Relative effectiveness of oral 25-hydroxyvitamin D₃ and vitamin D₃ in raising wintertime serum 25-hydroxyvitamin D in older adults

Background: The relative potency of 25-hydroxyvitamin D₃ to vitamin D₃ needs to be better defined so that food-composition tables can better reflect the true vitamin D nutritive value of certain foods. Objective: We performed a randomized, controlled intervention study in apparently healthy, free-living adults to investigate whether the intake of 25-hydroxyvitamin D₃ is 5 times more potent in raising serum 25-hydroxyvitamin D [25(OH)D] during winter compared with an equivalent amount of vitamin D₃. Design: A randomized, placebo-controlled, double-blind intervention study was conducted in adults aged ≥50 y (n = 56) who consumed a placebo, 20 μg vitamin D₃, or 7 or 20 μg 25-hydroxyvitamin D₃ daily throughout 10 wk of winter. Serum 25(OH)D was measured by using an enzyme-linked immunoassay, and serum albumin-corrected calcium (S-Ca) was assessed colorimetrically at the baseline, midpoint, and endpoint of the study. Results: The mean (±SD) increases (per microgram of vitamin D compound) in serum 25(OH)D concentrations over baseline after 10 wk of supplementation were 0.96 ± 0.62, 4.02 ± 1.27, and 4.77 ± 1.04 nmol · L^-1 · μg intake^-1 for the 20-μg vitamin D₃/d and 7- and 20-μg 25-hydroxyvitamin D ₃/d groups, respectively. A comparison of the 7- and 20-μg 25-hydroxyvitamin D₃/d groups with the 20-μg vitamin D ₃/d group yielded conversion factors of 4.2 and 5, respectively. There was no effect of treatment on S-Ca concentrations and no incidence of hypercalcemia (S-Ca >2.6 nmol/L). Conclusions: Each microgram of orally consumed 25-hydroxyvitamin D₃ was about 5 times more effective in raising serum 25(OH)D in older adults in winter than an equivalent amount of vitamin D₃. This conversion factor could be used in food-compositional tables for relevant foods. This study was registered at clinicaltrials.gov as NCT01398202.