1. The effect of zinc depletion on muscle function was tested in 8 male subjects. After receiving 12 mg Zn/day for 17 days, the subjects received 0.3 mg Zn/day for either 33 or 41 days. The subjects were then divided into two groups for zinc repletion. Group A subjects received overnight infusion of 66 mg of Zn on Day 1 and 10 and then were fed 12 mg Zn/day for another 16 days. Group B subjects were fed 12 mg Zn/day for 21 days. Peak force and total work capacity of the knee and shoulder extensor and flexor muscle groups were assessed using an isokinetic dynamometer at baseline, at two points during depletion, and at repletion. Plasma zinc levels decreased by an average of 67% during depletion and remained 9% below baseline after repletion. The peak force of the muscle groups was not found to be significantly affected by acute zinc depletion, however, shoulder peak force (strength) was found to be reduced by 9.2% in the extensor muscles. Total work capacity (muscle endurance) for the knee extensor muscles and shoulder extensor and flexor muscles declined significantly by 28.1%, 24.1% and 26.4%, respectively. This study demonstrates that muscle endurance, or total work capacity, declines rapidly with acute zinc depletion and the degree of the decline was correlated with the reduction in plasma zinc concentration.

Van Loan, MD, et al. The Effects of Zinc Depletion on Peak Force and Total Work of Knee and Shoulder Extensor and Flexor Muscles. Int J of Sport Nutr, June 1999, Vol. 9, No. 2, 125-135.

2. A study was conducted to determine the effects of magnesium supplementation on strength development during a double-blind, 7-week strength training program in 26 untrained subjects (14=placebo, 12= Mg), 18-30 years old. Pre and post peak quadriceps torque (leg press) measurements were made using an isokinetic dynamometer. The leg muscle strength of the magnesium supplemented group significantly increased by 26%, compared to only 10% for the placebo group.

Brilla, LR, et al. Effect of Magnesium Supplementation on Strength Training in Humans. J Am Coll Nutr, July 1992, Vol 11, No. 3, 326-329

3. Serum zinc levels were determined in 160 training athletes (103 males and 57 females). In 23.3% of male and 43% of female athletes, serum zinc was significantly below the "normal range".

Haralambie, G. Serum zinc in athletes in training. Int J Sports Med 2 (1981) 135-138.

4. Magnesium, zinc and copper status of 270 US Navy Sea, Air and Land (SEAL) trainees was determined from dietary intakes and biochemical profiles. The dietary intakes of 34% and 44% of the trainees were below the RDA for Mg and Zn, respectively. The blood plasma concentrations of Mg and Zn were significantly below the "normal range" for 23% and 24% of the trainees, respectively.

Sing A, et al. Magnesium, Zinc and Copper status of US Navy SEAL trainees. Am J Clin Nutr 1989;49:695-700.

5. Serum zinc levels were measured in 20 adolescent gymnasts (9 boys, 11 girls, age 12-15). They had 26% lower serum zinc levels (0.599 +/- 0.026 mg/l) when compared to 118 matched controls (0.810 +/- 0.014, p < 0.001). The gymnasts serum zinc levels were positively correlated with adductor strength (r=0.468, p < 0.05). The 11 of 20 gymnasts with serum zinc < 0.6 mg/L had lower insulin-like growth factor binding protein 3 levels than the others (2.326 +/- 0.064 vs 2.699 +/- 0.12, p < 0.01). This protein is supposed to reflect growth hormone activity. Thus, zinc is lowered in trained adolescent gymnasts and this reduction could play a role in abnormalities of growth or muscular performance.

Brun J, et al. Serum zinc in highly trained adolescent gymnasts. Bio Trac Elem Res, 1995, Vol. 47, 273-278.

6. Twenty-one professional football (soccer) players underwent a maximal exercise test on a cycloergometer, with progressively increasing workloads until VO2max. On the whole these subjects had low serum zinc because nine (43%) of them had a hypozincemia (0.54 +/- 0.01 mg/L) which suggested a zinc deficiency. The subjects with low serum zinc had a 26% lower power output (123 +/- 8.71 vs. 166.27 +/- 14.84 watts, p = 0.029) and exhibited a 35% higher increase in blood lactate (lactic acid) during exercise (7.51 +/- 0.81 vs. 5.57 +/- 0.33 mmol/L, p <0.04) resulting in a 24% lower 2 mmol lactate threshold (44.7 +/- 3.9% vs. 58.9 +/- 4.8% of maximal power output p < 0.04). In conclusion, this study suggests that zinc status may influence blood rheology (flow) during exercise by an effect related to lactate accumulation.

