Dietary supplementation with antioxidant micronutrients—such as beta-carotene, vitamin E, zinc, and vitamin A—to offset the age-associated decline in immune function of older adults has received much attention in the media. The premise of these studies is that supplementation with antioxidants reduces the deleterious effects of increased free radical production and, thereby, minimizes the age-related decline in the immune response and decreases the incidence of infections and cancer. Several studies have supported this view by showing positive effects of micronutrient supplementation on cell-mediated and humoral immunity and a decrease in the duration of infection. However, equally well-controlled studies have indicated that micronutrient supplementation in older adults produced only transient positive effects, adverse effects, or no effect on indices of immune function and protection from infectious disease. Differences in study design (i.e., the micronutrient tested, the dose given, and the duration of supplementation) probably account for these discrepant results. Although most evidence suggests that supplementation with moderate doses of micronutrients is not harmful, studies must be conducted to determine whether long-term supplementation produces any deleterious effects on the immune response of older adults. Likewise, future studies must investigate whether or not positive effects of these micronutrients are mediated by their antioxidant properties or by acting to maintain or enhance immune function in older adults by other mechanisms. Such studies are necessary so that the lay public, particularly older adults, understands the implications, and possible consequences, of using micronutrient supplementation as a method to offset the age-related decline in immune function.
In summary, an age-related decrease in immune function is well established in humans and several other mammalian species. This decrease has been consistently observed in T cell proliferation and is associated with a shift to a memory T-cell phenotype. Alterations in cytokine production, antibody responses, and innate immunity have also been observed in some cases. However, the dependence of these changes on T cell alterations must be established to clearly elucidate possible mechanisms of age-related changes in the immune response of elderly humans.
DONNA M. MURASKO ELIZABETH M. GARDNER
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