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Ergogenic Aids: Section Articles

Minerals as Ergogenic Aids

Volpe, Stella Lucia

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Current Sports Medicine Reports: July 2008 - Volume 7 - Issue 4 - p 224-229
doi: 10.1249/JSR.0b013e31817ed0e2
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Abstract

INTRODUCTION

Chromium, magnesium, zinc, and selenium are essential minerals and have roles for more than 300 metabolic reactions in the body. In general, these include their roles as antioxidants, in protein synthesis, in adenylate cyclase synthesis, in cellular energy production and storage, in glucose metabolism, in the preservation of cellular electrolyte composition, in cell growth and reproduction, in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) synthesis, and in the stabilization of mitochondrial membranes (1-7).

DIETARY REFERENCE INTAKES: A BRIEF OVERVIEW

The dietary reference intakes (DRI) for all known essential minerals for healthy people living in the United States were updated between 1997 and 2005 (8-12). Note that adequate intake (AI), recommended dietary allowance (RDA), estimated average requirement (EAR), and tolerable upper intake level (UL) are all under the DRI heading. The RDA is the dietary intake level that is sufficient for approximately 98% of healthy people living in the United States. The AI is a projected value that is used when the RDA cannot be established. The EAR is a value used to estimate the nutrient requirements of half of the healthy people in a group (8-12). The UL is the maximum quantity of a nutrient most persons can consume without adverse side effects (8-12). The DRIs for all nutrients may be found at the following Web site: www.iom.edu/Object.File/Master/21/372/0.pdf (accessed April 28, 2008).

In most cases, if energy intakes are sufficient, the mineral needs of athletes are analogous to healthy individuals; therefore, using the DRI for evaluating nutrient needs would be suitable. Some athletes, however, may have greater requirements as a consequence of disproportionate losses of nutrients in sweat and urine. For these athletes, supplementation may need to be considered on an individual basis to maintain normal body stores, not for ergogenic purposes. Many athletes, however, supplement their diets with minerals hoping they may provide an ergogenic effect to their athletic performance. Data on the ergogenic effects of mineral supplementation are equivocal. Fortunately, the UL has been established so that a person could consume of a given nutrient and avoid toxic effects.

ACCURATE DIETARY INTAKE ASSESSMENT

Although this review focuses on chromium, magnesium, zinc, and selenium, a total evaluation of an athlete's energy intake is essential. Although an athlete may consume the acceptable amount of micronutrients (with or without supplementation), if total energy (kilocalorie) needs are not being met, athletic performance will still be suboptimal. Clark et al. (13) examined the pre- and post-season intakes of macronutrients and micronutrients in Division I female soccer players. They report that, in spite of meeting total energy requirements, carbohydrate, vitamin E, folate, copper, and magnesium intakes were suboptimal (<75% of the DRI).

Further, the actual evaluation of an athlete's diet must be conducted properly to ensure accuracy (14). It is common for any individual to underreport dietary intake. Therefore, it is crucial that athletes are educated on how to precisely approximate portion sizes and fluid intake, the amount and frequency of snacking, any weight management practices they may perform, and alterations in their food patterns during and off seasons (14).

CHROMIUM AS AN ERGOGENIC AID

Chromium is a required trace mineral best known for its function in potentiating the effects of insulin. The DRI for chromium, established as an AI, is 25-35 µg·d−1 for adult women and men, respectively (11). Good food sources of chromium can be found in the Table.

T1-12
TABLE:
Good food sources of chromium, magnesium, zinc, and selenium.

For athletes who are in weight-monitored sports, such as wrestling, gymnastics, and rowing, chromium's use as a possible weight loss supplement has helped it gain popularity. Volpe et al. (16) assessed the effects of chromium picolinate supplementation in sedentary, overweight women to ascertain whether chromium truly had a weight loss effect. They performed a double-blind study in moderately obese women on body weight, body composition, and resting metabolic rate (RMR). They found that the supplementation of 400 µg·d−1 of chromium picolinate did not result in changes in weight loss, body composition (as assessed by under water weighing), or RMR (as assessed by indirect calorimetry) compared with women receiving an identical-looking placebo. This study was conducted in 44 women who had body mass indexes (BMI) from 27 to 41 and ranged in age from 27 to 51 yr. Other researchers also have reported no effect of chromium supplementation upon body weight or body composition in athletes (wrestlers) and non-athletes (17,18).

