The effect of estrogen deficiency on calcium balance in mature rats

The role of estrogen in the regulation of calcium balance is still poorly u nderstood. A calcium balance study was performed to examine the effects of estrogen status in relation to fecal calcium loss as a component of bone lo ss after oophorectomy (OOX) in the mature rat. The components of the classi c calcium balance were compared with calcium balance estimates obtained fro m whole body bone density. Six month or older Sprague Dawley rats were allo cated to either a sham-operated or OOX group and fed a 0.1% calcium diet. T he bone mineral density (BMD) and bone mineral content (BMC) were measured at baseline, 6 weeks, and 9 weeks. A calcium balance was done for 6 days be fore and 6 weeks post OOX. The fall in BMD from baseline to 9 weeks in the OOX group was significantly greater than in the sham-operated group. The ca lcium balance was more negative at baseline than at 6 weeks in both groups of animals because they had not adapted to the low calcium diet. However, t he increase in calcium balance was significantly less in the OOX animals th an in the sham-operated animals. The greater the rise in calcium balance fr om the baseline to the 6 weeks balance the less the fall in the calcium con tent of the whole body (Spearman correlation: r = 0.604 P = 0.008). The fal l in fecal calcium, but not urine calcium or calcium consumed, was negative ly correlated with the change in whole body BMC (Spearman correlation: feca l calcium r = -0.763 P = 0.001). Thus, the primary effect of estrogen defic iency on calcium balance in the mature rat appears to be calcium flux in th e bowel, rather than renal calcium handling.