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Nutrition and exercise: what your body needs.

People who exercise or play sports are always looking for something, anything, that will make their exertions easier and improve their game or workout. Will some food, diet, or supplement optimize your performance? Old myths and newfangled misinformation abound.

True or false:

1. Protein supplements and/or a high-protein diet will build muscle and boost energy.

2. Salt tablets are necessary to replace sodium lost in sweat.

3. Athletes need significantly more vitamins and minerals.

4. Drinking cold fluids during exercise is likely to give you cramps.

5. Eating a candy bar or other sugary foods right before exercising will give you energy.

All are myths. You'll find out why below and on page 6.

What an active body needs

An obvious difference between active and sedentary people is that active individuals expend more energy, so they do need to consume more calories, the exact number depending on age, body size and composition, activity, and level of training. Exercise does theoretically increase your need for a variety of vitamins (notably the B vitamins) and minerals that are involved in energy metabolism, but if you are eating a balanced diet, these needs are generally met by the food you are eating. A diet deficient in essential nutrients can not only make you feel weak and slow you down, it can also make you sick. Some athletes, such as young gymnasts, run into trouble because they try to lose weight; others manipulate their diet in ill-advised ways and thus shortchange themselves on nutrients. However, there's no evidence that exceeding the Recommended Dietary Allowances (RDAs) by taking vitamin or mineral supplements will improve performance. In some cases, large doses of supplements can prove detrimental.

The dietary needs of athletes and other active individuals are, with a few small adjustments, not very different from those recommended for all healthy people. Here's a basic outline of a balanced diet, discussed in detail below:

Carbohydrates: 60% to 70% of daily calories.

Protein: 12% to 15% of daily calories.

Fat: less than 25% to 30% of daily calories

Vitamins: supplied by a balanced diet containing at least five fruits and vegetables a day.

Minerals: supplied by a balanced diet. But active people should pay special attention to the minerals discussed below.

Fluids: see page 6.

How much carbohydrate is enough?

The more the better--at least when it comes to complex carbohydrates (starches). Carbohydrates, in the form of glucose in the blood or glycogen in the muscles or liver, are the body's major source of energy. Your body can't store large amounts of carbohydrates. For most people, a few hours of sustained, vigorous activity will deplete the body's stored carbohydrates; when this happens, they experience weakness, fatigue, and/or pain--a condition known as "hitting the wall." The training undertaken by endurance athletes such as marathoners is specially designed to help muscles utilize energy more efficiently and thus prolong the time before exhaustion.

Many athletes have sought ways to store, or "load," elevated amounts of glycogen in the muscles and liver in order to postpone the time to exhaustion. The classic "carbohydrate loading" regimen took a week and called for depleting the body's stores of glycogen through exercise and a low-carbohydrate diet, followed by rest and a very high carbohydrate intake. Most sports physiologists now recommend a simpler version of carbohydrate loading: during the two or three days before an endurance event, simply increase your intake of foods rich in complex carbohydrates (such as bread and pasta). Either way, the improvement in performance is usually small.

Recommendation: Whether you're physically active or not, carbohydrates should provide most of your calories--60% or more. These should be mostly complex carbohydrates (starches); no more than 10% to 15% of your total calories should come from simple carbohydrates (sugars). Endurance athletes should get even more of their calories from carbohydrates.

Protein: the more, the better, too?

No. Protein is not a concern for even the most active people, since the average American gets more than enough protein. The protein needs of sedentary and active people are about the same. The exceptions, according to recent studies, are weight lifters and endurance athletes, who do appear to need more protein. But these increased needs are easily met by a balanced diet. Despite all the hype about protein among athletes and coaches, the only people who may need to worry about protein are athletes who are on low-calorie diets. Excess protein won't build muscles--only exercise will.

Recommendation: The amount of protein in the typical American diet--accounting for 12% to 15% of total daily calories--is enough for active people. It's hard not to get that much, even if you're a semi-vegetarian, since not only meat, poultry, fish, and dairy products, but also grains, nuts, and legumes are good sources. Though many athletes swear by them, protein supplements, including those containing isolated amino acids (protein's building blocks), are unnecessary. They are also expensive, and may contain questionable additives. There's little or no evidence that consuming isolated amino acids offers any advantage over the protein in food--and they may create an imbalance and be toxic.

