Abstract
This chapter discusses the hormonal control of calcium metabolism in lactation. In lactating animals that secrete relatively large amounts of calcium in the milk, a number of adaptive changes in calcium metabolism occur. The efficiency of calcium absorption from the intestine increases twofold or more, and, in the rat, absorption may remain elevated for some time after weaning if dietary calcium intake is limited so that skeletal calcium stores are mobilized to meet the demands of milk secretion. Vitamin D and parathyroid hormone act in concert to mediate these changes. Vitamin D through its conversion to 1,25-(OH)2D3 stimulates calcium absorption, and both parathyroid hormone and the vitamin D metabolite enhance skeletal calcium mobilization. Both hormones are also closely related to the serum calcium level. An elevated blood level of parathyroid hormone, in lactating cows, is stimulated and maintained by a low serum calcium level, but may be required to prevent an excessive drop in serum calcium in response to the calcium drain into the milk. High circulating levels of calcitonin are observed in lactating rats and sheep and in some lactating women. The urinary excretion of calcium is reduced during lactation in several species.
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