In general, supplementation with individual vitamins, including the consumption of large doses of simple antioxidant mixtures is not recommended. Consuming megadoses of individual vitamins (common among athletes) is likely to do more harm than good. Because most vitamins function mainly as coenzymes in the body, once these enzyme systems are saturated, the vitamin in free form can become toxic. For example, very large doses of vitamin C are associated with urinary stone formation and impaired copper absorption, and megadoses of vitamin B6 can cause sensory neuropathy. Excess intakes of vitamin A by pregnant women can cause birth defects. Athletes should obtain complex mixtures of antioxidant compounds from increased consumption of fruits and vegetables.
Vitamin supplements are not necessary for athletes eating a well-balanced diet. Many athletes are, however, concerned about ensuring adequate intakes of vitamins, without the risks of oversupplementation. A daily intake of a low-dose vitamin preparation, supplying not more than the RDA, provides an adequate and safe level of vitamin intake, especially during periods of intensive training and carbohydrate loading before competition. Similarly, most athletes do not require mineral supplements, because their diet is already more than adequate to meet any increased requirements resulting from the effects of regular intensive exercise.
However, particular groups of athletes are at risk for marginal mineral intake. These athletes compete in sports events in which a low body weight is essential for success (e.g., gymnasts and dancers) or compete within certain body-weight categories (e.g., boxers, wrestlers, and weight lifters). Participants in such sports often train frequently and intensively but consume low-energy diets or undergo drastic weight-loss regimens to maintain or lose body weight before competition. The low-energy intakes (<8 MJ/day [<1900 kcal/day]) in these situations are likely to lead to an inadequate intake of essential minerals (and vitamins). Because many athletes are very young and still in a period of body growth and development, they can be detrimentally affected by micronutrient deficiencies. Specific recommendations can be given to athletes to ensure an adequate calcium intake when on an energy-restricted diet:
• Include three servings a day of low-fat dairy foods.
• Include these dairy foods in high-carbohydrate meals (e.g., skimmed milk on cereal).
• Eat fish with bones (e.g., sardines).
• Include calcium-enriched soy products.
• Eat leafy green vegetables (e.g., cabbage, broccoli, spinach).
Dietary recommendations can also be given to athletes to increase available iron intake on a high-carbohydrate diet:
• Eat foods rich in heme iron at least 4 times a week (e.g., liver or lean red meat).
• Eat iron-fortified foods (e.g., breakfast cereal).
• Include non-heme iron food sources (e.g., dried fruit, legumes, and green leafy vegetables).
• Combine nonheme-iron foods with meat or vitamin C-rich food (e.g., orange juice) to increase iron absorption.
• Avoid drinking tea at meals.
Other athletes who are at risk for marginal mineral intake are those who abstain from normal diets (i.e., consume extremely unbalanced diets with a low micronutrient density) and vegetarian athletes. Hence, micronutrient supplementation is recommended for such athletes.
Amenorrheic female athletes should certainly take calcium supplements, and other female athletes should consider taking calcium supplements, to ensure adequate calcium status and maintain healthy bones. Moderately elevated intakes of calcium do not appear to be harmful, possibly because the blood calcium concentration is under tight hormonal regulation, and moderate excesses can be excreted in the urine. Athletes who train and compete in hot environments should also consider increasing their intake of minerals (particularly iron, zinc, and magnesium) because mineral losses in sweat can be considerable. Even so, daily supplements of these minerals should not exceed 1 to 2 times the RDA. As with vitamins, excessive intakes of minerals can be toxic and can impair the absorption of other essential trace elements.
Poor diets are the main reason for any micronutrient deficiencies found in athletes, although, in certain cases, regular strenuous exercise contributes to the deficiency. With the possible exceptions of iron and calcium, micronutrient deficiencies can easily be corrected by eating a well-balanced diet. An inadequate knowledge of proper dietary practices, lack of time for food preparation, misleading advertisements for micronutrient supplements, and a lack of qualified dietary advice are possible reasons for suboptimal micronutrient intakes in athletes. Few studies have definitively documented beneficial effects of mineral or vitamin supplementation on exercise performance, except where supplementation was needed to correct an existing deficiency. Athletes who take micronutrient supplements are, for the most part, taking them to ensure good health, not to enhance sports performance. However, an unhealthy athlete is unlikely to perform to the best of his or her potential.