Is there an optimal vitamin D status for immunity in athletes and military personnel?

Abstract

Vitamin D is mainly obtained through sunlight ultraviolet-B (UVB) exposure of the skin, with a small amount typically coming from the diet. It is now clear that vitamin D has important roles beyond its well-known effects on calcium and bone homeostasis. Immune cells express the vitamin D receptor, including antigen presenting cells, T cells and B cells, and these cells are all capable of synthesizing the biologically active vitamin D metabolite, 1, 25 dihydroxy vitamin D. There has been growing interest in the benefits of supplementing vitamin D as studies report vitamin D insufficiency (circulating 25(OH) D < 50 nmol/L) in more than half of all athletes and military personnel tested during the winter, when skin sunlight UVB is negligible. The overwhelming evidence supports avoiding vitamin D deficiency (25(OH) D < 30 nmol/L) to maintain immunity and prevent upper respiratory illness (URI) in athletes and military personnel. Recent evidence supports an optimal circulating 25(OH) D of 75 nmol/L to prevent URI and enhance innate immunity and mucosal immunity and bring about anti-inflammatory actions through the induction of regulatory T cells and the inhibition of pro-inflammatory cytokine production. We provide practical recommendations for how vitamin D sufficiency can be achieved in most individuals by safe sunlight exposure in the summer and daily 1, 000 IU vitamin D-3 supplementation in the winter. Studies are required in athletes and military personnel to determine the impact of these recommendations on immunity and URI; and, to demonstrate the purported benefit of achieving 25(OH) D > 75 nmol/L.