I wrote to my brother, Dr. Kenneth King. The doctorate is in chemistry and he is in charge of Global Research and Development for a major drug company. Though you might want to have some idea of the credentials of the person answering this question. Hope it helps. Answer to your question. "There may be a conversion factor but it is certainly different for each type of substance measured in that manner, e.g. Vitamin A, Vitamin E and Growth Hormone [as well as many other hormones]. The mg is, as I'm sure you know, a measure of mass [On the surface of our planet mass and weight - a measure of the force of gravity - are nearly equal]. However, IU [international units] are a measure of potency. It is determined against a standard in a biological assay. If all substances were equally pure, then there would always be the same correlation between the IU and mgs. The potency assay was developed because when these substances were first being isolated, it was not possible to determine their purity with any degree of accuracy, so the mass was not particularly relevant. It was useful to determine relative purity, if I isolated 1 mg of material with 500 IU of potency, and you isolated 1 mg of material with 800 IU of potency, you had a more pure material than I did. We still didn't know how pure is absolute terms, only that you had done a better job than I did. Many substances of biological origin are still not 'pure' substances and therefore IU is the more meaningful number. For some - such as synthetic Vitamin A and E, where we know the structure and its absolute purity, it is not needed. We keep it anyhow because people seem to be familiar with it and 400 IU of Vitamin E means something to folks while 4.87 mg doesn't. Probably more than you wanted to know, but maybe not." Ary M. Anderson Macon-Piatt Regional Office of Education 1690 Huston Drive Decatur, IL 62526 217.872.3729 217.872.0239 fax [EMAIL PROTECTED] -- This is the ISTA-talk mailing list. To unsubscribe: <mailto:[EMAIL PROTECTED]> For more information: <http://www.ista-il.org/about/mail_list.html> To search the archives: <http://www.mail-archive.com/ista-talk@lists.csi.cps.k12.il.us/>
