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Recommendations for micronutrient intake in athletes

By Asker Jeukendrup and Michael Gleeson

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.

Key Points

  • Although vitamin and mineral supplementation may improve the nutritional status of athletes consuming marginal amounts of micronutrients from food and may improve performance in athletes with deficiencies, there is no convincing evidence to indicate that doses in excess of the RDA improve performance.
  • Vitamins and minerals are needed in the body for several important processes, including the growth and repair of body tissues, as cofactors in enzyme catalyzed metabolic reactions, for oxygen transport and oxidative metabolism, for immune function, and as antioxidants. Any sustained deficiency of an essential vitamin or mineral will cause ill health and an unhealthy athlete is extremely unlikely to perform to the best of his or her potential.
  • Vitamins are organic compounds that are needed in very small quantities in the diet. They are essential for specific metabolic reactions in the body and to promote normal growth and development. With the exception of vitamin D, which can be synthesized in the presence of sunlight, and vitamin K and some B-vitamins, which can be produced by the bacterial microflora of the gastrointestinal tract, vitamins are not produced by the human body and must be consumed in the diet.
  • Although vitamins do not directly contribute to energy supply, they play an important role in regulating metabolism, acting as reusable coenzymes in intermediary metabolism. A deficiency of some of the B-group vitamins, which act as cofactors of enzymes in carbohydrate (e.g., niacin, pyridoxine [B6], and thiamin [B1]), fat (e.g., riboflavin [B2], thiamin, pantothenic acid, and biotin), and protein (pyridoxine) metabolism, results in premature fatigue and inability to maintain a heavy training program. Other vitamins play a role in red and white blood cell production (folic acid and cobalamin [B12]) or assist in the formation of bones, connective tissue, and cartilage (e.g., vitamins C and D).
  • The water-soluble vitamins, C (ascorbic acid), thiamin, riboflavin, pyridoxine, niacin, pantothenic acid, and biotin are involved in mitochondrial energy metabolism. Folic acid and vitamin B12 are mainly involved in nucleic acid synthesis and, hence, are important for maintaining healthy populations of rapidly dividing cells in the body (e.g., red blood cells, immune cells, and the gut mucosa). Vitamin C is also an antioxidant.

This is an excerpt from Sport Nutrition.

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