Vitamin D deficiency (total 25-hydroxyvitamin D [25(OH)D] concentration <30nmol/L) is a global health concern reported in elite Irish athletes. The aim of this thesis was to investigate vitamin D status and the effects of supplementation on health and performance outcomes in athletes. In an observational study, blood samples (i?=92) were obtained from international Irish cricketers, boxers and Rugby sevens players. Some 86% were vitamin D sufficient (>50nmol/L) and those reporting vitamin D supplementation were 4.3 times more likely to be vitamin D sufficient than their non-supplemented counterparts. LL-37 concentrations were compared between athletes and a control group («=221). Although LL-37 concentrations were found to be significantly lower in athletes than controls, the study did not support the concept of vitamin D as a significant positive predictor of LL-37 in vivo. A double-blind, randomised, placebo-controlled trial (rc=42) was conducted [3000IU (75pg) vitamin D3 daily for 12-weeks via vitamin D or placebo oral spray solution]; in collegiate Gaelic footballers. Mean total 25(OH)D concentration increased from 47nmol/L to >80nmol/L. Vitamin D3 supplementation had no effect on the primary outcome measure VO2 max or secondary outcome measures skeletal muscle function, lung function, inflammation or bone turnover compared to placebo. A crossover trial comparing oral spray and capsule vitamin D supplementation [3000IU (75 pg) vitamin D3 daily for 4-weeks followed by a 10-week washout and crossover] was conducted in healthy adults (z?=22). Oral spray and capsule vitamin D3 supplementation were equally effective at increasing total 25(OH)D concentration. This thesis questions the purported performance benefits of vitamin D for the athletes studied. Vitamin D inadequacy is being effectively addressed in elite cohorts but remains prevalent in athletes without access to high performance nutritional support. Oral spray vitamin D3 supplementation is a suitable alternative to capsules for athletes and the general population to optimise wintertime vitamin D status.