The effects of different vitamin D-states on intestinal absorption of vitamin D3 and its metabolites in rats.

Rats were kept on a diet deficient in vitamin D for 2 months and were then randomly assigned to one of three groups with different supplies of vitamin D for 9 days. At the end of this period [3H]-cholecalciferol (5 microCi) was administered intragastrically and the serum radioactivity was recorded after various periods of time. The animals were kept in metabolic cages and urine and faeces were collected. After 3 days the animals were killed and the liver, kidney and fat tissue were investigated for radioactivity. The radioactivity was separated into different fractions of vitamin D by means of chromatography. The animals with vitamin D deficiency displayed higher levels of serum radioactivity, which were to a great extent confined to the fractions of the more polar metabolites (25(OH)D3 and 1,25(OH)2D3). There was significantly less radioactivity in the 3-day faecal collection from these animals. In general, there was a low urinary excretion of radioactivity. In the rats with sufficient vitamin D, most of the radioactivity remained as the parent substance and was also detected in increased amounts in the liver, kidney and fat tissue. The results suggest a discriminatory enterohepatic cycling of vitamin D and its more polar metabolites leading to an increased faecal loss of the pro-hormone in animals with a vitamin D surplus. It is proposed that the selectively effective enterohepatic reabsorption of the different metabolites may serve as a protective mechanism when body stores of vitamin D are overloaded.