Plasma levels of 25-hydroxyvitamin D₃ (25-OH-D₃) and 25-hydroxyvitamin D₂ (25-OH-D₂) in 758 Japanese healthy subjects (most of them adults) were determined by a high-performance liquid chromatographic method previously reported (6) and the following results were obtained:
1) The mean and standard deviation (M±SD) of the assayed values of 25-OH-D (sum of 25-OH-D₃ and 25-OH-D₂) was 23.8±10.1 ng/ml.
2) 25-OH-D₃ was detected in all the samples and the M±SD was 23.0±10.1 ng/ml. The plasma levels clearly showed the seasonal variation that the levels in summer were significantly higher than those in winter. Moreover, the plasma levels were significantly correlated with the amounts of UV light in solar radiation. These results strongly suggested that 25-OH-D₃ in plasma mainly originated from endogenous vitamin D₃ formed by photo-conversion of 7-dehydrocholesterol in skin.
3) 25-OH-D₂ was detected only in 18.3% of the plasma samples and the M±SD in the detected samples was 4.4±2.9 ng/ml which was much lower than those of 25-OH-D₃. The results suggested that few healthy Japanese are taking daily exogenous vitamin D₂ from multivitamin preparations or others.
4) The M±SD values of 25-OH-D₃ plasma levels in men and women were 26.2±10.4 and 19.3±8.0 ng/ml, respectively. The formers were significantly higher than the latters. The results were thought to be due to the reason that men might be outdoors for longer periods than women.
5) When age variation of plasma 25-OH-D₃ levels was examined, the levels in the twenties were significantly lower than the other generations. This was confirmed to be due to the low values observed in the female twenties group, but the detailed reason is unclear at the present time.
6) When 4 healthy volunteers were orally administered 400 I.U./day of vitamin D₂ every day for 8 weeks, maximum levels (average: 11.5 ng/ml) were observed at the 8 weeks and the levels gradually decreased after stopping the administration. The results suggested that the half life of 25-OH-D₂ in plasma might be 4-5 weeks.