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  • Title: Evidence that discrimination against ergocalciferol by the chick is the result of enhanced metabolic clearance rates for its mono- and dihydroxylated metabolites.
    Author: Hoy DA, Ramberg CF, Horst RL.
    Journal: J Nutr; 1988 May; 118(5):633-8. PubMed ID: 2835464.
    Abstract:
    Studies on the discrimination between ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) by chickens have led to conflicting conclusions. To investigate this problem in more detail, radiolabeled vitamin D and vitamin D metabolites were administered intravenously, which allowed determination of their relative plasma clearance rates. The study involved three groups of adult male chickens (five per group). Birds in group 1 were co-dosed with [3H]vitamin D2 and [3H]vitamin D3. Birds in group 2 were co-dosed with [3H]25-dihydroxyvitamin D2 and [3H]25-dihydroxyvitamin D3 and birds in group 3 were co-dosed with [3H]1,25-dihydroxyvitamin D3 and [3H]1,25-dihydroxyvitamin D2. The results indicated that the plasma turnover rate of [3H]vitamin D2 is 1.5 times faster than that of [3H]vitamin D3. Plasma turnover of the 25-hydroxylated metabolites differed, with [3H]25-dihydroxyvitamin D2 clearing faster (11 times) than [3H]25-dihydroxyvitamin D3. The largest difference appeared in the 1,25-dihydroxyvitamin D2 turnover rates with 1,25-dihydroxyvitamin D2 clearing approximately 33 times faster then [3H]1,25-dihydroxyvitamin D3. These data, therefore, indicate that discrimination against vitamin D2 sterols in the chick occurs primarily between steps in the metabolism of vitamin D and not at the point of metabolism or excretion of the parent vitamin.
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