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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
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.
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.
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 =
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 <0.05), and there was no significant difference in change in peak torque between
groups. After controlling for change in
25(OH)D and baseline 25(OH)D separately, supplementation with vitamin D was not associated with any
of the strength or power variables.
Supplementation of 50,000 IU of vitamin D3 per month improved vitamin D status but did not improve
the chosen measures of muscle strength and power in this group of female adolescent athletes. This
may be due in part to the small sample size and high baseline serum 25(OH)D concentrations seen in
Keywords: skeletal muscle strength, skeletal muscle power, athletes, dancers, gymnasts, vitamin D,