Skeletal muscle is a major target organ of the vitamin D endocrine system( Reference Bischoff-Ferrari, Borchers and Gudat 1 ). Whilst adolescence is a critical period for musculoskeletal development, few studies have investigated the effect of vitamin D status on muscle function and physical fitness in this age group( Reference Hazell, DeGuire and Weiler 2 , Reference Ward, Das and Berry 3 ). The aim of this study was to investigate the association between vitamin D status and muscle strength, muscle power and cardiorespiratory fitness (CRF) in adolescents.
A total of 1015, 12- and 15-y-olds from the Young Hearts Study 2000 had serum 25-hydroxyvitamin D (25(OH)D) concentrations quantified using enzyme linked immunoassay (OCTEIA®, IDS, Ltd., UK). Total skinfolds were used to calculate absolute fat free mass (FFM; kg) and FFM corrected for height (fat free mass index (FFMI); kg/m2). Sargent jump test results were used to calculate peak muscle power (kW)( Reference Sayers, Harackiewicz and Harman 4 ). Muscle strength (kg) was measured by hand grip dynamometry (Takei Scientific Instrument Company Limited, Japan) and VO2 max, a measure of CRF, was estimated from 20-metre shuttle test scores.
Regression analyses to compare the relationship between tertiles of serum 25(OH)D concentration (low, middle and high) and muscle strength, muscle power and cardiorespiratory fitness (VO2 max), controlling for season, FFMI, energy adjusted protein intake, pubertal status and physical activity scores. *P < 0·05.
Muscle strength and CRF were significantly higher in 15-y-old boys in the high tertile of serum 25(OH)D concentration (>75 nmol/L), compared to those in the low tertile (<50 nmol/L). This association was not evident in any of the other age-sex groups and there were no associations between serum 25(OH)D concentration and muscle power, in any age-sex group. These results confirm the relationship between vitamin D status and muscle strength and CRF; the difference in these relationships between age-sex groups highlights a fast-changing hormone profile during adolescence and warrants further research in this age group.
This work was funded by the Department for Employment and Learning. Serum 25(OH)D analysis was funded previously by the Higher Education Authority under its North-South programme. Ethical approval was obtained from the Research Ethics Committee, Queen's University Belfast and the study was conducted according to the guidelines laid down in the Declaration of Helsinki.
