The effects of a vitamin D randomised controlled trial on muscle strength and power in female adolescent athletes : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Human Nutrition at Massey University, Albany, New Zealand

Background: Vitamin D deficiency is widespread in the general public and emerging evidence has revealed it is common in athletic populations, particularly those who train indoors. Recent studies suggest that vitamin D deficiency is correlated with impaired skeletal muscle function; however there is limited evidence from randomised controlled trials that vitamin D supplementation can improve muscle strength and power in trained athletes. Objective: To investigate the effects of vitamin D3 supplementation on serum 25(OH)D concentrations and muscle strength and power in female adolescent athletes training predominantly indoors. Methods: Female adolescent dancers, gymnasts, and swimmers (n = 61) who trained regularly for at least five hours per week participated in this randomised double blind placebo controlled trial. Participants were stratified to receive 50,000 IU vitamin D3 or placebo every month for six months. Serum 25(OH)D concentrations, muscle strength (handgrip and isokinetic knee extensor and flexor torque), power (vertical jump), and anthropometric measurements were assessed at baseline and endpoint (n = 54). Results: At baseline, the median 25(OH)D concentration was 77.5 [63.5,92] nmol/L for the vitamin D group and 74 [64.5,88.5] nmol/L for the placebo group. Following six months of supplementation, serum 25(OH)D concentrations increased significantly in the vitamin D group (16.5 [7,46] nmol/L) (P = 0.001), but not the placebo group (-6.25 [-21,44] nmol/L). Peak torque (Nm) of the knee extensors in concentric and eccentric extension increased significantly for both groups (P