Lower cathelicidin concentrations in Irish athletes compared to controls: a role for vitamin D?

The biologically active vitamin D metabolite (1[alpha]-dihydroxyvitamin D) has been linked to regulation of human cathelicidin antimicrobial peptide (LL-37) in vitro. Recent studies in athletes indicate that vitamin D inadequacy is associated with lower concentrations of LL-37 and that this may increase risk of upper respiratory tract infection (URTI). PURPOSE: To compare circulating LL-37 concentrations between athletes and healthy controls and to establish if total 25-hydroxyvitamin D (25[OH]D) was a significant determinant of this antimicrobial peptide.METHODS: A total of 221 stored, fasted plasma samples from male and female athletes (elite rugby n=12, elite boxing n=12, Gaelic football n=77) and age and sex-matched healthy controls (n=120) were analysed in duplicate for LL-37 concentrations using an enzyme-linked immunosorbent assay. Total 25(OH)D concentrations were quantified by liquid chromatography-tandem mass spectrometry as part of the previous studies. Comparison of 25(OH)D and LL-37 concentrations between athletes and controls was made using Mann-Whitney U tests. Linear regression was used to test if 25(OH)D concentration was a significant determinant of LL-37 concentration after adjusting for season of sampling.RESULTS: Median [IQR] 25(OH)D concentrations were 49.76[39.37] and 28.90[23.55] nmol/L for athletes and healthy controls respectively, P<0.001. LL-37 concentrations were significantly lower in athletes compared to healthy controls (26.92[17.04] versus 47.91[59.93] ng/mL respectively, P<0.001). Nevertheless 25(OH)D was not found to be a significant determinant of LL-37 concentration in athletes ([beta]=0.052, P=0.611) or healthy controls ([beta]=0.170, P=0.224).CONCLUSION: Our preliminary findings demonstrate a significant difference in LL-37 concentration between athletes and the general population. Yet the significant positive association between 25(OH)D and LL-37 concentrations reported in endurance athletes may not apply to those competing in intermittent sports; possibly owing to a decreased likelihood of experiencing exercise-induced immunosuppression resulting from prolonged high-intensity exercise. Further studies are required to determine if lower LL-37 concentrations translate into athletes being at greater risk of URTI than the general population.