Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D

Background: Calcium absorption is generally considered to be impaired under conditions of vitamin D deficiency. but the vitamin D status that fully normalizes absorption is not known for humans. Objective: To quantify calcium absorption at two levels of vitamin D repletion. using pharmacokinetic methods and commercially marketed calcium supplements. Design: Two experiment, performed in the spring of the bear. one ear apart. In the first, in which participants were pretreated with 25-hydroxyvitamin D (250HD), mean serum 250HD concentration was 86.5 nmol/L: and in the other with no pretreatment. mean serum concentration was 50.2 nmol/L participants received 500 mg oral calcium loads as a part of a standard low calcium breakfast. A low calcium lunch was provided at mid-day. Blood was obtained fasting and at frequent intervals for 10 to 12 hours thereafter Methods: Relative calcium absorption at the two 250HD concentrations was estimated front the area under the curie (AUC) for the load-induced increment in serum total calcium. Results: AUC(9) (+/- SEM), was 3.63 mg, hr/dL +/- 0.234 in participants pretreated with 250HD and 2.20 +/- 0.240 in those not pretreated (P < 0.001). In briefs absorption was 65%, higher at serum 250HD levels averaging 86.5 nmol/L than at levels averaging 50 nmol/L (both value, within the nominal reference range fur this analyte). Conclusions: Despite the fact that the mean serum 250HD level in the experiment without supplementation,,as within the current reference ranges. calcium absorptive performance at 50 nmol/L was significantly reduced relative to that at a mean 250HD level of 86 nmol/L, Thus. individuals kith serum 25-hydroxyvitamin D levels at the low end of the current reference ranges may not be getting the fall benefit from their calcium intake. We conclude that the lower end of the current reference range is set too low.