Absorption and metabolism of labelled vitamin D3 in elderly subjects

: This study involved three adult men and six elderly individuals (3 men and 3 women). Adult subjects served as the control group. All subjects were given one dose of 10 microCi of tritiated vitamin D3 (approximately 15 IU) per os. Blood samples were taken 5 min., 1 h, 2 h, 3 h, 4 h, 6 h, 8 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h, 168 h and 192 h after administration of vitamin D3. The concentrations of tritiated vitamin D3 and 25-hydroxyvitamin D3, its principal metabolite, were measured in each sample after extraction and purification by high performance liquid chromatography (HPLC). In addition, two adult subjects and three elderly women subjects were given an identical dose of tritiated vitamin D3 approximately 10 days following the end of the initial experience. Blood samples were collected at the times indicated above and analyzed in identical fashion. The percentage of intestinal absorption of vitamin D3 and the amount of vitamin D3 metabolized to 25-hydroxyvitamin D3 were calculated as were the time constants for vitamin D3 and 25-hydroxyvitamin D3 disappearance curves. Review of these parameters confirmed that significant individual variations exist for intestinal absorption of vitamin D3. Study results did not furnish any evidence of a decrease in 25-hydroxylation of vitamin D3, nor any increase in the rate of disappearance of vitamin D3 or 25-hydroxyvitamin D3 which might explain the very low levels of 25-hydroxyvitamin D3, often found in aged persons.