During lactation in the rat, vitamin D is required for maintenance of a normal serum calcium level and maximal enhancement of active calcium transport in the duodenum. Vitamin D does not appear to be required for part of the adaptive increase in intestinal calcium transport or for calcium transport into the milk. The functions of vitamin D appear to be mediated by 1,25-(OH)2D3, the circulating level of which increases during lactation. Two days after sudden weaning, the serum level of 1,25-(OH)2D3 falls to levels below the pre-pregnant control level in parallel with a sharp increase in serum calcium; normal levels of calcium and 1,25-(OH)2D3 are observed one week after weaning. The initial stimulus for the increase in circulating 1,25-(OH)2D3 during lactation appears to be a small decrease in serum calcium which stimulates parathyroid hormone secretion, which in turn enhances synthesis of 1,25-(OH)2D3. Vitamin D is also required by the lactating rat to insure normal development of the suckling pup, since vitamin D deprivation during pregnancy and lactation causes significant decreases in body weight gain, in serum levels of 25-OHD3, calcium and phosphorus, in bone ash content and clear evidence of histological rickets by 20 days of age.
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