25-HYDROXYCHOLECALCIFEROL IN POULTRY NUTRITION

Vitamin D is a complex of secosteroids that must undergo metabolic alt erations 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, respectiv ely. Transport of vitamin D steroids after absorption is associated wi th 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-dihydro xy 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 DB P in poultry does not bind D-2 forms effectively, and therefore poultr y 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 enzym e in the kidney is subject to negative feedback regulation and is crit ical for formation of the active metabolite 1,25-(OH)(2)D-3. The intra cellular vitamin D receptor (VDR) specifically binds 1,25-(OH)(2)D-3 a nd 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(O H)(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 ha ve shown that 25-(OH)D-3 supports good fertility and hatchability, whe reas hens fed only 1,25-(OH)(2)D-3 did not have normal hatchability. L ikewise, 1,25-(OH)(2)D-3 seems to reach toxic levels at dietary concen trations only two to three times optimal dietary levels whereas feedin g 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 bo th cellular functions and embryonic development in chickens and turkey s when fed as the sole source of vitamin D-3.