Species Differences to Vitamin D, and D3
It was early established that, vitamin D2 and D3 showed about 40,000 USP units of vitamin D activity per milligram by the rat assay procedure. It was also found that the chick did not respond to a certain number of rat units of D2 as it did to the same number of units of D3' In fact, about 100 times the number of units ofD2 as ofD3 were found to be necessary to produce equal bone ash in assay chicks. Human beings do not show such a variation in response to the two forms of vitamin D. Different response is questioned by many. Jeans and other workers, however, felt that D3 by oral administration is about 1.5 times as active as D2 in human beings. Other forms of the vitamin have not been studied in man.
