Abstract
A current hypothesis suggests that the degree of Ca absorption is hormonally controlled via the feed-back regulation of 1,25-dihydroxycholecalciferol (1,25-(OH) 2D 3) production from 25-hydroxycholecalciferol (25-OHD 3) by kidney 1-hydroxylase. To test this hypothesis, dihydrotachysterol 3 (DHT 3), a steroid not requiring 1-hydroxylation for biological activity, was given to chicks as the only source of vitamin D-activity. As expected, DHT 3-treated chicks did not adapt to a calcium-deficient diet. However, both the efficiency of Ca absorption and net synthesis of CaBP were stimulated in DHT 3-treated chicks by a low phosphorus intake, providing evidence for an alternate pathway of control.
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