Why athletes may need more zinc in their diet
Although it plays a role in protein metabolism and growth, zinc has never really been associated with enhanced aerobic performance. But all that may be about to change following a recent American study carried out for the US Department of Agriculture.
Fourteen young healthy men were randomly divided into two groups, both fed a typical Western diet for a period of nine weeks. However, one group’s diet contained low-zinc foods, providing a daily intake of around 3.8mg per day, while the other included high-zinc foods, providing a daily intake of around 18.7mg per day. After a six-week washout period (during which both groups returned to their normal eating habits), the protocol was reversed for another nine weeks – ie the low-zinc group followed the high-zinc diet, and vice-versa.
At the beginning and end of the two nine-week dietary regimes, peak work capacity was determined by means of a graded, progressive cycling test and a prolonged sub-maximal test (70% VO2max for 45 minutes). The researchers also checked on blood levels of zinc and iron because very high levels of zinc intake can interfere with iron absorption, which could in turn impair oxygen transport and aerobic capacity. They also checked for levels of a zinc- dependent enzyme called carbonic anhydrase, which plays a key role in facilitating the removal of carbon dioxide from energy producing cells. Any reduction in this enzyme could theoretically cause a ‘bottleneck’ in aerobic energy production in cells by allowing carbon dioxide to back up.
Analysis of the results showed that peak oxygen uptake, carbon dioxide output and respiratory exchange ratio all decreased on the low-zinc diet, while the ventilatory demand for any given level of exercise increased. In plain English, the men used less oxygen, produced less carbon dioxide and breathed more strenuously during exercise on the low-zinc diet by comparison with the high-zinc diet. Blood haemoglobin was unchanged, but carbonic anhydrase levels dropped on the low-zinc diet.
These findings provide an explanation for previous findings of decreased muscle strength and endurance in adolescents and adults with low-zinc status. And the researchers conclude that low-zinc diets cause a drop in carbonic anhydrase levels, leading to impaired peak aerobic power, aerobic efficiency and ventilatory efficiency.
Given that many ‘Western’ diets are estimated to contain significantly less than the recommended daily intake of zinc (15mg per day), it seems prudent for anyone engaged in physical activity or sport to review their diet to make sure they are consuming sufficient quantities of this vital nutrient.
American Journal Of Clinical Nutrition 2005; 81:1045-1051