Relationship between 25-hydoxyvitamin D, muscle strength, and incidence of injury in healthy adults: a systematic review.

The purpose of this systematic review is to answer the following clinical questions in healthy adults: is vitamin D status related to (1) muscle strength? (2) or incidence of injury? A literature search was performed using Pubmed, SPORTDiscus, and Web of Science to capture relevant articles that have examined these outcomes. Inclusion criteria required studies to address at least one of the 2 questions stated above, enroll healthy human subjects with a mean age of 18 to 65 years of age, and include serum 25-hydoxyvitamin D measures. Study characteristics such as vitamin D status, study design, and study population were documented. Measured assessors and outcomes from all studies were extracted to answer at least one of the two questions. When applicable, data were used to compute effect sizes at a 95% confidence interval for comparisons across studies to answer the 2 questions. The results of these studies indicate a weak to moderate effect of higher 25-hydoxyvitamin D levels on greater muscle strength and reduced incidence of injury. Randomized controlled clinical trials examining these questions are scarce when compared with the popularity of vitamin D testing; therefore, future trials are necessary to advance our understanding and to clarify the effect vitamin D has on extraskeletal outcomes in healthy adults.

The effects of season-long vitamin D supplementation on collegiate swimmers and divers.

The purpose of this 6-month randomized, placebo-controlled trial was to determine the effect of season-long (September-March) vitamin D supplementation on changes in vitamin D status, which is measured as 25(OH) D, body composition, inflammation, and frequency of illness and injury. Forty-five male and female athletes were randomized to 4,000 IU vitamin D (n = 23) or placebo (n = 22). Bone turnover markers (NTx and BSAP), 25(OH)D, and inflammatory cytokines (TNF-alpha, IL-6, and IL1-ß) were measured at baseline, midpoint, and endpoint. Body composition was assessed by DXA and injury and illness data were collected. All athletes had sufficient 25(OH)D (> 32 ng/ml) at baseline (mean: 57 ng/ml). At midpoint and endpoint, 13% and 16% of the total sample had 25(OH)D < 32 ng/ml, respectively. 25(OH)D was not positively correlated with bone mineral density (BMD) in the total body, proximal dual femur, or lumbar spine. In men, total body (p = .04) and trunk (p = .04) mineral-free lean mass (MFL) were positively correlated with 25(OH)D. In women, right femoral neck BMD (p = .02) was positively correlated with 25(OH)D. 25(OH)D did not correlate with changes in bone turnover markers or inflammatory cytokines. Illness (n = 1) and injury (n = 13) were not related to 25(OH)D; however, 77% of injuries coincided with decreases in 25(OH)D. Our data suggests that 4,000 IU vitamin D supplementation is an inexpensive intervention that effectively increased 25(OH)D, which was positively correlated to bone measures in the proximal dual femur and MFL. Future studies with larger sample sizes and improved supplement compliance are needed to expand our understanding of the effects of vitamin D supplementation in athletes.