1. The distribution of vitamin D and its metabolites in human tissues has been studied by the combined use of radioactive cholecalciferol and biological assays of antiricketic activity in tissue extracts.

2. Injected radioactive cholecalciferol was cleared rapidly from the blood; unchanged vitamin D and various metabolites were detected subsequently in all tissues examined. The highest concentration of biological activity and radioactivity was in fat; this and, to a lesser extent, other tissues were shown to retain activity over a prolonged period of time.

3. Adipose tissue and voluntary muscle are the principal sites of storage of vitamin D in man. Pre-existing tissue pools of vitamin D can, in some circumstances, invalidate the use of radioactively labelled cholecalciferol to trace the pattern of body distribution.

4. Metabolically produced 25-hydroxycholecalciferol is also taken up from the blood into many tissues, probably by protein-binding.

5. Vitamin D is excreted in bile principally as more polar metabolites. Smaller amounts of cholecalciferol and 25-hydroxycholecalciferol are also excreted, and antiricketic activity has been demonstrated in the bile of individuals treated with vitamin D.

6. An excess of cholecalciferol (or of 25-hydroxycholecalciferol) in the blood appears to be eliminated by the physicochemical processes of partition into tissue lipid and binding to tissue proteins. It is inferred tentatively that an increased concentration of vitamin D or of 25-hydroxycholecalciferol in the liver also causes an increased biliary excretion of these substances, and an increased hepatic formation and elimination of more polar metabolites of the vitamin.

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