Acute Effects of Vitamin D3 Supplementation on Muscle Strength in Judoka Athletes: A Randomized Placebo-Controlled, Double-Blind Trial

Objective: Indoor athletes have been shown to be prone to vitamin D-3 deficiency. The aim of the study was to examine the acute effects of vitamin D supplementation on muscle function using isokinetic dynamometry. Design: Randomized placebo-controlled, double-blind study. Setting: Institutional. Participants: Adult male white national level judoka athletes (n = 22) who were involved in full-time training. Exclusion criteria were vitamin supplementation, overseas travel to sunny climes, and/or an injury incurred during the last 3 months before testing. Interventions: Subjects were randomly allocated to the treatment (150 000IU vitamin D-3) or placebo and given blinded supplements by an independent researcher. Participants were tested twice, 8 days apart, on a Monday morning before the start of judo training and after 2 days of rest. A 5 to 7 mL of blood sample was collected followed by isokinetic concentric quadriceps and hamstring muscle function assessments on the right leg at 30 and 200 degrees.s(-1). Main Outcome Measures: Repeated-measures analysis of variance was used to analyze isokinetic muscle force and serum 25(OH)D-3. Regression to the mean was used to examine changes in 25(OH)D-3 levels over the study period. Results: The treatment group demonstrated a significant increase in serum 25(OH)D levels (34%, P <= 0.001) and muscle strength (13%, P = 0.01) between days 1 and 8. No significant differences were found for the placebo group for the same period. Conclusions: A single bolus of 150 000IU vitamin D-3 had a significant positive effect on serum 25(OH)D levels and muscle function in vitamin D insufficient elite indoor athletes. Clinical Relevance: Serum 25(OH)D-3 levels of indoor athletes should be monitored throughout the year and especially during winter months. Beneficial responses, in muscle strength and serum 25(OH)D-3, to 1 dose of vitamin D-3 supplementation can be observed within 1 week of ingestion. Muscle strength is linked to serum 25(OH)D levels.