Creating an Iron Man - 10 essential nutrients for endurance athletes
Quite simply, athletes need more nutrients than less-active people. They demand more from their bodies than even average fitness buffs and so must compensate with the right nutrients from foods or supplements to keep performance-and recovery-at its peak.
The more intense the exercise or sport, the greater the body's nutrient needs. Athletes who participate in endurance sports--those that involve more than one hour of consistent activity-have specific needs because of what they demand from their bodies. For example, athletes lose more electrolytes-such as magnesium, potassium and sodium-through perspiration and must diligently replace them. The wear and tear of intense activity may necessitate increased intake of antioxidants such as vitamin E, which can help protect muscle cells from oxidative damage. Since muscle-tissue breakdown is common during intense exercise, athletes also need more proteins to repair the tissues.
To keep their bodies performing optimally, endurance athletes should be familiar with these 10 important nutrients.
The first seven essential supplements are the minerals calcium, iron, magnesium, potassium, selenium, sodium and zinc. Their benefits range from keeping bones strong to minimizing fatigue.
1. Calcium This may be the most important nutrient for an athlete. In a survey of more than 10,000 male and female athletes ages 7 to 50, fewer than half consumed 1,000 mg of calcium daily.1 The recommended dietary intake ranges from 1,000 to 1,500 mg/day depending on age and gender.
For female athletes, calcium intake is of particular concern. Excessive training-more than seven hours per week-may cause hormonal declines in young girls that can stop menstruation. This hormonal decline also compromises bone formation, possibly leading to premature, irreversible osteoporosis.2 Recent research shows that male endurance athletes of all ages experience testosterone deficits that also can cause osteoporosis.3
Athletes should monitor their calcium intake. Dairy foods can supply the required amounts unless sensitivities exclude them from the diet. But a diet without dairy foods requires supplements. All athletes should make sure they get 1,200 to 1,500 mg of calcium daily from food or supplements. Drinking a cup of skim milk, for example, provides about 300 mg of calcium.
2. Iron For the casual athlete who trains less than four hours per week, iron deficiency is no more of a concern than for a sedentary person. But athletes who train for six or more hours per week often have iron-deficiency anemia and should be checked yearly for the condition. Female athletes who are unable to correct such mild anemia through diet can benefit from supplements.4
Athletes use iron stores more quickly than non-athletes and, considering the neurologic effects of anemia on children and teens who engage in rigorous sports, adequate intake of iron is quite important.5 The recommended dietary allowance (RDA) for iron ranges from 10 to 15 mg/day-an amount easily acquired from food. In the absence of anemia, athletes shouldn't take any supplemental iron because it raises the risk of heart disease and colon cancer.
3. Magnesium This mineral is important in many ways. It is involved in adenosine triphosphate (ATP) production from fatty acid oxidation; ATP, present in all cells but particularly in muscle cells, stores energy. Magnesium is also involved in post-contractile muscular relaxation, bone remineralization and in phosphatidylglycerol (DPG) production, which is important to red blood cell formation. Low magnesium levels can acutely contribute to early fatigue, nausea and muscle cramps. Chronic magnesium deficiencies can lead to increased osteoporosis risk and anemia.6
Athletes lose magnesium through sweat and urine. This, combined with the fact that athletes' diets are usually low in magnesium, generally leads to the need for supplementation.7 Recommended intake for endurance athletes is 500 to 800 mg daily.8 Higher doses can cause diarrhea.
4. Potassium This mineral, present in intracellular fluid, is responsible for regulating total body water and stabilizing controlled and automatic muscle contractions. It is also lost through sweat and urine.
In a study of athletes running 40 minutes at 70o F, potassium loss was estimated at 435 mg/hour. The rate of potassium loss is approximately 200 mg/kg of weight lost during exercise.9
Cells release potassium into the bloodstream and serum levels rise with exercise, possibly instigating fatigue. Potassium supplementation after short events (less than two hours), and during and after long events, is warranted.10 For postactivity replacement, athletes should take about 435 mg/hour of exercise or 200 mg/kg of weight loss. As much as 150 mg/hour during activity can be tolerated by most athletes. Take potassium cautiously, because too much too quickly can cause cardiac arrest.
