Vitamins and Minerals
Micronutrients play an important role in energy production, hemoglobin synthesis, maintenance of bone health, adequate immune function, and protection of body against oxidative damage. They assist with synthesis and repair of muscle tissue during recovery from exercise and injury. Exercise stresses many of the metabolic pathways where micronutrients are required, and exercise training may result in muscle biochemical adaptations that increase micronutrient needs. Routine exercise may also increase the turnover and loss of these micronutrients from the body. As a result, greater intakes of micronutrients may be required to cover increased needs for building, repair, and maintenance of lean body mass in athletes.
The most common vitamins and minerals found to be of concern in athletes' diets are calcium and vitamin D, the B vitamins, iron, zinc, magnesium, as well as some antioxidants such as vitamins C and E, β-carotene, and selenium.[46,47,48,49,50] Athletes at greatest risk for poor micronutrient status are those who restrict energy intake or have severe weight-loss practices, who eliminate one or more of the food groups from their diet, or who consume unbalanced and low micronutrient-dense diets. These athletes may benefit from a daily multivitamin-and-mineral supplement. Use of vitamin and mineral supplements does not improve performance in individuals consuming nutritionally adequate diets.[46,47,48,50]
B Vitamins:Thiamin, Riboflavin, Niacin, Vitamin B6, Pantothenic Acid, Biotin, Folate, Vitamin B12
Adequate intake of B vitamins is important to ensure optimum energy production and the building and repair of muscle tissue.[48,51] The B-complex vitamins have two major functions directly related to exercise. Thiamin, riboflavin, niacin, pyridoxine (B6), pantothenic acid, and biotin are involved in energy production during exercise,[46,51] whereas folate and vitamin B12 are required for the production of red blood cells, for protein synthesis, and in tissue repair and maintenance including the CNS. Of the B vitamins, riboflavin, pyridoxine, folate, and vitamin B12 are frequently low in female athletes' diets, especially those who are vegetarian or have disordered eating patterns.[47,48]
Limited research has been conducted to examine whether exercise increases the need for the B-complex vitamins.[46,48] Some data suggest that exercise may slightly increase the need for these vitamins as much as twice the current recommended amount; however, these increased needs can generally be met with higher energy intakes. Although short-term marginal deficiencies of B vitamins have not been observed to impact performance, severe deficiency of vitamin B12, folate, or both may result in anemia and reduced endurance performance.[46,47,52] Therefore, it is important that athletes consume adequate amounts of these micronutrients to support their efforts for optimal performance and health.
Vitamin D is required for adequate calcium absorption, regulation of serum calcium and phosphorus levels, and promotion of bone health. Vitamin D also regulates the development and homeostasis of the nervous system and skeletal muscle.[53,54,55] Athletes who live at northern latitudes or who train primarily indoors throughout the year, such as gymnasts and figure skaters, are at risk for poor vitamin D status, especially if they do not consume foods fortified with vitamin D.[50,56,57] These athletes would benefit from supplementation with vitamin D at the DRI level (5 μg·d−1 or 200 IU for ages 19-49 yr).[54,56,58,59,60,61] A growing number of experts advocate that the RDA for vitamin D is not adequate.[53,62,63]
Antioxidants: Vitamins C and E, β-Carotene, and Selenium
The antioxidant nutrients, vitamins C and E, β-carotene, and selenium, play important roles in protecting cell membranes from oxidative damage. Because exercise can increase oxygen consumption by 10- to 15-fold, it has been hypothesized that long-term exercise produces a constant "oxidative stress" on the muscles and other cells leading to lipid peroxidation of membranes. Although short-term exercise may increase levels of lipid peroxide by-products, habitual exercise has been shown to result in an augmented antioxidant system and reduced lipid peroxidation.[50,65] Thus, a well-trained athlete may have a more developed endogenous antioxidant system than a sedentary person. Whether exercise increases the need for antioxidant nutrients remains controversial. There is little evidence that antioxidant supplements enhance physical performance.[49,50,64,66] Athletes at greatest risk for poor antioxidant intakes are those following a low-fat diet, restricting energy intakes, or limiting dietary intakes of fruits, vegetables, and whole grains.[29,66]
The evidence that a combination of antioxidants or single antioxidants such as vitamin E may be helpful in reducing inflammation and muscle soreness during recovery from intense exercise remains unclear.[42,67] Although the ergogenic potential of vitamin E concerning physical performance has not been clearly documented, endurance athletes may have a higher need for this vitamin. Indeed, vitamin E supplementation has been shown to reduce lipidperoxidation during aerobic/endurance exercise and have a limited effect with strength training. There is some evidence that vitamin E may attenuate exercise-induced DNA damage and enhance recovery in certain active individuals; however, more research is needed. Athletes should be advised not to exceed the tolerable upper intake levels (UL) for antioxidants because higher doses could be pro-oxidative with potential negative effects.[46,64,68]
Vitamin C supplements do not seem to have an ergogenic effect if the diet provides adequate amounts of this nutrient. Because strenuous and prolonged exercise has been shown to increase the need for vitamin C, physical performance can be compromised with marginal vitamin C status or deficiency. Athletes who participate in habitual prolonged, strenuous exercise should consume 100-1000 mg of vitamin C daily.[47,69,70]
Minerals: Calcium, Iron, Zinc, and Magnesium
The primary minerals low in the diets of athletes, especially female athletes, are calcium, iron, zinc, and magnesium. Low intakes of these minerals are often due to energy restriction or avoidance of animal products.
