Seasonal Variation of 25-Hydroxyvitamin D among non- Hispanic Black and White Pregnant Women from Three US Pregnancy Cohorts.

Background Vitamin D deficiency during pregnancy has been associated with increased risk of complications and adverse perinatal outcomes. We evaluated seasonal variation of 25-hydroxyvitamin D [25( OH) D] among pregnant women, focusing on patterns and determinants of variation. Methods Data came from three cohort studies in the US that included 2583 non- Hispanic Black and White women having prenatal 25( OH) D concentrations determined. Fourier time series and generalised linear models were used to estimate the magnitude of 25( OH)D seasonality. We modelled seasonal variability using a stationary cosinor model to estimate the phase shift, peak-trough difference, and annual mean of 25( OH) D. Results We observed a peak for 25( OH) D in summer, a nadir in winter, and a phase of 8 months, which resulted from fluctuations in 25( OH) D3 rather than 25( OH) D2. After adjustment for covariates, the annual mean concentrations and estimated peak-trough difference of 25( OH) D among Black women were 19.8 ng/mL [95% confidence interval ( CI) 18.9, 20.5] and 5.8 ng/mL [95% CI 4.7, 6.7], and for non- Hispanic White women were 33.0 ng/mL [95% CI 32.6, 33.4] and 7.4 ng/mL [95% CI 6.0, 8.9]. Conclusions Non- Hispanic Black women had lower average 25( OH) D concentrations throughout the year and smaller seasonal variation levels than non- Hispanic White women. This study's confirmation of 25( OH) D seasonality over a calendar year has the potential to enhance public health interventions targeted to improve maternal and perinatal outcomes. [ABSTRACT FROM AUTHOR]