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

Abstract

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 (NlCa): 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/m(2)) 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 NlCa 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 NlCa 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 NlCa 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.

Figures

FIGURE 1

Flow diagram of subjects in study. Excessive weight gain: >2 kg; hyperestrogenism: estradiol >80 pg/mL; low PTH (parathyroid hormone): 1500 mg/d. NlCa, normal calcium intake; HiCa, high calcium intake; WM, weight maintenance; WL, weight loss.

FIGURE 2

(A) Association between the rate of weight change and true fractional calcium absorption (TFCA) [high calcium group (HiCa): r = 0.46, P = 0.018; normal calcium group (NlCa): r = –0.05, P = NS] and (B) estimated amount (mg) of calcium absorbed (HiCa: r = 0.43, P = 0.028; NlCa: r = –0.01, P = NS) after 6 wk of dietary intervention in 57 postmenopausal overweight women. Diamonds and solid line represent HiCa (n = 26), open squares and dashed line represent NlCa (n = 31).