Supplementing with potassium during training does increase markers of recovery (primarily serum lactate and muscle hydration) but does not aid performance.10
5. Selenium Selenium is a free radical-scavenging nutrient that is concentrated in the lining of the GI tract and lungs, in the liver, and in skeletal muscle. It is essential to the production of antioxidant glutathione peroxidase (SeGPx). In an animal study, reducing muscular SeGPx increased cellular damage from prolonged exercise, supporting the theory that free radical-induced muscle damage causes muscle fatigue.11
Research shows selenium benefits athletes' immune function and helps repair cellular damage. Researchers studied the selenium supplementation effects on muscle SeGPx in 24 healthy nonsmoking males. Half took 240 mcg of sodium selenite; half took placebo. After cycling to exhaustion-durations ranged from 2.6 to 3.5 hours-the group that took selenium showed less cellular damage.12
Supplementation with 200 mcg of selenium is safe and warranted for endurance athletes.13
6. Sodium This element helps cells retain water and prevents dehydration. Sodium also enables the generation of ATP, which stores energy. For events lasting longer than five hours, especially in hot weather, dangerously low sodium, or hyponatremia, is a real concern. This especially applies to first-time or slower-running marathoners. Most organized events have aid stations with salty snacks. Anyone out for more than a few hours, especially on a warm day, should make sure to get some salt from snacks and fluid-replacement drinks.
A prospective study was performed on 36 athletes during a three- to four-hour triathlon and 64 athletes at an ironman race, which lasts between nine and 15 hours. No athletes were hyponatremic after the shorter race, but 27 percent were hyponatremic following the ironman. An average of 17 percent of the ironman participants required medical attention, most for hyponatremia.14
Extrapolated from that study, athletes should aim for 80 to 100 mg sodium per quart of hydrating beverage and 100 to 300 mg sodium per hour from other sources.
7. Zinc This mineral aids in post-exertion tissue repair and in the conversion of food to fuel. Both male and female athletes have lower serum zinc levels compared with sedentary individuals. Studies correlate endurance exercise with periods of compromised immunity -zinc depletion may be one reason.15
Those who train without days off lose zinc even more quickly. In a study of cyclists, researchers looked at zinc excretion via sweat. Half of the group underwent intense training for two months. Half underwent moderate training with two to three days off per week. Both groups were studied before and after. The exercising group showed increased zinc excretion while the control group showed no increase.16 The researchers believe altered zinc metabolism coupled with increased zinc excretion and stress levels lead to fatigue and decreased endurance.
Athletes should take 30 to 60 mg zinc daily.17 Zinc picolinate or monomethionate are most easily tolerated.18
Antioxidants to the rescue
Antioxidants are another set of nutrients that endurance athletes are wise to use.
8. Vitamin E For athletes, one of the most important antioxidants is vitamin E. Aerobic athletes may have an increased need for this vitamin because their cells undergo more oxidative damage. Research shows athletes have less cellular damage when they ingest more vitamin E.19 Aerobic exercise places additional demands on the molecular free radical scavengers of the body, and vitamin E is a well-known scavenger.
In a study of 30 top-class cyclists, five months of supplementation with natural vitamin E (alpha-tocopherol) at an 800-IU daily dose significantly decreased markers of oxidative damage to muscle tissue. However, vitamin E did not benefit athletic performance.
Studies evaluating vitamin E as an ergogenic, or performance aid, show no benefit.19 One possible exception is at higher altitudes where oxidative stress is more intense. A group of six mountain climbers took 400 mg synthetic vitamin E (dl-alpha-tocopherol acetate). During exertion at altitude, they showed less output of pentane and lactic acid-both markers of oxidative damage, but not suggestive of improved athletic performance. The athletes also showed a statistically significant increase in anaerobic threshold compared to a placebo group.20
The amount of vitamin E necessary to benefit athletes is not obtainable through diet. The jury is still out on natural vs. synthetic vitamin E, but endurance athletes should take 400 to 800 IU/day.
Protein and Glutamine
Without adequate protein and glutamine, athletes can feel the effects of reduced metabolism, poorer recovery times and increased susceptibility to infections.
9. Protein The RDA for protein is 60 mg per day for adults (specifically 0.8 g/kg of body weight/day). This recommendation, however, is based on the needs of sedentary individuals. Recent studies indicate that protein needs increase during strenuous activity, which applies to both strength and endurance athletes.21
Endurance athletes need more protein for different reasons than strength athletes do. Endurance athletes primarily use protein for maintaining aerobic metabolism, compared with the increased tissue-repair needs of strength athletes. When intake is inadequate, the body sequesters the needed proteins from lean tissue, which gives overtrained endurance athletes a gaunt appearance. A protein deficit also impairs an athlete's recovery and wound-healing ability.14
Researchers recommend endurance athletes eat 1.2 to 1.4 g/kg of body weight/day of protein.22 For a 155-pound athlete, this means a total of 85 to 100 g protein per day. Only a few studies recommend protein intake levels as high as 2 g/kg of body weight/day.23
10. Glutamine This amino acid increases the numbers of lymphocytes and macrophages, which contribute greatly to immune function. Prolonged exercise consistently lowers glutamine levels., and when glutamine levels are low these immune cells show depressed activity.24 Glutamine supplementation reduces vulnerability to infections after prolonged exercise, though a few studies examining this phenomenon at lower exercise intensity levels have not shown benefit.25
Oral glutamine replacement after exercise can lower infection risk. In one study, 200 runners and rowers were given placebo or 2,000 mg glutamine two hours after exercise. In the seven days following the exercise, 81 percent of the glutamine-supplemented group were infection-free compared to 49 percent in the placebo group.26
A supplement that provides 2 g glutamine daily is a wise choice for athletes in training.26
Athletes who train strenuously have greater nutritional needs than less active people. Adequate nutrients can mean quicker recovery time, lower infection rates, less fatigue, and ultimately, can help athletes reach their desired performance levels.
