Weight loss and calcium intake influence calcium absorption in overweight postmenopausal women

Background: Weight loss (WL) reduces bone mass and increases fracture risk. Mechanisms regulating calcium metabolism during WL are unclear. Objective: The objective was to assess the effect of 6 wk of WL at 2 different amounts of calcium intake [normal (NICa): 1 g/d; high (HiCa): 1.8 g/d] on true fractional calcium absorption (TFCA), bone turnover, and bone-regulating hormones in overweight postmenopausal women. Design: Seventy-three women (body mass index, 26.9 ± 1.9 kg/m²) were recruited either to consume a moderately energy-restricted diet (WL group) or to maintain their body weight [weight-maintenance (WM) group] and were randomly assigned to either the HiCa or the NICa group in a double-blind manner. Subjects underwent weekly diet counseling, and measurements were taken at baseline and after 6 wk. Results: Fifty-seven women completed the study and had a baseline TFCA of 24.9 ± 7.4%. Energy restriction significantly decreased the total calcium absorbed (P < 0.05) in the WL group (n = 32) compared with the WM group (n = 25; analysis of covariance). Regression analysis showed that a greater rate of weight loss suppressed TFCA and the total calcium absorbed (P < 0.05) in the HiCa group. The women in the NICa WL group absorbed inadequate amounts of calcium (195 ± 49 mg/d), whereas the women in the HiCa WL group absorbed adequate amounts (348 ± 118 mg/d). Parathyroid hormone explained 22% of the variance in calcium absorbed in the NICa group only. Conclusions: We suggest that WL is associated with elevated calcium requirements that, if not met, could activate the calcium-parathyroid hormone axis to absorb more calcium. Normal intakes of calcium during energy restriction result in inadequate total calcium absorption and could ultimately compromise calcium balance and bone mass.