DOES VITAMIN D METABOLITE MEASUREMENT HELP PREDICT 25(OH)D CHANGE FOLLOWING VITAMIN D SUPPLEMENTATION?

Variability in 25-hydroxyvitamin D (25(OH)D) change following vitamin D supplementation exists. Vitamin D metabolite measurement might assist in predicting 25(OH)D response and also contribute to defining vitamin D adequacy. This study assessed utility of vitamin D metabolite measurements to predict 25(OH)D response and explored the relationship between parathyroid hormone (PTH) and a "vitamin D composite index" comprised of the sum of serum 25(OH) D, cholecalciferol (vitamin D ) and 24,25 dihydroxyvitamin D (24,25(OH) D). Sixty-two postmenopausal women were randomized to daily vitamin D 1,800 IU or placebo for 4 months. Blood was drawn at baseline and after 1 and 4 months. Serum 25(OH)D, vitamin D , and 24,25(OH) D were measured by liquid chromatography tandem mass spectroscopy. Free 25(OH)D and PTH were measured by enzyme-linked immunosorbent assay. Repeated measures analysis of variance and regression analyses were performed. Baseline 25(OH)D was positively correlated (P