Abstract
The purpose of this study was to determine longevity of vitamin D status following cessation of vitamin D3 supplementation, 2 and 3years after a 1-year randomised, double-blind placebo controlled trial and to investigate possible predictive factors. Caucasian non-smoking postmenopausal women randomised to ViCtORY (2009-2010), who had not taken vitamin D supplements since the trial ended, were invited to attend follow-up visits. Total 25-hydroxyvitamin D (25OHD) and 24,25-dihydroxyvitamin D (24,25OH2D) were measured by dual tandem mass spectrometry of serum samples following removal of protein and de-lipidation; the original randomised controlled trial (RCT) samples were re-analysed simultaneously. Vitamin D-binding protein (VDBP) was measured by monoclonal immunoassay. In March 2012 and March 2013, 159 women (mean (SD) age 67.6 (2.1)years) re-attended, equally distributed between the original treatment groups: daily vitamin D3 (400 IU, 1000IU) and placebo. One month after the RCT ended (March 2010), the proportion of women in placebo, 400IU and 1000IU vitamin D3 groups, respectively, with 25OHD<25nmol/L was 15, 0 and 0 (chi-square p<0.001, n=46, 44, 54). After 2years (March 2012), it was 22, 4 and 4% (p=0.002, n=50, 48, 57); after 3years, it was 23, 13 and 15% (p=0.429, n=48, 45, 52). The respective proportions of women with 24,25OH2D<2.2nmol/L were 50, 2 and 2% (1month, p < 0.001, n=46, 44, 54); 42, 33 and 12% (2years, p=0.002, n=50, 48, 57); and 45, 27 and 29% (3years, p=0.138, n=47, 45, 51). VDBP was a predictor of circulating 25OHD longevity (beta for VDBP in μg/mL 0.736; 95% CI 0.216-1.255, p=0.006) but not 24,25OH2D. Four hundred international units or 1000IU of daily vitamin D3 showed benefits over placebo 2years after supplementation ceased in keeping 25OHD>25nmol/L.
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