What Does "IU" Mean in Vitamins?

What Does "IU" Mean in Vitamins?
Last Updated: Jan 24, 2014 | By Rebecca Slayton
http://www.livestrong.com/article/408282-what-does-iu-mean-in-vitamins/
An international unit, or IU, is used to measure fat-soluble vitamins. Vitamins A, D and E are measured using international units to calculate recommended intakes, deficiencies and toxicities for healthy people. Recommendations vary based on age and gender.
Dietary Reference Intakes
Dietary reference intakes, or DRIs, are used to assess the nutrient intakes of healthy individuals. Recommended dietary allowance, or RDA, is the daily average intake that is sufficient to meet the nutrient requirements of healthy people. If an RDA cannot be established due to insufficient evidence, an adequate intake, or AI, is set to assure nutrient adequacy. Tolerable upper intake level, or UL, is the maximum daily intake of a nutrient that can cause adverse health effects.
Vitamin A
Vitamin A in your body is responsible for healthy vision, reproduction and cellular communication. You obtain it through diet in two forms: retinols and carotenoids. All dietary sources of vitamin A are converted to retinol in the body. Since different compounds of vitamin A vary in their bioactivity, they are measured using retinol activity equivalents, or RAEs. You must know the source of vitamin A to correctly convert it to international units. One international unit of vitamin A is equivalent to 0.3 micrograms of retinol, 0.15 micrograms of beta carotene from dietary supplements, 0.05 micrograms of beta carotene from food and 0.025 micrograms of alpha carotene or beta cryptoxanthin. This means the RDA for vitamin A of 3,000 IU intake for adults is equivalent to 900 micrograms in RAE.
Vitamin D
You can obtain vitamin D from food, supplements and sun exposure. Vitamin D must undergo two chemical reactions in your body before your body can use it. When vitamin D enters the body, it is converted to calcidiol by the liver. The kidneys then convert calcidiol to calcitriol. Vitamin D is crucial for bone growth and maintenance since it promotes calcium absorption. It also aids in cell growth, reduction of inflammation and neuromuscular and immune functions. The RDA for male and female adults is 600 IU of vitamin D daily.
Vitamin E
Vitamin E is found naturally in foods such as seeds, nuts, leafy greens and vegetable oils. Fortified foods, like cereal, also contain vitamin E. Vitamin E is an antioxidant that helps to prevent or delay chronic diseases caused by free radicals. While vitamin E naturally occurs in eight different forms, alpha-tocopherol is the only form used for referencing human requirements. Men and women should get 22.4 IU of vitamin E, according to the RDAs.