Changes in the concentrations of vitamin D and its metabolites in plasma of healthy subjects orally given physiological doses of vitamin D
2 by multivitamin or vitamin D liquid preparations were determined and the bioavailability of vitamin D was studied. Separative assay on the D
2 and D
3 compounds of vitamin D, 25-hydroxyvitamin D (25-OH-D), 24R, 25-dihydroxyvitamin D [24, 25(OH)
2D], and 1 α, 25-dihydroxyvitamin D [1, 25(OH)
2D] was performed in plasma of eight healthy male volunteers. When the concentrations of vitamin D and its metabolites in plasma of volunteers were assayed after daily oral administration of 400 IU of vitamin D
2 in a form of multivitamin tablet for 1 week, the variations of vitamin D
3 and its metabolites in plasma levels were very small. In contrast, the concentrations of 25-OH-D
2 and 1, 25(OH)2D
2 slightly increased after the administration, while neither vitamin D
2 nor 24, 25(OH)
2D
2 was detected. A single dose of 4, 000 IU of vitamin D
2 was orally given to the volunteers in a form of a vitamin D liquid preparation and the hourly variations were observed during 24 h. These concentrations of vitamin D2, 25-OH-D
2, and 1, 25(OH)2D
2 were slightly higher than those of the repeated doses. The result suggests that even the high dose of 4, 000 IU has little effect on the plasma levels of vitamin D
2 and its metabolites by a single dose, indicating a low risk for hypervitaminosis D.
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