Influence of dietary vitamin D₃ on the circulating concentration of its active metabolites in the chick and rat

Plasma concentrations of 25-hydroxyvitamin D₃ (25-OHD₃) and lα,25-dihydroxyvitamin D₃ (lα,25-(OH)₂D₃) in growing chicks and weanling rats were measured by a new radioreceptor assay to determine the effects of varying dietary levels of vitamin D₃. The plasma concentration of 25-OHD₃ fell from 14.1 ng/ml in 1-day-old chicks to undetectable levels after 3 weeks on a rachitogenic diet. Circulating lα,25-(OH)₂D₃ hormone also decreased from 8.9 ng/100 ml to undetectable levels at 3 weeks in these chicks. Chicks receiving an optimal supplement of vitamin D₃ (1.4 IU/g diet) for three to four weeks had plasma 25-OHD3 and lα,25-(OH)₂D₃ levels of 21–35 ng/ml and 5.1–7.5 ng/100 ml, respectively. Nutritional supplementation with a 50-fold excess of vitamin D₃ (70 IU/g diet) elicited a substantial increase in plasma 25-OHD₃ to 87–130 ng/ml, while plasma lα,25-(OH)₂D₃ was not increased. Increasing dietary calcium from 1.4 to 2.8% did not alter the circulating level of vitamin D₃ metabolites in chicks fed 1.4 IU of vitamin D₃/g diet. Direct measurement of the renal 25-OHD₃-lα-hyclroxylase, in vitro, showed that lowering dietary calcium or exclusion of vitamin D₃ stimulated the biosynthesis of lα,25-(OH)2D3, but raising calcium did not alter the enzyme activity. It is concluded that the circulating concentration of the lα,25-(OH)₂D₃ hormone in the chick is unaffected by abnormally high intakes of vitamin D₃ or calcium, but the renal production of the hormone increases during vitamin D₃ or calcium deprivation. Additional studies in rats fed a diet supplemented with either 2 or 1000 IU of vitamin D₃/g verify that the circulating concentration of 25-OHD₃ is markedly increased when the dietary intake of vitamin D₃ is elevated. Moreover, lα,25-(OH)₂D₃ is not increased under these conditions, but actually falls significantly when the dietary level of vitamin D₃ is raised from 2 to 1000 IU/g. These studies in both the chick and rat indicate that dietary vitamin D₃ excess enhances circulating 25-OHD₃ probably because the vitamin D₃-25-hydroxylase enzyme is not stringently controlled. The fact that the circulating lα,25-(OH)₂D₃ is not concomitantly increased may reflect either decreased synthesis or increased utilization of the lα,25-(OH)₂D₃ sterol.