… the ‘International Unit’ (IU), a unit of measurement for the amount of a substance (e.g., vitamins) based on measured biological activity, is not part of the International System of Units used in physics and chemistry?
The precise definition of one International Unit (IU) differs from substance to substance and is established by international agreement. There is no equivalence among different substances: e.g., one IU vitamin E does not contain the same number of milligrams as one IU vitamin A, vitamin C or vitamin D.