One more time, THE PRACTICE of expressing the strength of a vitamin preparation in "units" is a relic t of the days before isolation of the pure chemical substance. Units represent the first approximation attempts at dealing quantitatively with the new factor in question, and were a convenient device to express the activity of a preparation containing an unknown amount of an uncharacterized material. The availability of crystalline vitamins made it possible and desirable to work in terms of milligrams or micrograms of the pure substance. The accuracy of any assay is enhanced when a standard product is available for comparison. Standard vitamin preparations were first made available by the Permanent Commission on Biological Standardization of the Health Organization of the League of Nations and distributed through National Control Centers to qualified investigators. Its functions have been assumed by the Expert Committee on Biological Standardization of the World Health Organization. These standards are widely used for research and to prepare work ing standards for use by manufacturers and enforcement agencies. International standards have been established for vitamins A, B1, C,D and E. An "International Unit" is, by definition, based on the activity of an International Standard preparation. The United States Pharmacopoeia Convention has been designated as the Control Center for the United States. Its Committee on Reference Standards has extended the work of the original Commission until USP standards for ascorbic acid, calcium pantothenate, choline chloride, nicotinamide, nicotinic acid, pyridoxine hydrochloride, riboflavin, thiamine hydrochloride, eight "essential" amino acids, vitamin A and vitamin D (forms for both rat and chick assay) are now available. (Address USP Reference Standards, 46 Park Ave., New York 16, N. Y.) When an International Standard exists, the U.S.P. standard is compared and brought as closely as possible into agreement. Therefore, a "USP unit" is equal to an "International Unit." The existence of adequate standards has proved a boon, both to research workers and to Federal and state control officials. Vitamin A 1 I.U. = 1 USP unit the vitamin A activity of .344 micrograms of crystalline vitamin A acetate, corresponding to .30 micrograms of vitamin A alcohol. Provitamin A: beta-carotene 1 I.U. = 0.6 micrograms of beta-carotene. The International Standard is a solution in vegetable oil, containing 200 I.U. per gram. Vitamin D 1 I.U. = 1 USP unit= the vitamin D activity of 0.025 micrograms of crystalline vitamin D3. Products assayed against this new standard (official May 1, 1951) will have only 93.4 per cent of the unit dosage? shown when assayed against the last USP Cod Liver Oil Reference Standard. An even greater difference is shown when the new USP Standard is used for assay of chicken feeds. Products assayed against the new standard show about 75 per cent of the unitage shown when assayed against the last USP Cod Liver Oil Reference Standard. In order to differentiate between the present A.O.A.C. Chick Unit, and the new Unit which is 33 per cent larger, the term "International Chick Unit" has been adopted. Other vitamin units The activity of other vitamins is expressed as the weight in milligrams or micrograms of the various chemically pure materials. One still occasionally encounters values for vitamins other than A and D listed in "units". For reference use, we list these below. Thiamine: 1 I.U. = the thiamine activity of 3.0 micrograms of crystall.ne thiamine hydrochloride. Ascorbic acid: 1 I.U. = the vitamin C activity of .05 mg. of 1-ascorbic acid. Vitamin E: 1 I.U. = the vitamin E activity of 1 milligram of synthetic, racemic alphatocopherol acetate. -- This is the CPS Science Teacher List. To unsubscribe, send a message to <[EMAIL PROTECTED]> For more information: <http://home.sprintmail.com/~mikelach/subscribe.html>. To search the archives: <http://www.mail-archive.com/science%40lists.csi.cps.k12.il.us/>
