Vitamin D and adolescents : estimation of evidence-based dietary requirements and health outcomes related to vitamin D status.

Vitamin D deficiency and inadequacy are worldwide public health concerns and occur across all age, sex and ethnic groups, with significant implications for human health. Adolescents are a population group at high risk of low vitamin D status, yet the evidence base for establishing vitamin D requirements remains weak. The primary aim of this Thesis was to estimate the dietary vitamin D intakes required to maintain serum 25-hydroxyvitamin D [25(OH)D] concentrations above specific cut-off thresholds (25, 30, 40 and 50 nmol/l) during the winter-time in white Caucasian adolescents residing in the UK (51°N). Secondary aims were to: 1) investigate vitamin D status in relation to musculoskeletal and cardiometabolic health outcomes and; 2) explore familial associations in vitamin D status, dietary intakes and musculoskeletal health in mother-child pairs. This was achieved via a dose-response trial in 110 adolescents (14-18 years), who were randomly allocated to receive 0 (placebo), 10 or 20 μg vitamin D3 daily for 20 weeks during the winter-time. A final aim of this Thesis was to assess the vitamin D status and prevalence of vitamin D deficiency in a small study of African Caribbean and South Asian adolescent and young adult females (16-25 years) in comparison to their white Caucasian counterparts. It was estimated that vitamin D intakes of 10.1, 13.1, 23.3 and ~30 μg/day would maintain serum 25(OH)D concentrations > 25, 30, 40 and 50 nmol/l respectively in 97.5% of the adolescents. Adolescents with serum 25(OH)D concentrations below 50 nmol/l had significantly lower radial trabecular volumetric bone mineral density and presented with a poorer cardiometabolic profile, with greater waist circumference and higher triglyceride and glucose concentrations than their more replete counterparts. However cardiometabolic risk was lost after controlling for potential confounders. Positive, sex-specific associations were found for serum 25(OH)D concentrations and radial mass, total area and cortical volumetric bone mineral density in mother-child pairs, but not for dietary vitamin D and calcium intakes. A high prevalence of vitamin D deficiency was found in African Caribbean and South Asian females (serum 25(OH)D < 30 nmol/l: 75% and 63% respectively) and serum 25(OH)D concentrations in these ethnic minority populations was half that of their white Caucasian counterparts (22.7 [14.5, 34.0], 22.8 [19.2, 37.1] and 44.4 [36.6, 60.3] nmol/l respectively). In conclusion, dietary vitamin D intakes of between 10 and ~30 μg/day are required by adolescents during the winter-time to maintain serum 25(OH)D concentrations > 25-50 nmol/l, depending on the cut-off threshold selected. These data will allow for future refinement of evidence-based dietary requirements for adolescents. Further studies are now urgently needed to investigate the vitamin D requirements of ethnic minority populations via targeted dose-response vitamin D trials. Finally, the beneficial effects of maintaining serum 25(OH)D concentrations above 50 nmol/l on musculoskeletal health parameters requires confirmation in further randomised trials