Although chromium supplementation may not result in weight loss, could it provide an ergogenic effect in athletes or sedentary individuals with respect to its role in enhancing insulin signaling and insulin-mediated glucose uptake? Volek et al. (19) evaluated the effects of 600 µg·d−1 of chromium picolinate in 16 overweight men (with an average BMI of 31.1 ± 3.0) who were assigned randomly to either the chromium group or a placebo. The participants were supplemented (or given the placebo) for 4 wk, and after that time, they performed a supramaximal bout of cycling ergometry to deplete their stores of muscle glycogen. After this supramaximal exercise bout, the researchers fed the participants high glycemic index carbohydrates for the following 24 h. The participants then had muscle biopsies taken at rest, immediately post-exercise, and both 2 and 24 h post-exercise. Volek et al. (19) report no differences in glucose or insulin concentrations between the groups. They concluded that 4 wk of chromium supplementation did not improve glycogen synthesis during recovery after high-intensity exercise followed by a high-glycemic index feeding.

Crawford et al. (20) evaluated the effects of 600 µg of niacin-bound chromium on the body composition of 20 overweight African-American women. This was a cross-over design study, whereby participants in one group received a placebo for 2 months followed by chromium supplementation for an additional 2 months, while the other group received the chromium supplementation first, followed by the placebo. They reported that the supplement resulted in significantly more fat loss, while concomitantly preserving lean body mass. Although the former group did lose more body fat and retain more lean body mass while taking the chromium supplement compared with the placebo, the group that took the chromium supplement first, followed by the placebo, lost more body fat and retained more lean body mass while on the placebo versus the chromium. All study participants were placed on a diet and exercise program during the study, suggesting that the resulting improvements could have stemmed from their dietary and exercise programs rather than the chromium supplementation.

In a larger study of 158 moderately obese subjects, the addition of a multi-mineral supplement, including chromium picolinate, significantly increased the rate of fat loss while maintaining lean body mass compared with a placebo (21). This study also failed to demonstrate any specific effects of chromium picolinate because the supplement contained several other minerals that could have resulted in the significant difference. Thus, although the sample size was fairly large, it is difficult to draw solid conclusions from this study regarding the contribution that chromium picolinate made to the results.

Based upon the data presented from previous studies, it does not appear that chromium (picolinate or nicotinate) has ergogenic effects upon exercise performance, especially with regard to weight loss or increases in lean body mass.

MAGNESIUM AS AN ERGOGENIC AID

The DRI for magnesium for adults is 310-420 µg·d−1. However, magnesium intake is often below these recommendations, especially as people age (8). Good food sources of magnesium can be found in the Table. Decreased magnesium intake has been related to an increased risk of metabolic syndrome and type II diabetes mellitus (22,23). In addition, stressors such as type II diabetes mellitus and exercise may deplete magnesium, which, together with a suboptimal dietary magnesium intake, may negatively impact normal metabolism and physical performance (1).

Cinar et al. (24) conducted a 4-wk supplementation study to assess the effects of magnesium supplementation in exercising and sedentary individuals and compared this with a control group. Thirty healthy participants, 18-22 yr, were divided into the following three groups: untrained participants given 10 mg·kg−1·d−1 of magnesium, participants who trained in tae kwon do for 90-120 min per day for 5 d per week and were given 10 mg·kg−1·d−1 of magnesium, and a non-supplemented group who exercised the same amount of time as the second group. Subjects in the supplemented groups had significant increases in erythrocyte and hemoglobin concentrations, concluding that magnesium would then improve exercise performance. This conclusion is too strong based on the findings, and more research needs to be conducted over a longer period of time, assessing both biochemical markers and exercise performance, to fully ascertain the impact magnesium has as an ergogenic aid. For example, Finstad et al. (25) reported that 4 wk of 212 mg·d−1 of magnesium oxide improved ionized magnesium concentrations. However, exercise performance and recovery were not improved in physically active women (magnesium versus placebo). Further, in a meta-analysis, Newhouse and Finstad (26) reported no effect of magnesium supplementation upon exercise performance.