Calcium. Calcium is especially important for growth, maintenance and repair of bone tissue, maintenance of blood calcium levels, regulation of muscle contraction, nerve conduction, and normal blood clotting. Inadequate dietary calcium and vitamin D increase the risk of low bone mineral density and stress fractures. Female athletes are at greatest risk for low bone mineral density if energy intakes are low, dairy products and other calcium-rich foods are inadequate or eliminated from the diet, and menstrual dysfunction is present.[47,52,55,71,72,73]
Supplementation with calcium and vitamin D should be determined after nutrition assessment. Current recommendations for athletes with disordered eating, amenorrhea, and risk for early osteoporosis are 1500 mg of elemental calcium and 400-800 IU of vitamin D per day.[50,72,73]
Iron. Iron is required for the formation of oxygen-carrying proteins, hemoglobin and myoglobin, and for enzymes involved in energy production.[50,74] Oxygen-carrying capacity is essential for endurance exercise as well as normal function of the nervous, behavioral, and immune systems.[64,74] Iron depletion (low iron stores) is one of the most prevalent nutrient deficiencies observed among athletes, especially females. Iron deficiency, with or without anemia, can impair muscle function and limit work capacity.[47,58,75,76] Iron requirements for endurance athletes, especially distance runners, are increased by approximately 70%.[58,74] Athletes who are vegetarian or regular blood donors should aim for an iron intake greater than their respective RDA (i.e., >18 mg and >8 mg, for men and women respectively).
The high incidence of iron depletion among athletes is usually attributed to inadequate energy intake. Other factors that can impact iron status include vegetarian diets that have poor iron availability, periods of rapid growth, training at high altitudes, increased iron losses in sweat, feces, urine, menstrual blood, intravascular hemolysis, foot-strike hemolysis, regular blood donation, or injury.[50,75,77] Athletes, especially women, long-distance runners, adolescents, and vegetarians should be screened periodically to assess and monitor iron status.[75,77,78]
Because reversing iron deficiency anemia can require 3-6 months, it is advantageous to begin nutrition intervention before iron deficiency anemia develops.[47,75] Although depleted iron stores (low serum ferritin) are more prevalent in female athletes, the incidence of iron deficiency anemia in athletes is similar to that of the nonathlete female population.[50,75,77] Chronic iron deficiency, with or without anemia, that results from consistently poor iron intake can negatively impact health, physical, and mental performance and warrants prompt medical intervention and monitoring.[76,78]
Some athletes may experience a transient decrease in serum ferritin and hemoglobin at the initiation of training due to hemodilution after an increase in plasma volume known as "dilutional" or "sports anemia" and may not respond to nutrition intervention. These changes seem to be a beneficial adaptation to aerobic training, which do not negatively impact performance.
In athletes who are iron-deficient, iron supplementation not only improves blood biochemical measures and iron status but also increases work capacity as evidenced by increasing oxygen uptake, reducing heart rate, and decreasing lactate concentration during exercise. There is some evidence that athletes who are iron-deficient but do not have anemia may benefit from iron supplementation.[50,75] Recent findings provide additional support for improved performance (i.e., less skeletal muscle fatigue) when iron supplementation was prescribed as 100-mg ferrous sulfate for 4-6 wk. Improving work capacity and endurance, increasing oxygen uptake, reducing lactate concentrations, and reducing muscle fatigue are benefits of improved iron status.
Zinc. Zinc plays a role in growth, building and repair of muscle tissue, energy production, and immune status. Diets low in animal protein, high in fiber and vegetarian diets, in particular, are associated with decreased zinc intake.[50,52] Zinc status has been shown to directly affect thyroid hormone levels, BMR, and protein use, which in turn can negatively affect health and physical performance.
Survey data indicate that a large number of North Americans have zinc intakes below recommended levels.[74,75,79] Athletes, particularly females, are also at risk for zinc deficiency. The impact of low zinc intakes on zinc status is difficult to measure because clear assessment criteria have not been established and plasma zinc concentrations may not reflect changes in whole-body zinc status.[47,79] Decreases in cardiorespiratory function, muscle strength, and endurance have been noted with poor zinc status. The UL for zinc is 40 mg. Athletes should be cautioned against single-dose zinc supplements because they often exceed this amount, and unnecessary zinc supplementation may lead to low HDL cholesterol and nutrient imbalances by interfering with absorption of other nutrients such as iron and copper. Further, the benefits of zinc supplementation to physical performance have not been established.
Magnesium. Magnesium plays a variety of roles in cellular metabolism (glycolysis, fat, and protein metabolism) and regulates membrane stability and neuromuscular, cardiovascular, immune, and hormonal functions.[47,55] Magnesium deficiency impairs endurance performance by increasing oxygen requirements to complete submaximal exercise. Athletes in weight-class and body-conscious sports, such as wrestling, ballet, gymnastics, and tennis, have been reported to consume inadequate dietary magnesium. Athletes should be educated about good food sources of magnesium. In athletes with low magnesium status, supplementation might be beneficial.
Sodium, Chloride, and Potassium Sodium is a critical electrolyte, particularly for athletes with high sweat losses.[80,81,82,83] Many endurance athletes will require much more than the UL for sodium (2.3 g·d−1) and chloride (3.6 g·d−1). Sports drinks containing sodium (0.5-0.7 g·L−1) and potassium (0.8-2.0 g·L−1), as well as carbohydrate, are recommended for athletes especially in endurance events (>2 h).[50,80,82,83]
Potassium is important for fluid and electrolyte balance, nerve transmission, and active transport mechanisms. During intense exercise, plasma potassium concentrations tend to decline to a lesser degree than sodium. A diet rich in a variety of fresh vegetables, fruits, nuts/seeds, dairy foods, lean meats, and whole grains is usually considered adequate for maintaining normal potassium status among athletes.[32,83]
The authors thank the reviewers for their many constructive comments and suggestions. The reviewers were not asked to endorse this position or the supporting paper.
Cite this: Nutrition and Athletic Performance - Medscape - Mar 01, 2009.