To study the metabolism of vitamin D
3 in dairy cows, the serum levels of vitamin D
3 metabolites were determined in healthy dairy cows raised on normal ration. Newborn, young and adult cows and vitamin D
3 administrated adult cows were used in the experiment. Serum levels of 1α, 25-dihydroxyvitamin D
3 and 24(R), 25-dihydroxyvitamin D
3 in the newborn calves were higher compared with those in young and adult cows, whereas serum 25-hydroxyvitamin D
3 levels was not changed in those three groups. On the other hand, daily administration of 10, 000 IU vitamin D
3 to adult cows resulted in the increases of serum 25-hydroxyvitamin D
3 and 24(R), 25-dihydroxyvitamin D
3 levels. However, serum 1α, 25-dihydroxyvitamin D
3 levels in those cows were not different from those in adult cows which did not receive vitamin D
3. Whereas single administration of 150, 000 IU vitamin D
3 showed a slight increase in serum 25-hydroxyvitamin D
3 and 24(R), 25-dihydroxyvitamin D
3, and a significant increase in serum 1α, 25-dihydroxyvitamin D
3. Correlation between serum vitamin D
3 metabolites and serum Ca, P or Mg concentration was not observed. However, correlation was observed between 24(R), 25-dihydroxyvitamin D
3 and 25-hydroxyvitamin D
3. Serum 1α, 25-dihydroxyvitamin D
3 was maintained relatively constant level, except for newborn calves and adult cows treated with 150, 000 IU vitamin D
3. However, serum levels of 25-hydroxyvitamin D
3 and 24(R), 25-dihydroxyvitamin D
3 vatied in a certain degree directly proportional to intake of vitamin D
3.
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