Perhaps magnesium may be more effective in individuals with heart disease who exercise. Pokan and colleagues (27) studied individuals with coronary heart disease based upon previous research showing that magnesium supplementation plays a role in and may improve endothelial function, exercise tolerance, and exercise-induced chest pain (28) in patients with coronary artery disease. In a double-blind, placebo-controlled trial, Pokan et al. (27) randomly assigned 53 male participants with stable coronary artery disease to either an oral magnesium (15 mmol twice per day [72 mg mg·d−1]) (N = 28, 61 ± 9 yr) or a placebo (N = 25, 58 ± 10 yr) for 6 months. Compared with the placebo, 6 months of magnesium supplementation significantly increased intracellular magnesium levels, maximal oxygen consumption, and left ventricular heart function (as measured by ejection fraction). Based upon these data, the authors concluded that magnesium supplementation is effective in improving exercise performance and heart function in stable men with coronary artery disease. A longer supplementation trial should be conducted to verify these results.

A number of researchers have attested to the benefits of increased dietary magnesium intake and/or magnesium supplementation on the metabolic syndrome, insulin resistance, and type II diabetes mellitus (29-34). Although prevention of type II diabetes mellitus is extremely important, treating it is equally as important. Rodríguez-Morán and Guerrero-Romero (35) evaluated whether oral magnesium supplementation (50 mL of a magnesium chloride solution, containing 50 g of magnesium chloride per 1000 mL of solution) improved insulin sensitivity as well as metabolic control in individuals with both type II diabetes mellitus and decreased serum magnesium concentrations (≤0.74 mmol·L−1 [<1.8 mg·dL−1]). Sixty-three participants qualified for this 16-wk, randomized, double-blind, placebo-controlled trial. To assess insulin sensitivity, homeostasis model analysis for insulin resistance (HOMA-IR) was used, while glucose concentrations and glycosylated hemoglobin (HbA1c) were used to evaluate metabolic control. At 16 wk, the individuals who received magnesium supplementation had a significantly greater serum magnesium concentration than control subjects (0.74 ± 0.10 [1.8 ± 0.24 mg·dL−1] versus 0.65 ± 0.07 mmol·L−1 [1.58 ± 0.17], P = 0.02). Those who were supplemented also significantly improved insulin sensitivity and metabolic control, based upon a lower HOMA-IR index, lower fasting blood glucose levels, and lower HbA1c compared with control participants.

Although short-term, this study clearly indicates the impact of magnesium supplementation on managing type II diabetes mellitus. Nonetheless, although this and other cited studies offer positive results of magnesium supplementation on the metabolic syndrome, insulin resistance, and type II diabetes mellitus, it appears that an individual must have hypomagnesemia for supplementation to be effective. This could be the reason for a lack of ergogenic effects of magnesium in many athletes; if they begin with normal serum magnesium levels, providing more through supplementation will not improve status, and hence, will not impact performance.

ZINC AS AN ERGOGENIC AID

The DRI for zinc, established as an RDA, ranges from 8 to 11 mg·d−1 for adult women and men, respectively (11). Good food sources of zinc are listed in the Table. Plasma zinc concentrations have been shown to decline with acute stress such as exercise. This decrease is thought to be related to an increased uptake of zinc by the liver and bone marrow for synthesis of acute phase proteins. Strenuous exercise induces an acute phase response; however, previous researchers have reported both declines and increases in plasma zinc levels after strenuous exercise (36-41). The disparity in results may have differed due to variations in the timing of blood draws, fitness status of subjects, exercise intensity, type, and duration, as well as zinc status. To date, no one has used stable isotopes of zinc to assess the kinetics of zinc metabolism after an exhaustive exercise bout. Therefore, Volpe et al. (42) evaluated short-term changes in zinc kinetics, using the stable isotope Zn70, to define the kinetics of zinc metabolism after an acute, strenuous bout of exercise in sedentary men. They performed a cross-over design study in 12 healthy, sedentary men, 25-35 yr. Zn70 was infused 10 min after an exhaustive exercise bout (cycle ergometry) or at rest. Plasma zinc concentrations significantly decreased after exercise, with a mean nadir of 13.9% + 4.1% observed at 70 min post-exercise. Based on their Zn70 data, Volpe et al. (42) reported increases in the size of the rapidly exchangeable plasma zinc pool and the liver zinc pool, representing a shift of plasma zinc into the interstitial fluid and liver after exercise, possibly mirroring the acute stress response of strenuous exercise.

