Magnesium, Zinc, and Chromium

In contrast to the macronutrients (proteins, carbohydrates, and fats) that are consumed daily in large amounts (tens and hundreds of grams), micronutrients, such as magnesium, zinc, and chromium, are ingested in much smaller amounts (thousandths and millionths of a gram). The relative paucity of these micronutrients, both in the diet and in the body, suggests their importance in the regulation of whole-body metabolism, including energy utilization and work performance.

The biological importance of magnesium, zinc, and chromium is revealed by the various metabolic processes in which these elements regulate biological function. Magnesium, a ubiquitous element that plays a fundamental role in many cellular reactions, is involved in more than 300 enzymatic reactions in which food is metabolized and new products are formed. Some important examples include glycolysis, fat and protein metabolism, adenosine triphosphate synthesis, and second messenger system. Magnesium also serves as a physiological regulator of membrane stability and in neuromuscular, cardiovascular, immune, and hormonal function.

Another intracellular cation, zinc, is required for more than 300 enzymes from many species. Zinc-containing enzymes participate in many components of macronutrient metabolism, particularly cell replication. In addition, some zinc-containing enzymes, carbonic anhydrase and lactate debydrogenase, are involved in exercise metabolism, and another enzyme, superoxide dismutase, protects against free radical damage.

Recent attention has been directed to the element chromium. Trivalent chromium is required for the maintenance of normal glucose metabolism in animals; thus, chromium may act as a cofactor for insulin action. This insulinogenic characteristic of chromium has prompted the suggestion that chromium has an anabolic function.

Attempts to examine the relationships between physical activity and magnesium, zinc, or chromium nutritional status have been hampered generally by limitations in experimental design. Overall, the evidence to suggest that physically active individuals have either impaired magnesium or zinc status or improve performance with supplementation of these minerals is equivocal because nutritional status was not examined.

The effects of chromium supplementation on human physical performance are current topics of interest. Initial studies, in which collegiate football players were supplemented with 200 _ug chromium as chromium picolinate, reportedly found a significant increase in fat-free mass with a concomitant decrease in body fat during resistance training based on anthropometric assessment of body composition. Subsequent studies that used more accurate methods of body composition assessment failed to document beneficial effects of generalized chromium supplementation (200 _ug/d) on strength gain or body composition in young adults.

The indiscriminate use of mineral supplements may be hazardous. Ingestion of magnesium supplements in amounts exceeding 500 mg/d often results in gastrointestinal disturbances, particularly diarrhea, and may adversely affect phosphate balance. Supplemental zinc (50 mg/d) can inhibit absorption of copper from the diet. Consumption of supplemental zinc in amounts ranging from 17 to 50 mg/d can prevent an exercise-induced increase in high-density lipoprotein cholesterol (HDL) concentration. Excessive intake of supplemental zinc (160 mg/d) can decrease HDL. Chromium supplementation (200 _ug/d as chromium picolinate) may reduce transferrin saturation and induce iron deficiency. Therefore, the use of mineral supplements is not recommended unless under the guidance of a physician or a registered dietician.

The effect of nutritional supplementation on physical performance has been examined in crossover design experiments. Based on laboratory and field performance tests, there was no measurable effect of nutritional supplementation in well-nourished individuals. These findings support the hypothesis that there is no beneficial effect of nutritional supplements on performance when athletes consume diets adequate in essential nutrients.

Because of the importance of magnesium, zinc, and chromium for the maintenance of health and the development of physical fitness, physically active individuals should consume a balanced diet. If an individual is concerned about the nutritional quality of the diet, he or she should seek advice from a registered dietician who is experienced in working with people with active lifestyles.

1. Fogelholm M. Vitamin and mineral status in physically active people. Publications of the social insurance institution, ML: 118. Turku, Finland, 1992.

2. Haymes EM. Trace minerals and exercise. In: Wolinsky I and Hickson JF, eds. Nutrition in exercise and sport. Boca Raton, FL: CRC Press, 1994. pp.223-44.

3. Lefavi RG, Anderson RA, Keith RE, Wilson GD, McMillan JL, Stone MH. Efficacy of chromium in athletes: emphasis on anabolism. Int J Sports Nutr 1992;2:111-22.

4. Lukaski HC. Micronutrients (magnesium, zinc and copper): are mineral supplements needed for athletes? Int J Sports Med 1995;5:S74-S83.

5. Weight LM, Myburgh KH, Noakes TD. Vitamin and mineral supplementation: effect on the running performance of trained athletes. Am J Clin Nutr 1988;47:192-5.