Oral spray wintertime vitamin D₃ supplementation has no impact on inflammation in Gaelic footballers

Vitamin D inadequacy [total 25(OH)D <50 nmol/L] is widespread in athletes. The biologically active metabolite, 1,25-dihydroxyvitamin D, may be involved in regulating inflammation although in vitro findings have not been consistently replicated in human intervention trials. This study, conducted at a latitude of 55°N, aimed to assess inflammatory biomarkers in Gaelic footballers before and after a wintertime vitamin D₃ intervention. Samples from a 12-week double-blind, randomized, placebo-controlled trial, in which 42 Gaelic footballers received 3000 IU (75 μg) vitamin D₃ daily or placebo via oral spray solutions, were analysed for a range of inflammatory biomarkers. Cytokines (interleukin-8 and tumor necrosis factor-α), cathelicidin and high sensitivity C-reactive protein were quantified by multiplex assay, enzyme-linked immunosorbent assay and clinical biochemistry, respectively. White blood cell, lymphocyte, and neutrophil concentrations were determined by full blood profile. Data on total 25-hydroxyvitamin D, measured by LC-MS/MS, were available from the previous study. Vitamin D₃ supplementation significantly increased mean total 25-hydroxyvitamin D concentrations from 47 to 84 nmol/L (P = 0.006); yet this had no effect on white blood cell count (P = 0.699), lymphocyte (P = 0.694), neutrophil (P = 0.594), interleukin-8 (P = 0.334), tumor necrosis factor-α (P = 0.587), cathelicidin (P = 0.745) or high sensitivity C-reactive protein concentration (P = 0.621) compared to placebo. 12-weeks vitamin D₃ supplementation did not impact the immune profile of Gaelic footballers. This is likely because biomarkers were within their respective normal range or at a concentration similar to that of the general population at baseline. Future studies are encouraged to use inflammation as their primary outcome measure and recruit athletes at risk of compromised immunity.