Vitamin D insufficiency in adolescent males in Southern Tasmania: prevalence, determinants, and relationship to bone turnover markers

There are limited data on vitamin D insufficiency in healthy children. The aim of this study was to describe the prevalence and determinants of vitamin D insufficiency and its association with bone turnover in adolescent boys (N=136, mean age 16 years). Sun exposure and physical activity were assessed by questionnaire. Vitamin D stores were assessed by serum 25-hydroxyvitamin D₃ (25[OH]D₃). Bone turnover was assessed by bone-specific alkaline phosphatase (BAP) and urinary pyridinoline (PYR) to creatinine (Cr) ratio (mmol PYR/µmol Cr). The mean 25(OH)D₃ level was low (44 nmol/l; 68% <50 nmol/l; range, 16–87) and was associated with self-reported sun exposure on winter weekends (r=0.23, p=0.01), school holidays (r=0.22, p=0.01), and weekdays (r=0.17, p=0.05). It was also associated with number of sports (r=0.34, p<0.001) and vigorous activity (r=0.22, p=0.01) but not television, computer, and video watching (r=-0.04, p=0.68). In multivariate analysis, number of sports but not total sun exposure remained significantly associated with 25(OH)D₃. Furthermore, 25(OH)D₃ was significantly associated with BAP in cutpoint analysis (cutpoint 55 nmol/l, p=0.03) but not continuous analysis (r=–0.12, p=0.16) and PYR in both forms (r=−0.23, p=0.01, cutpoint 43 nmol/l, p=0.01). In conclusion, vitamin D insufficiency is common in healthy adolescent boys in winter in our setting, is primarily derived from sports-related sun exposure, and is associated with bone turnover markers. These data suggest that a 25(OH)D₃ level of at least 43–55 nmol/l is required for optimal bone health in children.