Metabolism of Vitamin D3-3H in Human Subjects: Distribution in Blood, Bile, Feces, and Urine*

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Abstract
Vitamin D3-3H has been administered intravenously to seven normal subjects, three patients with biliary fistulas, and four patients with cirrhosis. Plasma D3-3H half-times normally ranged from 20 to 30 hours. in vivo evidence that a metabolic transformation of vitamin D occurs was obtained, and a polar biologically active vitamin D metabolite was isolated from plasma. Urinary radioactivity averaged 2.4% of the administered dose for the 48-hour period after infusion, and all the excreted radioactivity represented chemically altered metabolites of vitamin D. The metabolites in urine were mainly water-soluble, with 26% in conjugated form. From 3 to 6% of the injected radioactivity was excreted in the bile of subjects with T-tube drainage and 5% in the feces of patients having no T-tube. The pattern of fecal and biliary radioactivity suggested that the passage of vitamin D and its metabolites from bile into the intestine represents an essential stage for the fecal excretion of vitamin D metabolites in man. Abnormally slow plasma disappearance of vitamin D3-3H in patients with cirrhosis was associated with a significant decrease in the quantity and rate of glucuronide metabolite excretion in the urine.

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