Chemical Composition, but Not Specific Surface Area, Affects Calcium Retention of Nanostructured Calcium Compounds in Growing Rats.

Background: Low dietary calcium intake and bioavailability may adversely affect bone health. Reducing the size of calcium compounds increases their specific surface area (SSA, expressed as m2/g) and may increase calcium dissolution and bioavailability.Objective: We investigated the influence of SSA and chemical composition on the bioavailability of calcium and compared in vitro calcium dissolution with in vivo absorption.Methods: Calcium dissolution was measured in 0.1 M phosphoric acid, whereas color and pH changes of foods were assessed as indicators for potential sensory performance. Calcium absorption, retention, and fractional retention were measured over a 5-d balance study in growing Sprague-Dawley male rats after 21 d of feeding. Femoral and vertebral bone mineral density (BMD) and extensive tissue histology were assessed at study end. The influence of SSA on calcium bioavailability was assessed by comparing the groups fed pure calcium carbonate (CaCO3) with increasing SSAs of 3, 36, and 64 m2/g (CaCO3_3, CaCO3_36 and CaCO3_64), whereas chemical composition was assessed by comparing the smallest CaCO3_64, a 50:50 wt:wt percent solution mixture of CaCO3 and hydroxyapatite_94, and pure hydroxyapatite_100.Results: In vivo, fractional calcium retention from hydroxyapatite_100 (mean ± SEM: 54.86% ± 0.95%/5 d) was significantly greater than from CaCO3_64 (49.66% ± 1.15%/5 d) (P = 0.044). Increasing SSA of the pure CaCO3 did not significantly improve calcium retention. Across all 5 groups, there were no significant differences in BMD or tissue calcification by histology. In vitro calcium dissolution did not correlate with SSA or calcium absorption. In selected food matrixes, hydroxyapatite_100 caused less color change and/or smaller pH increase than did the other calcium compounds.Conclusions: Our findings suggest that chemical composition rather than SSA is a predictor of nanostructured calcium bioavailability and that in vitro dissolution of nanostructured calcium does not predict in vivo absorption. Although its phosphorus content may limit use in some populations, nanostructured hydroxyapatite may be a promising calcium compound for food fortification.