Vitamin D plays a vital role in regulating immunity via the up-regulation of antimicrobial peptides and proteins which have a broad range of activities against micro-organisms and may be involved in the direct inactivation of viruses. Vitamin D is crucial in activating and controlling the T-cell antigen receptor and can influence cytokine production during periods of infection.
Several studies have demonstrated a negative correlation between vitamin D status and incidence of respiratory illness in adults. A recent 2013 study monitored 239 endurance athletes during a four month winter training period. Blood was collected from the athletes at the beginning and the conclusion of the study, in addition, participants were required to keep weekly training and daily illness logs.
At the beginning of the observation period 38% of the athletes were vitamin D deficient. By the end of winter, 55% were considered vitamin D deficient.
Of the athletes with optimal vitamin D status, 27% experienced one or more upper respiratory tract infection (URTI) as opposed to 67% of the vitamin D deficient group. Thus, vitamin D status influenced the prevalence of URTI episodes and furthermore results showed that symptoms were worse, and lasted longer, in those who were vitamin D deficient.
Secretory IgA (sIgA), is a key marker of mucosal immunity. This was also measured every four weeks throughout the winter period. The research showed that athletes who had optimal vitamin D levels also had significantly higher IgA secretion rates. The vitamin D deficient group tended to have the lowest concentrations of sIgA. Therefore, it was concluded that "low vitamin D status could be an important determinant of URTI risk in endurance athletes and mucosal as well as systemic immunity may be modified via vitamin D-dependent mechanisms."
- He CS, Handzlik M, Fraser WD, et al. Incidence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes. Exerc Immunol Rev 2013;19:86-101. [Abstract]