Skeletal changes during pregnancy and lactation in the rat: effect of different levels of dietary calcium.

It is now well known that the skeleton serves as a store of calcium and phosphorus, and that a part of the mineral requirements of lactation is met by bone resorption. In an earlier communication (Warnock & Duckworth, 1944) the literature was surveyed and the role of the spongiosa as a source of minerals for lactation in the rat was studied under conditions of adequate calcium intake. Certain interesting questions about the resorption of bone in lactation and its subsequent repair after lactation remain unanswered. In the first place, the extent to which the response of the skeleton to lactational needs is modified by differences in the level of calcium intake is unknown. Secondly, the extent to which cancellous and compact bone are individually resorbed in lactation and the degree to which these changes can be modified by alterations in calcium intake have not been studied. Thirdly, the extent to which calcium intake affects the capacity of cancellous bone and compact bone to recover after depletion does not seem to have been examined. Fourthly, it is not clear whether resorption of bone is the only process by which minerals are released from skeletal stores to meet the needs of lactation in the calciumdeficient animal. The possibility of demineralization of bone, that is of extraction of the mineral component without resorption of the organic matrix, must also be considered. Finally, it seems that no study has been made of changes in physical dimensions of bone in pregnant and lactating animals receiving diets differing in their calcium content. The present experiment was designed to answer these questions and to afford an opportunity to study the effects of different dietary calcium levels on the reproductive performance of rats and their ability to rear their young. The diets were composed of natural feeding-stuffs, selected for their low calcium content. Most earlier investigators in this field used synthetic diets.