Supplemental Vitamin D Increased Serum Total 25-Hydroxyvitamin D in the US Adult Population During 2007-2014

Background: Data from the 2007-2010 NHANES suggested that vitamin D supplements contributed to increased serum concentrations of 25-hydroxyvitamin D [25(OH)D] in the US population. Objectives: We sought to determine whether 25(OH)D continued to increase during NHANES 2011-2014 and whether associations of 25(OH)D with preselected covariates differed across time periods. Methods: For this study, 25(OH)D was measured in adults (>= 20 y) using LC-MS/MS. Descriptive and regression analyses were stratified by survey period to investigate the effects of age, race-Hispanic origin, sex, season, BMI, dietary vitamin D, and vitamin D-containing supplements. A multiple linear regression model was used to assess 25(OH)D changes between two 4-y survey periods, namely 2007-2010 and 2011-2014. Results: We observed several significant concomitant increases between 2007-2010 and 2011-2014: unadjusted mean 25(OH)D increased by 2.7 nmol/L (95% CI: 0, 5.4 nmol/L; P= 0.048), the percentage of persons taking any vitamin D-containing supplements increased 2.9% (95% CI: 0.03, 5.5%; P= 0.0314), and the percentage of persons taking high-dose (>= 1000 IU/d) vitamin D-containing supplements increased 8.6% (95% CI: 6.9, 9.9%; P < 0.0001). With covariate adjustment, the increase in 25(OH)D from 2007-2010 to 2011-2014 was no longer statistically significant [1.4 nmol/L (95% CI: -3.0, 0.23 nmol/L; P= 0.09)]. After adjustments, several large differences in 25(OH)D remained, namely non-Hispanic blacks had 25(OH)D 22 nmol/L lower than that of non-Hispanic whites, and users of vitamin D-containing supplements >= 1000 IU/d had 25(OH)D 31 nmol/L higher than that of nonusers. Conclusions: After adjusting for vitamin D supplement dose, the overall adjusted increase in 25(OH)D was no longer statistically significant, suggesting that changes in US adults' 25(OH)D concentrations between NHANES periods 2007-2010 and 2011-2014 may primarily be associated with changes in vitamin D supplementation.