Longevity of daily oral vitamin D3 supplementation: differences in 25OHD and 24,25(OH)2D observed 2 years after cessation of a 1-year randomised controlled trial (VICtORy RECALL).

To determine how long vitamin D lasts after supplementation ceases, the marker of status was measured 2 and 3 years after a 1-year trial. Compared to placebo, the proportion of vitamin D-deficient women was still lower, if they had taken daily vitamin D3, after 2 years, indicating its longevity.

INTRODUCTION: The purpose of this study was to determine longevity of vitamin D status following cessation of vitamin D3 supplementation, 2 and 3 years after a 1-year randomised, double-blind placebo controlled trial and to investigate possible predictive factors.

METHODS: 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.

RESULTS: 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, 1000 IU) and placebo. One month after the RCT ended (March 2010), the proportion of women in placebo, 400 IU and 1000 IU vitamin D3 groups, respectively, with 25OHD < 25 nmol/L was 15, 0 and 0 (chi-square p < 0.001, n = 46, 44, 54). After 2 years (March 2012), it was 22, 4 and 4% (p = 0.002, n = 50, 48, 57); after 3 years, it was 23, 13 and 15% (p = 0.429, n = 48, 45, 52). The respective proportions of women with 24,25OH2D < 2.2 nmol/L were 50, 2 and 2% (1 month, p < 0.001, n = 46, 44, 54); 42, 33 and 12% (2 years, p = 0.002, n = 50, 48, 57); and 45, 27 and 29% (3 years, 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.

CONCLUSION: Four hundred international units or 1000 IU of daily vitamin D3 showed benefits over placebo 2 years after supplementation ceased in keeping 25OHD > 25 nmol/L.