AIM: The aims of this study were to investigate changes in vitamin D status during two years in adolescence, and whether lifestyle changes were associated with serum 25-hydroxyvitamin D (s-25(OH)D) at follow-up.

METHODS: Fit Futures is a longitudinal study at 69°N in Norway. Participants had their s-25(OH)D levels analysed in their first and third year of upper secondary school (median age 16 and 18 years), in Fit Futures 1 (FF1) and Fit Futures 2 (FF2), respectively. Self-reported lifestyle habits were registered through questionnaires. The association between lifestyle changes and s-25(OH)D levels at follow-up were calculated by regression analyses, controlling for baseline s-25(OH)D levels.

RESULTS: Longitudinal data were available for 309 girls and 280 boys. The proportion of adolescents with s-25(OH)D < 50 nmol/L were 73.7% in FF1 and 77.1% in FF2, while the proportion <30 nmol/L constituted 35.7% in FF1 and 40.9% in FF2. Of those with s-25(OH)D < 30 nmol/L (severe vitamin D deficiency) in FF1, 73.3% remained severely deficient in FF2. Among boys, an increase in ultraviolet (UV)-exposure was significantly associated with higher s-25(OH)D levels in FF2 [beta; CI (nmol/L)] [12.9; 9.1, 16.7]. In girls, decreased vitamin/mineral supplement intake was significantly associated with lower s-25(OH)D at FF2 [-6.7; -10.2, -3.1], while increased UV [10.8; 7.0, 14.7] and combined hormonal contraceptive (CHC) exposure [12.1; 6.0, 18.1] in FF2 was significantly associated with higher s-25(OH)D levels in FF2.

CONCLUSION: Severe vitamin D deficiency was prevalent throughout adolescence. Lifestyle changes may alter s-25(OH)D levels in this age group.