Size at birth, adult intestinal calcium absorption and 1,25(OH)2 vitamin D

Background: Adult bone mineral status is modified by early environmental influences, but the mechanism of this phenomenon is unknown. Intestinal calcium absorption and vitamin D metabolism are integrally involved in bone metabolism and may be programmed during early life. Aim: To examine the early-life influences on calcium absorption and its control in 322 post-menopausal female twins. Methods: Intestinal calcium absorption was assessed by the stable strontium (Sr) method. Serum PTH, 25(OH) and 1,25(OH)(2) vitamin D were measured and recalled birth weight recorded. Results: Fractional intestinal Sr absorption (alphaSr) was correlated with serum 1,25(OH)(2) vitamin D (p < 0.001), but not with 25(OH) vitamin D. Birth weight was inversely associated with serum 1,25(OH)(2) vitamin D (p = 0.04), the association being independent of serum calcium, phosphate, creatinine and PTH. Birth weight was inversely correlated with alphaSr (p = 0.03), this association being independent of age, season, customary calcium intake and serum 25(OH) vitamin D; however, when serum 1,25(OH)(2) vitamin D was added into the model, the association became non-significant, suggesting that the association was partially mediated via serum 1,25(OH)(2) vitamin D. Discussion: We found a significant inverse association between birth weight and intestinal calcium absorption that is partially explained by an association between serum 1,25(OH)(2) vitamin D and birth weight. This suggests a mechanism whereby the intra-uterine environment might affect adult skeletal status.