Studies of recent years have shown beyond a doubt that there are several chemically distinct forms of vitamin D. Nevertheless, this vitamin is still spoken of in the singular, chiefly as a convenience and because its multiple nature was not recognized for about a decade after the first form was discovered. Custom limits the application of the term vitamin D to antirachitic sterol derivatives and to the unidentified antirachitic components of fish oils and other foods which are supposed to belong to this group.
Of the several forms of vitamin D which have been recognized to date, two are known to be of prime importance in medicine. These are activated ergosterol and activated 7-dehydro-cholesterol. Other forms undoubtedly contribute to the total antirachitic effectiveness of certain agents, and it may be that the importance of some is greater than now appears. But for practical purposes, one may consider that all antirachitic