Khaled S, et al. Serum zinc and blood rheology in sportsmen (football players. Clin Hemo and Micro 17 (1997) 47-48.

7. Ten collegiate basketball players serum mineral levels were measured before official practice began and immediately following the competitive season. Diets were monitored and remained the same throughout the four month period. Mean serum values for Mg and Zn decreased pre-season to post- season by 16% and 41%, respectively.

Lefavi RG, et al. Reduced serum mineral levels in basketball players after season. Med and Sci in Sports and Exer. Vol. 27, No. 5, May 1995

8. Twelve professional volleyball players and 12 control subjects were studied to determine the effects of daily physical training on serum, sweat and urine zinc concentrations. The professional athletes trained every day in two sessions, one in the morning (work in the gym for 2 hours) and another in the afternoon (specific work on the sports field for 3 hours). Simultaneously, 12 male volunteer university students, who were moderately trained, participated as the control group. The study was conducted over a period of 10 weeks. Pre-post tests were made using a progressive bicycle ergometer (increasing 30 W every 3 minutes to reach a maximum tolerated power). Pre-post blood samples were obtained at rest and immediately following exercise. After ten weeks of training, the professional athletes showed a significant increase in 24 hour urinary zinc excretion (22% greater losses), in contrast to a slight decrease (2% less) in the controls. The athletes also showed a very significant increase in the zinc loses in sweat compared to the controls. The athletes sweat zinc concentrations increased by an astounding 300%, compared to only 30% increases in the control group. The athletes serum zinc levels decreased by 4%, compared to a 2% decrease in the control group. Finally, the post exercise cortisol levels of the athletes significantly increased by 93%, compared to only an 18% increase in the controls. The authors stated that the athletes "cortisol levels increased in response to the exercise work load stress, and this behavior seems to be related to muscular damage". The authors went on to say that "It seems that the changes in zinc metabolism found in the study may be damage, increased protein turnover and increased zinc excretion (via sweat and urine). Because strenuous exercise during a period of competition can induce a "catabolic state" and has been shown to increase skeletal muscle protein turnover, it is likely that urine zinc is derived from muscle tissue". The authors concluded by saying that "Zinc supplementation and/or stress control appear to be indicated in athletes. In our practical opinion, we think that alterations in zinc metabolism with increases in zinc excretion and stress levels lead to a situation of latent fatigue with a decrease of endurance".

Cordova A, et al. Effect of training on zinc metabolism: changes in serum and sweat concentrations in sportsmen. Ann Nutr Metab, 1998 42:5, 274-82.

9. Plasma zinc, iron, copper and selenium levels were measured in 66 Navy SEAL trainees before and after a 5 day period of sustained physical and psychological stress called "Hell Week". The trainees pre-post plasma zinc levels decreased by 33%.

Singh A, et al. Biochemical indices of selected trace minerals in men: effect of stress. Am J Clin Nutrition 1991; 53:126-31.

10. Nine healthy, male subjects (18-40 years) were supplemented daily with 365 mg of magnesium as aspartate for 14 days. Before and after the supplementation period each subject performed a rigorously identical one hour ergometer exercise. The magnesium supplementation significantly reduced the subjects plasma levels of the catabolic "stress" hormone cortisol by an average of 25% (P < 0.025), which remained decreased during the exercise. The magnesium also significantly lowered the subjects' heart rates throughout the exercise period by an average of 8% (P < 0.03). Golf SW, et al. Plasma aldosterone, cortisol and electrolyte concentrations in physical exercise after magnesium supplementation. Clin Chem Clen Biochem, 1984, Vol. 22, pp. 717-721.

11. Medical students were tested to determine the acute effect of zinc supplementation on cortisol levels. The test was started at 7:00 AM after a 12 hour fast. Serial blood samples were collected from an experimental zinc group and controls at 30 minute intervals for 240 minutes. A subgroup of 7 subjects (3 men, 4 women) ingested 25 mg of zinc immediately after the baseline collection and their cortisol levels were compared to 8 matched controls who received a placebo. The control group started out with an average cortisol level of 11 mcg/dL and fell to 9 mcg/dL at 240 minutes, which is an 18% reduction. The zinc supplemented group started out with an average cortisol level of 16 mcg/dL and significantly dropped to 6.5 mcg/dL, which is a 59% reduction. In summary, the zinc supplemented group had a 41% greater reduction in cortisol levels compared to controls. The fact that zinc inhibits basal cortisol secretion in humans may be related to a direct blockade of cortisol synthesis and secretion in the adrenal cortex.

