Determinants and longitudinal changes of serum 25-hydroxyvitamin D during basic military training in female New Zealand Army recruits

Suboptimal vitamin D status is reported to negatively impact military readiness.This longitudinal study aimed to describe vitamin D status during basic military training (BMT) and identify determinants of serum 25-hydroxyvitamin D (25(OH)D) in female recruits in the New Zealand Army.Serum 25(OH)D was measured during week one (baseline) and week 16 (end) of BMT (n = 87) and further characterised by ethnicity and season.Following univariate analysis, age, body mass index (BMI), ethnicity, season, exercise, and serum ferritin were analysed as determinants of 25(OH)D at baseline using a hierarchical linear regression model.From baseline to end, mean SD 25(OH)D was 102.5 33.6 vs 67.4 22.6 nmol/L (p < 0.001) for BMT commenced in summer and 67.4 21.5 vs 73.8 18.9 nmol/L (p = 0.033) for BMT commenced in winter.Regardless of the season basic training commenced, less than one-third of participants overall had sufficient ( 75 nmol/L) vitamin D status at the end.Age, BMI, ethnicity, and season explained 48.2% of the variance in 25(OH)D at baseline.Wintertime and being of Pacific or Mori ethnicity were the strongest negative determinants of 25(OH)D at baseline.These results suggest seasonal UVB exposure is a major determinant of 25(OH)D and Pacific and Mori ethnic groups are particularly at risk of suboptimal vitamin D status.Education, screening, and early supplementation are recommended to prevent suboptimal vitamin D status during BMT.