Vitamins are organic substances, meaning they consist of carbon, hydrogen and oxygen -- all necessary for life. The human body needs 13 essential vitamins, meaning they are vitamins the body cannot produce and must get from foods. The nutrition labels found on foods tell you the percentage of the daily recommended amount of the vitamin it contains. This is based on the recommended daily intake, set by the National Institutes of Medicine. Although the recommended daily intake for most vitamins uses a measure of mass or weight, such as milligrams or micrograms, some vitamins like vitamin E, A and D are measured in international units, or IU.
What is IU?
International unit is a measurement commonly used in pharmacology. It describes the amount of a particular substance based on the biological activity of the substance. A committee formed by the World Health Organization for Biological Standardization arbitrarily sets the international unit for each substance. Once set, the IU level of that particular substance is internationally accepted.
Use of IU
Any substance that induces biological activity can be assigned an IU. This includes vitamins, minerals, hormones, medications, vaccines and blood products. The vitamins that use IU occur in several different forms that all induce varying levels of biological activity. Using IU for dosing provides a way to normalize and quantify these similar forms of the vitamin so that the consumer knows that, no matter which form exists in their supplement, they are getting the same effects. Because the IU value is arbitrarily set for each vitamin, this unit cannot be used to compare two different vitamins; meaning that 1 IU of vitamin E is not equal to 1 IU of vitamin A.
Vitamin E
Vitamin E exists in eight different chemical forms alpha-, beta-, gamma-, and delta-tocopherol and alpha-, beta-, gamma-, and delta-tocotrienol. Natural food sources of vitamin E may contain any of these forms. Once absorbed through the small intestine, the liver takes up the various forms of vitamin E but only re-secretes alpha-tocopherol, making this form the most important for humans. Vitamin E supplements usually contain only alpha-tocopherol, but some contain a mix of the other forms. Because the majority of vitamin E supplement labels provide the dosing information in IU, the National Institutes of Health Office of Dietary Supplements provides the recommended daily intake for vitamin E in both mg and IU. Adolescents and adults older than the age of 14 need 15 mg of vitamin E, or 22.4 IU per day.
Vitamin A
Vitamin A describes a group of compounds including preformed vitamin A found in animal based foods and provitamin A carotenoids found in plant-based foods. Preformed vitamin A, known as retinol, is the active form of vitamin A. Previtamin A carotenoids include the plant substances beta-carotene, alpha-carotene and beta-cryptoxamthin which get converted into retinol in the body. Other forms of vitamin A include lycopene, lutein and zeaxanthin. Because each form of vitamin A promotes different levels of biological activity in the body, the daily intake and supplement labels use IU instead of micrograms. The National Institutes of Health Office of Dietary Supplements lists the daily recommended intake for females older than the age of 14 as 2,310 IU and males older than 14 as 3,000 IU.
Vitamin D
Vitamin D exists in two main forms; vitamin D-2 known as ergocalciferol and vitamin D-3 known as cholecalciferol. Vitamin D-2 comes from plant sources while vitamin D-3 comes from animal sources. In synthetic supplements, manufacturers make vitamin D-2 through the ultraviolet irradiation of yeast while they create vitamin D-3 by the irradiation of lanolin -- oil from sheep. These two forms differ slightly in their chemical structure and because they induce different levels of activity in the body the dosing for vitamin D occurs in IU rather than micrograms. The National Institutes of Health Office of Dietary Supplements specifies that children and adults between the ages of 1 and 70 require 600 IU of vitamin D per day. Those older than 70 should increase their intake to 800 IU per day to support the increased need for vitamin D and calcium to keep bones strong.
References
- National Institutes of Medicine; Food and Nutrition Board: Daily Recommended Intake Summary Listing; 2004
- National Institutes of Health Office of Dietary Supplements: Vitamin D; February 2011
- National Institutes of Health Office of Dietary Supplements: Vitamin E; December 2009
- National Institutes of Health Office of Dietary Supplements: Vitamin A; April 2006
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