4.3.1. Anti-Inflammatory Vitamin D

There has been increasing interest in the benefits of supplementing vitamin D during the winter months as studies demonstrate vitamin D insufficiency (25(OH)D < 50 nmol/L) in more than half of all athletes living at northerly latitudes [259]. It is important to avoid vitamin D deficiency (25(OH)D < 30 nmol/L) in order to maintain immunity and prevent URI, particularly in settings where profound vitamin D deficiency is common [260]. URI tend to be more prevalent in female endurance athletes when engaging in similarly high training loads than their male counterparts (~11 h/week of moderate-vigorous activity) [261,262]. Evidence supports an optimal circulating 25(OH)D of 75 nmol/L to prevent URI in athletes and military personnel and, furthermore, to enhance immune function and to induce anti-inflammatory actions through the induction of regulatory T cells and the inhibition of pro-inflammatory cytokine production [263]. Reducing inflammation is a key mechanism that can improve age-related skeletal muscle changes through either direct catabolic effects or indirect mechanisms (e.g., higher GH and IGF-1 concentrations, less anorexia) [264]. A recent study in sarcopenic older adults participating in a 12-week controlled resistance training program found a significant beneficial effect of daily supplementation with whey protein (22 g), essential amino acids (including 4 g leucine), and vitamin D (100 IU) compared to placebo, with a gain of 1.7 kg in fat free mass, significant decreases in C-reactive protein concentrations, and significant increases in IGF-I concentrations, accompanied by a reduced risk of malnutrition [265]. Although the authors were not able to assess the effects of vitamin D supplementation separately from essential amino acid supplementation, this study suggests that whey protein, essential amino acid and vitamin D supplementation, together with resistance training, can reduce inflammatory markers and improve anabolic markers in sarcopenic elderly. Interestingly, long-term

supplementation with calcium-vitamin D fortified milk may negate these benefits because of the concomitant gains in fat mass [266]. Despite the decline in vitamin D production with aging, vitamin D sufficiency can be achieved in older athletes by regular sunlight exposure in the summer and daily 1000 IU vitamin D3 supplementation in the winter months [267].
