Vitamin D Status and Performance in Semi-Professional Male Rugby Union Players: A Cross-Sectional Analysis

Abstract

Background: Low vitamin D status is common amongst the athletic population internationally. Sufficient vitamin D is important for bone health and recent research also suggests an importance for physical performance, with associations found between serum 25-hydroxyvitamin D concentrations and muscle strength and physical performance in healthy populations. However, these associations have not been assessed in New Zealand athletes. Objectives: The objectives were to 1) evaluate the vitamin D status of semi-professional male rugby union players in Otago and Southland, New Zealand; 2) within this group, to assess associations between serum 25-hydroxyvitamin D concentrations and specific athletic performance measures; and 3) to identify potential predictors of serum 25-hydroxyvitamin D concentrations in this group of New Zealand athletes. Design: Cross-sectional secondary data analysis of a randomised, placebo-controlled, double-blinded intervention study. Methods: Fifty-seven semi-professional male rugby union players residing in Otago and Southland, New Zealand (latitude: 45-47° S) completed baseline measures including: a demographic questionnaire, sun exposure questionnaire, serum 25-hydroxyvitamin D analysis and standard New Zealand Rugby Union performance tests during the months of autumn (March to May), 2011. Results: The mean serum 25-hydroxyvitamin D concentration was 94 nmol/L (range, 57-131 nmol/L). No participant had a serum 25-hydroxyvitamin D concentration of <50 nmol/L. There were no significant associations between serum 25-hydroxyvitamin D concentrations and the specific athletic performance measures after adjusting for body mass and training group: Yo-Yo Intermittent Recovery Test (p = 0.36), 30m sprint (p = 0.11), predicted one repetition maximum (1RM) bench pull (p = 0.09), predicted 1RM weighted reverse-grip chin-up (p = 0.11) and predicted 1RM bench pull (p = 0.14). Those of self-identified Pacific ethnicity had significantly lower serum 25-hydroxyvitamin D concentrations compared to New Zealand European (p <0.001) and Māori (p = 0.003). Looking at potential predictors of serum 25-hydroxyvitamin D concentrations including: sun exposure, body mass, BMI, percent body fat, ethnicity and sunscreen use, self-identified Pacific ethnicity was a predictor of serum 25-hydroxyvitamin D concentrations (p = 0.001). The final regression model of the aforementioned potential predictors of serum 25-hydroxyvitamin D concentrations explained 34% (p = 0.01) of the variability in circulating 25-hydroxyvitamin D. Conclusion: This study concludes that this group of semi-professional male rugby union players from the southernmost parts of New Zealand (Otago and Southland) were vitamin D replete during autumn, based on the current New Zealand Ministry of Health cut off of >50 nmol/L. There were no associations between serum 25-hydroxyvitamin D concentrations and the specific measures of athletic performance, suggesting that athletic performance is not associated with circulating 25-hydroxyvitamin D. Participants of self-identified Pacific ethnicity had significantly lower serum 25-hydroxyvitamin D concentrations compared to those of self-identified New Zealand European and Māori ethnicities. Self-identified Pacific ethnicity was a significant predictor of serum 25-hydroxyvitamin D concentrations.