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2. Voss LA, et al. Exercise-induced loss of bone density in athletes. J Am Acad Orthop Surg 1998 Nov-Dec;6(6):349-57.
3. Bennell KL, et al. Effect of altered reproductive function and lowered testosterone levels on bone density in male endurance athletes. Br J Sports Med 1996 Sep;30(3):205-8.
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5. Weaver CM, et al. Exercise and iron status. J Nutr 1992 Mar;122(3 Suppl):782-7.
6. Altura BM, et al. Magnesium depletion impairs myocardial carbohydrate and lipid metabolism and cardiac bioenergetics and raises myocardial calcium content in-vivo: relationship to etiology of cardiac diseases. Biochem Mol Biol Int 1996 Dec;40(6):1183-90.
7. Lukaski HC, et al. Micronutrients (magnesium, zinc, and copper): are mineral supplements needed for athletes? Int J Sport Nutr, 1995;5 Suppl:S74-83.
8. Seelig M. Magnesium deficiency in the pathogenesis of disease. New York: Plenum Press; 1980.
9. Wenk C, et al. Methodological studies of the estimation of loss of sodium, potassium, calcium and magnesium through the skin during a 10 km run. Z Ernahrungswiss 1993 Dec;(4):301-7.
10. Tarnopolsky MA, et al. Mixed carbohydrate supplementation increases carbohydrate oxidation and endurance exercise performance and attenuates potassium accumulation. Int J Sport Nutr 1996 Dec;(4):323-36.
11. Venditti P. Effect of training on antioxidant capacity, tissue damage, and endurance of adult male rats. Int J Sports Med 1997 Oct;18(7):497-502.
12. Tessier F, et al. Muscle GSH-Px activity after prolonged exercise, training, and selenium supplementation. Biol Trace Elem Res, 1995 Jan-Mar;47(1-3):279-85.
13. Persson-Moschos M, et al. Plasma selenoprotein P levels of healthy males in different selenium status after oral supplementation with different forms of selenium. Eur J Clin Nutr 1998 May;52(5):363-7.
14. Hiller WD, et al. Medical and physiological considerations in triathlons. Am J Sports Med 1987 Mar;(2):164-7.
15. Cordova A. Behaviour of zinc in physical exercise: a special reference to immunity and fatigue. Neurosci Biobehav Rev 1995 Fall;19(3):439-45.
16. Cordova A, et al. Effect of training on zinc metabolism: changes in serum and sweat zinc concentrations in sportsmen. Ann Nutr Metab 1998;42(5):274-82.
17. Barrie SA, et al. Comparative absorption of zinc picolinate, zinc citrate and zinc gluconate in humans. Agents Actions 1987;21(1-2):223-8.
18. Rohde T, et al. Effect of glutamine supplementation on changes in the immune system induced by repeated exercise. Med Sci Sports Exerc 1998 Jun;30(6):856-62.
19. Rokitzki L, et al. Alpha-tocopherol supplementation in racing cyclists during extreme endurance training. Int J Sport Nutr 1994 Sep;4(3):253-64.
20. Simon-Schnass I, et al. Influence of vitamin E on physical performance. Int J Vitam Nutr Res 1988;58(1):49-54.
21. Lemon PW, et al. Do athletes need more dietary protein and amino acids? Int J Sport Nutr 1995 Jun;5 Suppl:S39-61.
22. Shephard, RJ, et al. Immunological hazards from nutritional imbalance in athletes. Exerc Immunol Rev 1998;4:22-48.
23. Rohde T, et al. The immune system and serum glutamine during a triathlon. Eur J Appl Physiol 1996;74(5):428-34.
24. Newsholme EA, et al. The proposed role of glutamine in some cells of the immune system and speculative consequences for the whole animal. Nutrition 1997 Jul-Aug; 13(7-8):728-30.
25. Rohde T, et al. Effect of glutamine supplementation on changes in the immune system induced by repeated exercise. Med Sci Sports Exerc 1998 Jun;30(6):856-62.
26. Castell LM, et al. Does glutamine have a role in reducing infections in athletes? Eur J Appl Physiol 1996;73(5):488-90.
Alan Christianson, N.D., has a naturopathic private practice in Scottsdale, Ariz.