Do athletes need more fat than others?

Not at all. It's true that your body's fat stores are its most concentrated and abundant form of energy, and that stored fat serves as the primary fuel during prolonged aerobic exercise. But even lean people have more than enough fat stores for energy production. And you don't need to eat fat to develop your body stores of fat: any carbohydrates and protein you don't use, as well as extra dietary fat, are stored as fat. A high-fat diet is not only unhealthy, but it may also impair performance, since the fat may take the place of the carbohydrates needed for high-intensity exercise.

Recommendation: For all people, fat should supply less than 25% to 30% of daily calories (with saturated fat accounting for less than 10%). Avoid fatty foods right before exercise, since it takes a long time to digest and absorb them.

Why are antioxidants important for athletes?

A number of studies, including those conducted by Lester Packer at the University of California at Berkeley, have shown that vigorous exercise increases the production of cell-damaging free radicals in the body, which may contribute to muscle soreness and inflammation after a workout. Oxygen consumption goes up dramatically during vigorous exercise, and free radicals are natural by-products of the processing of oxygen. In addition, because exercise makes you breathe faster and more deeply, it boosts your intake of ozone and other pollutants that can produce free radicals and injure cells.

Nutrients called antioxidants limit the damage done by free radicals in the body. Some promising preliminary studies have found that exercisers who take large doses of antioxidants, notably vitamins C and E, protect themselves against damage by free radicals. However, there is still no clinical evidence that such supplements enhance athletic performance.

Recommendation: Although some exercise physiologists are already recommending that athletes and other active people take large doses of supplements of vitamin C and beta carotene, it's premature to do so. But it certainly can't hurt to eat fruits and vegetables rich in these nutrients. Since it is hard to get lots of vitamin E from foods, and evidence is accumulating about its potentially protective effect against heart disease and various cancers, many scientists now recommend vitamin E supplements for everyone (see Wellness Letter, February 1993).

Which are the key minerals for athletes?

There are about 22 essential minerals, but here are seven that are important for physically active people:

Iron is vital for oxygen transport. Endurance athletes tend to have a higher incidence of iron depletion, which may impair performance. This iron shortfall (sometimes called sports anemia) has been attributed to a variety of factors, including the increased elimination of iron during prolonged exercise and the breakdown of red blood cells from, for instance, the impact of the feet on the ground. Dieters, menstruating women, and strict vegetarians are at greatest risk for an iron deficit, so if they are physically active, they need to be doubly certain that they are consuming enough iron. But don't take iron pills without consulting your doctor.

Sodium and potassium help maintain the body's water balance. Though these minerals are lost through perspiration, the losses are actually quite small and can usually be replenished by a normal diet. Salt tablets are unnecessary.

Chromium is important not only in the metabolism of carbohydrates and fat, but also in the regulation of blood sugar. Many Americans do not consume enough chromium (good sources are prunes, whole wheat, peanuts, apples, mushrooms, oysters, and wine). Because athletes may have a greater need for chromium, some have started taking chromium supplements, even though there's no scientific evidence that these can boost performance.

Zinc is also necessary for energy metabolism. Most Americans do not consume enough zinc to meet the RDA. Since physical activity leads to increased loss of zinc in sweat and urine, athletes may be especially prone to a zinc shortfall. But there's little or no evidence the zinc supplements enhance athletic performance. Good food sources include meat, eggs, seafood, and whole-wheat products.

Calcium is essential for strong bones, particularly for women, who are at risk for osteoporosis later in life.

Magnesium is crucial in muscle contractions. Though endurance athletes may become deficient in this mineral, a recent study from South Africa found that supplements had no effect on muscle activity or athletic performance.

Recommendation: If you eat a balanced diet rich in fruits, vegetables, and grains, as well as lean meats, fish, and low-fat dairy products, you'll probably have an adequate mineral intake. Athletes should make sure they eat foods that supply adequate amounts of chromium and zinc. Female athletes, in particular, should make sure they consume foods rich in iron and calcium.
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Title Annotation:includes related articles
Publication:The University of California, Berkeley Wellness Letter
Date:May 1, 1993
Words:1630
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