One of zinc's many functions is its role in the conversion of thyroxine (T4) to the more active triiodothyronine (T3). T3 is involved with the body's overall metabolism, and hence, it could impact exercise performance and overall health. Maxwell and Volpe (43) assessed the effects of zinc supplementation upon plasma zinc, serum ferritin, plasma T3 and T4, serum free T3 and T4, thyroid-stimulating hormone (TSH) concentrations, and RMR in two zinc-deficient, physically active women who were supplemented with 26.4 mg·d1 of zinc (as zinc gluconate) for 4 months. They reported that zinc deficiency was clinically corrected in both subjects, while serum ferritin concentration declined to classify both subjects as borderline iron deficient. At 4 months, total T3 concentrations increased in one participant, while all thyroid hormone concentrations increased in the other. RMR increased in both participants at 4 months. Because this was a case study, statistical analyses could not be conducted; however, zinc supplementation appeared to be directly responsible for the increase in plasma zinc and decline in serum ferritin concentrations in both subjects and to have a favorable effect upon total T3 concentrations and RMR.

In a similar study, Kilic (44) evaluated how exercise affects thyroid hormone and testosterone levels in sedentary men receiving oral zinc for 4 wk. Ten sedentary men (19.47 ± 1.7 yr) received 3 mg·kg−1·d−1 zinc sulfate for 4 wk. Thyroid hormone and testosterone concentrations were assessed at rest and after bicycle ergometry before and after zinc supplementation. He reported that exercise decreased thyroid hormone and testosterone concentrations in sedentary men; however, zinc supplementation seemed to prevent the decrease in these hormones. Kilic concluded that "administration of a physiologic dose of zinc can be beneficial to performance." Because both Kilic's and Maxwell and Volpe's studies consist of such few participants, longer-term studies with more participants are required to elucidate the true benefits of zinc supplementation upon thyroid hormone status and exercise performance.

SELENIUM AS AN ERGOGENIC AID

Selenium is best known for its role as an antioxidant. The DRI for selenium is 55 µg·d−1 for males and females 14 yr and older (10). See the Table for good food sources of selenium. Because of the role selenium plays in the antioxidant system, it would seem that it could be an ergogenic aid because of the increased free radical production that occurs with exercise.

Margaritis et al. (45) evaluated the impact of selenium supplementation compared with a placebo on the changes caused by endurance training. The researchers evaluated mitochondrial activity of succinate dehydrogenase and cytochrome c oxidase, the myosin heavy chain expression in muscle fibers, and their association with aerobic performance. This was a double-blind, placebo-controlled trial with 24 male volunteers who participated in a 10-wk endurance training program. The selenium-supplemented group received 180 µg·d−1 of organic selenium (selenomethionine). Selenium had no effect on endurance training-induced adaptations in these participants.

In a similar study, Tessier et al. (46) assessed changes in the blood glutathione antioxidant system (selenium-dependent) in response to exercise and training. This was a 10-wk, double-blind, placebo-controlled study. Both the selenium supplemented and placebo groups were endurance-trained during the 10-wk study. They did not report any ergogenic effects as a result of the selenium supplementation.

To date, there are not many published studies on selenium supplementation and exercise; therefore, this is an area where more research is required. Based upon the aforementioned studies, however, it does not appear that selenium supplementation will be beneficial for exercise performance.

CONCLUSION

This review evaluates the ergogenic effects of four minerals: chromium, magnesium, zinc, and selenium. Based upon this review, it appears that supplementation with these minerals does not result in positive effects upon exercise performance. A systematic approach to the study of minerals and exercise performance is needed. This approach needs to use the same protocol to evaluate whether minerals can be effective ergogenic aids. It would require longer supplementations periods, control of exercise settings, multi-center trials, men and women participants, elite and recreational athletes, and precise measures of mineral status. Not until tightly controlled trials are conducted can we definitively state whether minerals have an ergogenic effect upon performance; however, based upon the present data, it appears that minerals, and in particular, chromium, magnesium, zinc, and selenium, do not have ergogenic effects.

