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You Asked for It: Question Authority

Nieman, David C. Dr.PH, FACSM

ACSM'S Health & Fitness Journal: July/August 2004 - Volume 8 - Issue 4 - pp 3-4
Departments: You Asked for It: Question Authority

Examines the effects of regular exercise on diet.

David C. Nieman, Dr.PH., FACSM, is professor and director of the Human Performance Laboratory, Appalachian State University, in Boone, NC, an active researcher, and author of several textbooks on health and fitness.

Q: If you start a regular exercise program, will you also feel like eating better?

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A: Not really. Exercise in and of itself has a minor influence on the quality of the diet. However, with increasing amounts of exercise, people tend to eat more, increasing their overall intake of essential vitamins and minerals. Before exploring this further, let's review what "eating better" means.

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The Prudent Diet

People in the United States tend to eat too much energy in the form of fat, especially the hard saturated fat common in animal products. They also have diets too low in starch and fiber, found in plant foods like whole grains, fruits, and vegetables. Such diets explain why people in the United States have high rates of obesity, heart disease, high blood pressure, stroke, diabetes, and some forms of cancer.

It should be noted that diseases caused by vitamin and mineral deficiencies are rare in this country and other developed nations. Very few people die anymore from scurvy (vitamin C deficiency), pellagra (niacin deficiency), or beriberi (riboflavin deficiency).

For all individuals, whether physically active or inactive, a "prudent diet" is recommended for general health and prevention of disease. The calorie composition of a diet that provides adequate energy and nutrients and also reduces one's risk for chronic diseases is 45% to 65% carbohydrates (sugars and starch), 20% to 35% fat, and 10% to 35% protein (1). This type of diet is advocated for fitness enthusiasts (those exercising 3 to 5 days per week, 20 to 30 minutes per session) and nearly all athletes, including those in most individual, dual, and team sports and power events (such as weightlifting or track and field).

For the competitive endurance athlete, (who trains more than 90 minutes a day in such sports as running, swimming, and cycling) several adaptations beyond the prudent diet are beneficial, including a higher percentage of carbohydrate, less fat, and more water. To increase carbohydrate and lower fat intake, most experts recommend that athletes consume less visible fat (e.g., margarine, oil, salad dressing, and mayonnaise), fewer high-fat dairy products (e.g., most cheeses, whole milk, butter, and cream cheese), less high-fat meat (e.g., fried meats, bacon, corned beef, ground beef, ham, sausages, and processed meats), and more grain products (e.g., pasta, bagels, breads, brown rice, and cereals), tubers (potatoes and yams), legumes (e.g., kidney beans or pinto beans), dried fruits (e.g., raisins and dates), fresh fruits, and fresh vegetables. For the vast majority of athletes, vitamin and mineral supplements are not needed if the diet is varied and balanced.

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What Does Exercise Have to Do with the Kind of Food I Eat?

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Two types of studies are available in answering this important question: comparisons of the diets of athletes and nonathletes, or physically active and inactive people, and exercise training studies, where inactive people are assigned to exercise and control groups to determine whether people initiating exercise spontaneously improve the quality of their diets.

Many reports on the diets of athletes and nonathletes are available. In general, most researchers have found that athletes, especially those in endurance sports like running and cycling, have slightly healthier diets than nonathletes. The contrast is more impressive for the most serious athletes (2).

Although there are notable exceptions, many athletes feel that a good diet gives them a competitive edge, and therefore they try to keep their carbohydrate intake higher and their fat intake lower than normal. It is interesting, however, that most groups of athletes have been found to be eating more fat and less carbohydrate than is recommended for their athletic endeavors.

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What about people who exercise regularly but are not competitive athletes? Most studies that have compared physically active and inactive people have shown that, indeed, people who exercise regularly do have better diets. One problem with the group comparison studies, however, is that researchers are unable to sort out why serious exercisers have superior diets to their sedentary peers. Is it because the exercise spontaneously prompts them to eat better? Or could it be that people motivated enough to exercise regularly also are disciplined enough to choose healthy foods?

One way of providing answers to these questions is to randomly divide sedentary subjects into exercise and nonexercise groups, and then follow them for several months, measuring their diets along the way to see if quality changes. This is a difficult study to conduct, but most investigators have concluded that moderate exercise (e.g., brisk walking 3 to 5 days a week for 20 to 45 minutes a session) is an insufficient stimulus to cause people to make meaningful changes in the quality of their diets (3-5).

It is interesting, however, that as the exercise continues and is increased to higher levels, people start to eat more. Nutritionists feel that this is one of the chief benefits of regular and vigorous or long-duration (more than 45 minutes per day) exercise: people take in more food and energy, increasing their chances of getting all of the recommended amounts of vitamins and minerals (1).

So in answer to your question, keep exercising, but don't be fooled that you will automatically improve the quality of your diet. Eating better is hard work and will take a conscious effort on your part.

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1. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, D.C.: National Academies Press, 2002.
2. Fogelholm, M. Micronutrients: interaction between physical activity, intakes and requirements. Public Health Nutrition 2:349-356, 1999.
3. Nieman, D.C., L.M. Onasch, and J.W. Lee. The effects of moderate exercise training on nutrient intake in mildly obese women. Journal of the American Dietetic Association 90:1557-1562, 1990.
4. Donnelly, J.E., E.P. Kirk, D.J. Jacobsen, et al. Effects of 16 mo of verified, supervised aerobic exercise on macronutrient intake in overweight men and women: the Midwest Exercise Trial. American Journal of Clinical Nutrition 78:950-956, 2003.
5. Butterworth, D.E., D.C. Nieman, R. Perkins, et al. Exercise training and nutrient intake in elderly women. Journal of the American Dietetic Association 93:653-657, 1993.
© 2004 American College of Sports Medicine