Seasonal variation in vitamin D status of Auckland intermediate (11-12 years) school children : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Nutrition and Dietetics at Massey University, Auckland, New Zealand

Abstract

Vitamin D is an essential pro-hormone required for calcium homeostasis, bone mineralisation, and wider immune and metabolic functions. In New Zealand, vitamin D deficiency and insufficiency remain public-health concerns, particularly among children. Sun-safe behaviours, limited dietary sources and seasonal variation in ultraviolet beta (UVB) exposure are some of the factors that contribute to risk. This research investigated vitamin D status and UVB exposure from summer and winter in Auckland intermediate school children. A longitudinal repeated-measures study (the DISC study) was conducted in 100 intermediate school children aged 11–12 years. Serum 25-hydroxy-vitamin D (s-25(OH)D) was assessed using dried-blood spots in March (after summer exposure) and September (after winter exposure). UVB exposure was measured using personal dosimeters worn for 10 days at each timepoint. Anthropometric data and self-reported behaviours were also collected. Statistical analyses examined seasonal differences in s-25(OH)D and UVB dose, and predictors of seasonal change. Mean s-25(OH)D concentrations fell from 90 ± 22 nmol/L in summer to 65 ± 18 nmol/L in winter, a mean percentage change of -26±17% (p < 0.0001). The proportion of children classified as vitamin D deficient (< 50 nmol/L) rose from 3 % to 20 %, and those insufficient (< 75 nmol/L) from 21 % to 50 % from summer to winter. Mean UVB dose decreased from 9.2±5.0 Standard Erythemal Doses (SED) in summer to 3.1±1.1 SED in winter, a mean decrease of -6.1±28 SED (p < 0.0001). Greater seasonal reductions in s-25(OH)D occurred in children with higher summer concentrations (≥75 nmol/L), -30±17 vs -9.1±11 nmol/L (p = 25% for boys and >30% for girls), -29±15 vs 24±19 nmol/L (p = 0.04). Both summer s-25(OH)D and body-fat percentage independently predicted winter decline (R² = 0.45, p < 0.001), suggesting impaired mobilisation of vitamin D from adipose stores rather than altered metabolism. These findings suggest substantial seasonal variation in vitamin D status among Auckland school-aged children and highlight adiposity as a significant factor for insufficiency. Despite high exposure potential in summer, half of the cohort was insufficient by winter, indicating limited capacity to maintain optimal vitamin D status year-round. These results emphasise the need for evidence-based public-health guidance balancing sun-safety and vitamin sufficiency, consideration of targeted supplementation or fortification strategies, and possible tailored guidance for specific at-risk groups. Understanding and addressing determinants of vitamin D insufficiency in childhood is essential to protect long-term musculoskeletal, immune, and metabolic health of this population group.