IU (International Unit) in pharmacology is a quantity of a substance such as vitamins, hormones, mineral elements, toxins, that produces a specified effect when tested according to an internationally accepted biological procedure.
When you deal with commom units, let's say using the Metric System, you say 1 liter of water, or 1 liter of milk, or 1 liter of oil, all 3 volumes will be the same even though their weights will be different due to density difference. The same applies when you speak of 1 kg of sugar, 1 kg of salt or 1 kg of cheese. But when we speak of International Units (I U) in Pharmacology, these units are determined based at the biological activities. For example, let's take some vitamins:
Vitamin A 1 IU: all-trans retinol: 0.300 micrograms
Vitamin D 1 IU: Vitamin D crystalline: 0.025 micrograms
Vitamin E 1 IU: dl-alphatocopherol acetate(s.): 1 milligramm
The I U is analogous with the United States Pharmacopeia unit (USP) that also is based on measured biological activities. The use of IU is mostly connected with nutrition and nutritional daily needs or requirements for human species and also for animals and all kind of living organisms, aiming to provide one adequate diet that will include the needed amount of a substance, mineral, factor, essencial to life. So, if a specified factor is suspected of being essential for the growth or maintenance of an organism, a systematic series of procedures at the biological level must be used to determine its function, effects of deprivation, and quantitative requirements in various organisms. The results will then be expressed in terms of I U of these factors. For extention the IUs are used to characterize the amount of substances that are lethal, etc.
Source: Argonne National Laboratory
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