Energy Restriction Reduces Fractional Calcium Absorption in Mature Obese and Lean Rats

Bone loss has been associated with weight reduction in humans (1–4) and in animal models (5–7), and may be greater in leaner subjects (3,8). However, the mechanisms regulating bone mobilization because of energy restriction (EnR)4 and the importance of initial body weight remain unclear. Bone-regulating hormones such as 1,25 dihydroxy-vitamin D, parathyroid hormone (PTH) and estradiol (E2) may be altered during EnR (5,9,10) and thus be associated with changes in Ca absorption. For example, there is evidence that vitamin D accumulates in adipose tissue, and its release during weight loss may increase circulating levels (11). Serum PTH may rise with weight reduction (9), and the increased Ca-PTH axis activity may be driven by a decrease in Ca absorption due to EnR. In addition, intestinal Ca absorption is influenced by serum E2 (12–14), and E2 may decline with EnR (5,6), but the relationship between EnR, E2 and Ca absorption is not known. Finally, there may also be an association between weight loss, increased glucocorticoids and decreased calcium absorption (15). We hypothesized that EnR in mature lean and obese rats would decrease Ca absorption and that this could be explained by changes in bone-regulating hormones.