Absorption of Ca and P by active transport in the small intestine is regulated by calcitriol, which is the active form of vitamin D (1,25-dihydroxycholecalciferol) and the hormones calcitonin and PTH. One-alpha-hydroxycholecalciferol (1-α-OH-D₃) and 25-hydroxycholecalciferol (25-OH-D₃) are vitamin D metabolites that may be added to diets for pigs. Because 1-α-OH-D₃ is already hydroxylated at the 1-position, only the first hydroxylation in the liver at the 25-position is needed to convert the metabolite to calcitriol. Likewise, because the 25-OH-D₃ is already hydroxylated at the 25-position, only the second hydroxylation in the kidney at the 1-position is needed if this metabolite is used. It is possible that supplementation of diets with 25-OH-D₃ or 1-α-OH-D₃ increases absorption and retention of Ca and P by increasing the conversion efficiency to calcitriol compared with the conversion of cholecalciferol to calcitriol. It is possible that the effects on Ca and P balance differ between 25-OH-D₃ and 1-α-OH-D_3­, but research to test this hypothesis has not been reported.