Vitamin D(3) Supplementation Reduces the Symptoms of Upper Respiratory Tract Infection during Winter Training in Vitamin D-Insufficient Taekwondo Athletes: A Randomized Controlled Trial

Vitamin D insufficiency may be associated with increased risk of upper respiratory tract infection (URTI) in athletes. This study examined the effects of vitamin D(3) supplementation on salivary immune functions and symptoms of URTI in vitamin D-insufficient taekwondo athletes. Twenty-five male taekwondo athletes, aged 19–22 years with vitamin D insufficiency [serum 25-hydroxyvitamin-D concentrations (25(OH)D, 31.3 ± 1.39 nmol/L)], participated in this study. They were randomized to receive 5000 IU/day of vitamin D(3) (n = 13) or placebo capsule (n = 12) during 4 weeks of winter training. Blood samples were collected two times (pre- and post-tests) for analyzing serum 25(OH)D concentration while salivary samples were obtained three times (pre-, mid-, and post-tests) for secretory immunoglobulin A (SIgA) and lactoferrin analyses. The symptoms of URTI were reported daily during the intervention. Serum 25(OH)D concentration significantly increased by 255.6% in the vitamin D group, whereas in the placebo group it did not change (p < 0.001). While the significant increase in SIgA was observed in both groups (p < 0.001), elevated salivary lactoferrin level in response to winter training was found only in the placebo group (p = 0.011). The change in serum 25(OH)D concentration was negatively associated with total URTI symptoms (r = −0.435, p = 0.015). Vitamin D(3) supplementation may be effective in reducing the symptoms of URTI during winter training in vitamin D-insufficient taekwondo athletes.