Brandao-Neto J, et al. Zinc acutely and temporarily inhibits adrenal cortisol secretion in humans. Bio Trace Elem Res, 1990, Vol. 24, 83-89.

12. Nine runners urine zinc and chromium levels were measured on a run day and compared to the levels on a non run day. The runners daily losses of zinc in urine were 50% greater on a run day compared to a non run day.

Anderson, R. Strenuous running. Bio Trac Elem Res, Vol. 6 (1984) 327- 336.

13. A percentage of testosterone is converted to dihydrotestosterone (DHT) by the enzyme 5a-reductase. An invitro study was conducted to determine the inhibition of 5a-reductase activity by zinc sulphate and azelaic acid. When added at concentrations of 3 or 9 mmol/l, zinc was a potent inhibitor of 5a-reductase activity. At a high concentration of 15 mmol/l, zinc completely inhibited 5a-reductase. The addition of vitamin B-6 potentiated the effect of zinc and resulted in a two-fold increase in the inhibition of 5a-reductase. A moderate concentration of 1.5 mmol/l of zinc in combination with 0.025% of vitamin B-6 inhibited the 5a-reductase activity by 90%. The zinc and vitamin B-6 combination may be effective at limiting DHT production and could represent a potential therapeutic agent in the treatment of androgen related pathology.

Stamatiadis D, et al. Inhibition of 5a-reductase activity in human skin by zinc and azelaic acid. Brit J of Derm, 1988, Vol. 119, pp. 627-632.

14. Androgen metabolism and aromatization, androgen and estrogen receptor binding and circulating levels of reproductive hormones were studied in zinc deficient rats. The zinc deficient group had significantly lower serum concentrations of testosterone (2.8 +/- .07 nmol/L) compared to the controls (8.7 +/- .07 nmol/L). This represents a remarkable 68% reduction in circulating testosterone levels. Scatchard analysis of the receptor binding data showed a significantly higher number of estrogen receptors in the zinc deficient group (36.6 +/- 3.4 fmol/mg protein) than in controls (23.3 +/- 2.4 fmol/mg protein) and a significantly lower number of androgen binding sites in rats fed the zinc deficient diet (6.7 +/- o.7 fmol/mg protein) than in controls (11.3 +/- 1.2 fmol/mg protein). To summarize, zinc deficiency caused a 41% reduction in the number of androgen binding sites and a 57% increase in the number of estrogen receptors. These findings indicate that zinc deficiency significantly reduces circulating testosterone concentrations and modifies sex hormone receptor levels.

Om AS, et al. Dietary zinc deficiency alters 5 alpha-reduction and aromatization of testosterone and androgen and estrogen receptors. J Nutr, 1996, Apr, 126:4,842-8.

15. Androgen binding was studied in zinc deficient rats. The experimental group of animals were maintained on a zinc deficient diet for 3 months. Scatchard analysis of the data revealed that the number of androgen binding sites in the zinc deficient rats was 31 +/- 5.2 fmol/mg cytosol protein. This was significantly lower than that (84 +/- 11.5 fmol/mg protein) of the controls. This 63% decrease in the number of androgen receptor cites in the zinc deficient state indicates that this metal is extremely important in the androgen binding process in the target cells.

Chung KW, et al, Androgen receptors in ventral prostate glands of zinc deficient rats. Life Sci,1986, Jan 27, 38:4, 351-356.

16. Nine men participated in an 85 day zinc depletion/repletion study divided into 3 metabolic periods: 18 day baseline, a 44 day depletion, and a 23 day repletion. 12 mg of zinc per day was fed to the men during baseline and were held constant after adjustments during the baseline period. Plasma zinc declined from 77.1 +/- 0.03 mcg/dl at baseline to 28.1 +/- 0.07 mcg/dl at depletion; concentrations returned to 77.9 +/- 0.03 mcg/dl at repletion. Total body weight, fat, fat-free mass (FFM), and bone mineral did not change during depletion, but total body water increased 5.3% +/- 1.9%, or about 2 kg or 4.4 lbs (P <0.05) by the end of the depletion and returned to baseline values at the end of repletion. The percent water in FFM increased from 71% +/- 1 to 75% +/- (P <0.05) at the end of depletion and was associated with a small decrease in body protein. The data suggest that zinc depletion impairs water balance.

Sutherland B, et al, Effect of experimental zinc depletion on body composition and basal metabolism in men. The FASEB Journal, Mar. 10, 1995, Volume 9, Number 4.