored in liver and muscle cells. Glycogen stores are used up, however, in about 90 minutes or so of moderate-intensity exercise -- if no additional carbohydrates are taken in before or during the workout. When glycogen stores run low, muscles start burning protein (mainly by cannibalizing themselves) for energy. The result is fatigue, "bonking", or "hitting the wall."
There's not much you can do to expand your glycogen stores beyond this 90-minute capacity. You do need to replenish your stores with carbohydrate-rich snacks after exercising, and to maintain them with carbohydrate-adequate meals every day. If they're not adequately replenished, you'll just hit that "wall" even faster! You also need those carbs to fuel tissue repair and to help build up muscle mass. A reasonable guideline is 6 - 7 grams of carbohydrate/kilogram of body weight/day, if you're exercising at moderate intensity for an hour daily. You'll also need to eat enough protein, of course: about one gram/kilogram/day to maintain muscle mass, more to build it up.
And then there's your skeleton. Regular weight-bearing exercise is needed to build-up bone strength. Over-exercising, however, can actually decrease bone density. A number of other factors can as well, including excessive intake of salt, caffeine, soft drinks, alcohol, Vitamin A and protein. Caloric restriction, or inadequate caloric intake for exercise needs, is particularly harmful to bones.
Poor nutrition has direct metabolic effects on bone growth and repair. In addition, low body weight itself prevents bone strengthening, by not providing adequate mechanical support for bone motion. Most importantly for women, however, is a critical balance between exercise, nutrition and hormonal function. Adequate estrogen levels are crucial for building calcium into bones. Over-exercising quickly leads to decreased estrogen production and loss of bone density. If you're not on birth control pills, estrogen loss will be signaled first by amenorrhea, or loss of your periods. In this setting, amenorrhea is a clear warning sign that you are over-exercising or under-eating -- or both. Birth control pills will not replace this lost estrogen. And because you'll still get your "pill periods" (and so won't have an early warning sign), you might need medical and nutritional assessment to determine your risk for osteoporosis.
We exercise for a variety of reasons. Most are health-promoting, but unfortunately, not all. Like eating disorders, some exercising is in no small part driven by our cultural preoccupations with distorted, idealized body images and lifestyle expectations. This is particularly true for "obligatory" or "compulsive exercisers," who repeatedly exercise beyond the requirements of good health or training, often beyond the limits of what is safe. For them, exercise is no longer fun or satisfying, no longer even a free choice. It becomes something necessary and essential, an obligation to meet, in spite of injuries, too much time taken away from other activities, and possibly damaged relationships.
Many people who struggle with disordered eating also struggle with compulsive exercising. It becomes a way to burn calories, to "gain permission" to eat, to "relieve the guilt" of having eaten or binged. Exercise becomes another way to purge, another way to try to take control and hide from emotional pain. Quite appropriately, this kind of exercise has been called "Anorexia Athletica". It cries out for the same multi-disciplinary, long-term treatments required by all eating disorders.
So: try to eat a rounded, well-balanced diet, and Happy (Moderate) Exercising!
Jeff Benson, M.D.
Dudley Coe Health Center
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