Yearlong fluctuations of vitamin D status, intake, and health outcomes in university students: A prospective longitudinal study.

Most university students had insufficient 25(OH)D concentrations throughout the academic year.

• Participants (n = 49) completed four assessments in October, January, March, and July
• Blood sampling was conducted at each timepoint for 25(OH)D measurement
• Vitamin D insufficiency persisted across all four measurement points during the academic year

Vitamin D supplementation increased 25(OH)D concentrations only after 3 months, with levels declining when supplementation decreased.

• The study tracked supplementation behavior across four timepoints over an academic year
• A meaningful increase in serum 25(OH)D was not observed until after 3 months of supplementation
• When participants reduced or stopped supplementation, 25(OH)D concentrations subsequently declined
• Vitamin D intake correlated with 25(OH)D concentration

CRP concentration peaked in March despite the greatest frequency of infections occurring in January.

• CRP was measured at all four timepoints: October, January, March, and July
• The temporal dissociation between infection frequency peak (January) and CRP peak (March) was observed
• No significant associations were found between 25(OH)D and CRP

Body composition changed significantly across the academic year, with percent fat mass decreasing from January to March and fat-free mass being higher in March and July than in October and January.

• Body composition was assessed via plethysmography at each of the four timepoints
• Percent fat mass decreased between the January and March assessments
• Fat-free mass (kg) was significantly higher in March and July compared to October and January
• No significant associations were observed between 25(OH)D and body composition measures

Cortisol concentration was highest in October, and depressive symptoms were greatest in October, while cognitive performance improved after October.

• Cortisol was measured via blood sampling at all four timepoints
• Depressive symptoms were assessed via questionnaire at each timepoint
• Cognitive function was evaluated through cognitive testing at each timepoint
• No significant associations were observed between 25(OH)D and either cognition or depression

CYP2R1 rs10741657 GG homozygotes had significantly higher vitamin D intake than AG + AA genotype carriers, but no significant difference in 25(OH)D concentrations was observed between genotypes.

• Vitamin D intake in GG homozygotes was 58.0 ± 43.0 µg/d versus 25.3 ± 34.6 µg/d in AG + AA genotypes (P = 0.0081)
• Despite consuming significantly more vitamin D, GG homozygotes did not achieve significantly higher serum 25(OH)D concentrations
• This suggests CYP2R1 GG homozygotes may require higher intake levels of vitamin D to achieve comparable 25(OH)D concentrations to those with AG + AA genotypes