References

1. Bohl, C.H., and S.L. Volpe. Magnesium and exercise. Crit. Rev. Food Sci. Nutr. 42:533-563, 2002.
2. Ganapathy, S., and S.L. Volpe. Zinc, exercise, and thyroid hormone function. Crit. Rev. Food Sci. Nutr. 39:369-390, 1999.
3. Kobla, H.V., and S.L. Volpe. Chromium, exercise, and body composition. Crit. Rev. Food Sci. Nutr. 40:291-308, 2000.
4. Volpe, S.L. Calcium, zinc, iron, and exercise in women. Top. Clin. Nutr. 14:43-52, 1999.
5. Volpe, S.L. Micronutrient requirements for athletes. Clin. Sports Med. 26:119-130, 2007.
6. Volpe, S.L., and J. Soolman. Minerals for weight loss - fact or fiction? ACSM Health Fit. 11:1-7, 2007.
7. Volpe, S.L. Magnesium, the metabolic syndrome, insulin resistance and type 2 diabetes mellitus. Crit. Rev. Food Sci. Nutr. 48:293-300, 2008.
8. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press, 1997.
9. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press, 1998.
10. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press, 2000.
11. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press, 2000.
12. Food and Nutrition Board of the Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academy Press, 2005.
13. Clark, M., D.B. Reed, S.F. Crouse, and R.B. Armstrong. Pre-and post-season dietary intake, body composition, and performance indices of NCAA division I female soccer players. Int. J. Sport Nutr. Exerc. Metab. 13:303-319, 2003.
14. Magkos, F., and M. Yannakoulia. Methodology of dietary assessment in athletes: concepts and pitfalls. Curr. Opin. Clin. Nutr. Metab. Care. 6:539-549, 2003.
15. Hellerstein, M.K. Is chromium supplementation effective in managing type II diabetes? Nutr. Rev. 56:302-306, 1998.
    16. Volpe, S.L., H.-W. Huang, K. Larpadisorn, and I.I. Lesser. Effect of chromium supplementation on body composition, resting metabolic rate, and selected biochemical parameters in moderately obese women following an exercise program. J. Am. Coll. Nutr. 20:293-306, 2001.
    17. Pasman, W.J., M.S. Westerterp-Plantenga, and W.H. Saris. The effectiveness of long-term supplementation of carbohydrate, chromium, fibre and caffeine on weight maintenance. Int. J. Obes. Rel. Met. Dis. 21:1143-1151, 1997.
    18. Walker, L.S., M.G. Bemben, D.A. Bemben, and A.W. Knehans. Chromium picolinate effects on body composition and muscular performance in wrestlers. Med. Sci. Sports Exerc. 30:1730-1737, 1998.
    19. Volek, J.S., R. Silvestre, J.P. Kirwanet al, et al. Effects of chromium supplementation on glycogen synthesis after high-intensity exercise. Med. Sci. Sports Exerc. 38:2102-2109, 2006.
    20. Crawford, V., R. Scheckenbach, and H.G. Preuss. Effects of niacin-bound chromium supplementation on body composition in overweight African-American women. Diabetes Obes. Metab. 1:331-337, 1999.
    21. Hoeger, W.W.K., C. Harris, E.M. Long, and D.R. Hopkins. Four-week supplementation with a natural dietary compound produces favorable changes in body composition. Adv. Ther. 15:305-313, 1998.
    22. He, K., K. Liu, M.L. Davigluset al, et al. Magnesium intake and incidence of metabolic syndrome among young adults. Circulation. 13:1675-1682, 2006.
    23. Murakami, K., H. Okubo, and S. Sasaki. Effect of dietary factors on incidence of type 2 diabetes: a systematic review of cohort studies. J. Nutr. Sci. Vitaminol. (Tokyo). 51:292-310, 2005.
    24. Cinar, V., M. Nizamlioglu, R. Mogulkoc, and A.K. Baltaci. Effects of magnesium supplementation on blood parameters of athletes at rest and after exercise. Biol. Trace Elem. Res. 115:205-212, 2007.
    25. Finstad, E.W., I.J. Newhouse, H.C. Lukaski, J.E. Mcauliffe, and C.R. Stewart. The effects of magnesium supplementation on exercise performance. Med. Sci. Sports Exerc. 33:493-498, 2001.
    26. Newhouse, I.J., and E.W. Finstad. The effects of magnesium supplementation on exercise performance. Clin. J. Sport Med. 10:195-200, 2000.
