Zinc is an essential nutrient that has an important role in the growth and development of many cells, tissues, and the immune system. Zinc is considered a cofactor for over 300 metalloenzymes and one of the symptoms of zinc deficiency is an increased frequency of bacterial and viral infections. The effects of zinc and immune function havebeen well studied. The cellular and molecular effects of zinc on immunity are beyond the scope of this review and the reader is referred to several recent reviews for additional information.
Zinc deficiency may impair nonspecific and specific immune responses. Nonspecific defenses that may be impaired by zinc include NK cytotoxicity, macrophage phagocytosis, neutrophil oxidative burst, and chemotaxis. Other observations include thymic atrophy, decreased DTH, reduction in peripheral T-cell count, decrease in T helper cell and T cytotoxic cell functions, and reduced antibody production. It is apparent that zinc deficiency affects multiple aspects of host immunity and therefore it is not surprising that an increased frequency of infection is observed in deficient individuals. Zinc supplementation of deficient individuals has been shown to reduce the incidence and severity of infection, especially in children.
Although it is clear that malnourished individuals suffer impaired immunity due in part to zinc deficiency, the question of relevance to normal well-nourished healthy individuals is whether zinc supplementation can enhance immunity. Several studies in the elderly have shown that zinc supplementation is associated with an increased number of T cells, improved delayed dermal hypersensitivity, DTH response, and antibody production to tetanus vaccine. The elderly may suffer from mild zinc deficiency and therefore may benefit from supplementation.
Recently, there has been an interest in the potential role of zinc supplementation in reducing the duration of symptoms associated with the common cold. Some studies have demonstrated that zinc gluconate lozenges administered within 24 hours of the onset of symptoms of the common cold reduced the duration of cold symptoms. However, this is not a consistent finding and a recent meta-analysis that reviewed a number of clinical trials concluded that solid evidence for the effectiveness of zinc lozenges in reducing the duration of common colds is lacking.
Immune Effects and Exercise
Many athletes may be tempted to try zinc supplementation while experiencing symptoms of the common cold in the hope that the duration of cold symptoms will be reduced. Short-term supplementation is probably not harmful, however, high-dose zinc supplementation is associated with immunosuppression. It is not known if zinc supplementation can reduce the incidence of infection in healthy well-nourished individuals. Some evidence supports the concept that zinc supplementation may be considered potentially beneficial during episodes of the common cold, in terms of decreased symptoms or duration. However, there is no evidence that zinc can reduce the incidence of the common cold among healthy individuals.
One potential area of consideration for some athletes regarding zinc is the finding that some athletes, particularly endurance athletes, may suffer from mild zinc deficiency This population may be similar to elderly individuals regarding mild zinc deficiency and therefore may benefit from supplementation. However, to our knowledge this possibility has not been tested. Also, keep in mind that the elderly experience an age-associated decline in immune responsiveness and therefore zinc supplementation of younger individuals may not show similar benefits.
One study did compare the postexercise immune response in those individuals who consumed zinc and placebo. The subjects consuming zinc did not experience an exercise-induced increase in neutrophil production of ROS. It is possible that this change may alter exerciseÂinduced tissue damage. It is not clear at this time how the change in neutrophil function may relate to susceptibility to infection or disease outcome. Again, further research is certainly warranted considering that some athletes may experience mild zinc deficiency and adequate zinc intake is essential for optimal immune response and resistance to infection.