25-hydroxycholecalciferol in poultry nutrition

Vitamin D is a complex of secosteroids that must undergo metabolic alterations to reach optimal biological activity. The parent compounds 1) ergocalciferol (D-2) and 2) cholecalciferol (D-3) can be synthesized in the leaves of many plants or in the skin of most animals, respectively. Transport of vitamin D steroids after absorption is associated with vitamin D binding proteins (DBP). In general, the relative binding affinities of the vitamin D steroids are: 25-hydroxy vitamin D-3 [25-(OH)D-3] = 24,25-dihydroxy vitamin D-3 [24,25-(OH)(2)D-3] 25,26-dihydroxy vitamin D-3 [25,26-(OH)(2)D-3] > 25-hydroxy vitamin D-2 (25-(OH)D-2) > 1, 25-dihydroxy vitamin D-3 [1,25-(OH)(2)D-3] > vitamin D-3 The DBP in poultry does not bind D-2 forms effectively, and therefore poultry can not use this form of vitamin D adequately. The concentration of 25-(OH)D, in blood seems to be well correlated with dietary vitamin D intake or exposure to ultraviolet light. The 1 alpha hydroxylase enzyme in the kidney is subject to negative feedback regulation and is critical for formation of the active metabolite 1,25-(OH)(2)D-3. The intracellular vitamin D receptor (VDR) specifically binds 1,25-(OH)(2)D-3 and is necessary for cellular action. Increased levels of two to three orders of magnitude are required for 25-(OH)D-3 to compete with 1,25(OH)(2)D-3 for binding on VDR Feeding studies with 25-(OH)D-3 suggest it has nearly twice the activity of vitamin D-3. Hatchability studies have shown that 25-(OH)D-3 supports good fertility and hatchability, whereas hens fed only 1,25-(OH)(2)D-3 did not have normal hatchability. Likewise, 1,25-(OH)(2)D-3 seems to reach toxic levels at dietary concentrations only two to three times optimal dietary levels whereas feeding 25-(OH)D-3 for extended periods at levels 8 to 10 times requirement seems to have no adverse effects. It seems that 25-(OH)D-3 is the most active metabolite of vitamin D-3, ultimately capable of supporting both cellular functions and embryonic development in chickens and turkeys when fed as the sole source of vitamin D-3.