    27. Pokan, R., P. Hofmann, S.P. von Duvillardet al, et al. Oral magnesium therapy, exercise heart rate, exercise tolerance, and myocardial function in coronary artery disease patients. Br. J. Sports Med. 40:773-778, 2006.
    28. Chubanov, V., T. Gudermann, and K.P. Schlingmann. Essential role for TRPM6 in epithelial magnesium transport and body magnesium homeostasis. Pflugers Arch. 451:228-334, 2005.
    29. Kumeda, Y., and M. Inaba. Metabolic syndrome and magnesium. Clin. Calcium. 15:97-104, 2005.
    30. Soltani, N., M. Keshavarz, B. Minaiiet al, et al. Effects of administration of oral magnesium on plasma glucose and pathological changes in the aorta and pancreas of diabetic rats. Clin. Exp. Pharmacol. Physiol. 32:604-610, 2005.
    31. McCarty, M.F. Magnesium may mediate the favorable impact of whole grains on insulin sensitivity by acting as a mild calcium antagonist. Med. Hypotheses. 64:619-627, 2005.
    32. Lopez-Ridaura, R., W.C. Willett, E.B. Rimmet al, et al. Magnesium intake and risk of type 2 diabetes in men and women. Diabetes Care. 27:134-140, 2004.
    33. Guerrero-Romero, F., and M. Rodríguez-Morán. Low serum magnesium levels and metabolic syndrome. Acta Diabetol. 39:209-213, 2002.
    34. Song, Y., J.E. Manson, J.E. Buring, and S. Liu. Dietary magnesium intake in relation to plasma insulin levels and risk of type 2 diabetes in women. Diabetes Care. 27:59-65, 2004.
    35. Rodríguez-Morán, M., and F. Guerrero-Romero. Oral magnesium supplementation improves insulin sensitivity and metabolic control in type 2 diabetic subjects. A randomized, double-blind controlled trial. Diabetes Care. 26:1147-1152, 2003.
    36. Anderson, R.A., M.M. Polansky, and N.A. Bryden. Acute effects of chromium, copper, zinc, and selected clinical variables in urine and serum of male runners. Biol. Trace Elem. Res. 6:327-336, 1984.
    37. Aruoma, O.I., T. Reilly, D. MacLaren, and B. Halliwell. Iron, copper, and zinc concentrations in human sweat and plasma; the effect of exercise. Clinica Chimica Acta. 177:81-88, 1988.
    38. Lukaski, H.C., W.W. Bolonchuk, L.M. Klevay, D.B. Milne, and H.H. Sandstead. Changes in plasma zinc content after exercise in men fed a low-zinc diet. Am. J. Physiol. 247:E88-E93, 1984.
    39. Mundie, T.G., and B. Hare. Effects of resistance exercise on plasma, erythrocyte, and urine Zn. Biol. Trace Elem. Res. 79:23-28, 2001.
    40. Ohno, H., K. Yamashita, R. Doiet al, et al. Exercise-induced changes in blood zinc and related proteins in humans. J. Appl. Physiol. 58:1453-1458, 1985.
    41. Singh, A., B.L. Smoak, K.Y. Pattersonet al, et al. Biochemical indices of selected trace minerals in men: effect of stress. Am. J. Clin. Nutr. 53:126-131, 1991.
    42. Volpe, S.L., N.M. Lowe, L.R. Woodhouse, and J.C. King. Effect of maximal exercise on the short-term zinc kinetics of zinc metabolism in sedentary males. Br. J. Sports Med. 41:156-161, 2007.
    43. Maxwell, C., and S.L. Volpe. Effect of zinc supplementation on thyroid hormone function: a case study of two college females. Ann. Nutr. Metab. 51:188-194, 2007.
    44. Kilic, M. Effect of fatiguing bicycle exercise on thyroid hormone and testosterone levels in sedentary males supplemented with oral zinc. Neuro. Endocrinol. Lett. 28:681-685, 2007.
    45. Margaritis, I., F. Tessier, E. Prou, P. Marconnet, and J.F. Marini. Effects of endurance training on skeletal muscle oxidative capacities with and without selenium supplementation. J. Trace Elem. Med. Biol. 11:37-43, 1997.
    46. Tessier, F., I. Margaritis, M.J. Richard, C. Moynot, and P. Marconnet. Selenium and training effects on the glutathione system and aerobic performance. Med. Sci. Sports Exerc. 27:390-396, 1995.
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