The variety of different measurements used for vitamins and other nutrients can make interpreting nutrition information difficult. The IU, or international unit, provides a standard unit of measure for individual vitamins that applies in all countries, making nutrition information easier to understand.
IU
Each vitamin or biologically active substance has a specific, internationally agreed upon definition of one IU, according to the USDA. The definition of 1 IU differs from nutrient to nutrient. In other words, 1 IU of vitamin B12 does not contain the same number of milligrams as 1 IU of folate. Some vitamins exist in multiple forms, each of which the body absorbs differently. For example, the body can convert retinol or beta-carotene to vitamin A. You can get 1 IU of vitamin A from 0.3 mcg of retinol or 0.6 mcg of beta-carotene.
Considerations
Rather than simply measuring the amount of a nutrient in weight, the IU measures the effectiveness of a certain quantity or preparation of that nutrient. Any dose of 1 IU of a given nutrient should produce a consistent biological effect. Conversion factors for converting from IU to mg or mcg vary from nutrient to nutrient. For some nutrients, no definite conversion exists between international units and units of mass because different preparations of the substances may vary in their biological effect.
Other Common Measurements
Nutrition labels often express nutrient quantities in metric units of mass such as micrograms and milligrams: 1,000 micrograms, or mcg, equal 1 milligram, or mg. Dietary guidelines often express recommended daily intakes in milligrams and micrograms as well.
RDA and AI
RDA stands for recommended dietary allowance, a dietary guideline established by the Institute of Medicine. The RDA indicates the average daily dietary intake levels needed for 97 to 98 percent of healthy individuals to meet their nutritional needs. The Institute of Medicine bases RDA values on scientific evidence. If insufficient scientific evidence exists to establish a recommended dietary allowance for a particular nutrient, the Institute of Medicine develops an adequate intake, or AI, value. AI values represent the mean intake for breastfed infants and should cover the nutritional needs of all individuals in later life stage and gender groups. A lack of scientific evidence prevents the USDA from establishing a percentage of individuals covered by the AI value.
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