Vitamin A occurs in several different forms. Retinol is one form Vitamin A. This is converted to retinal or to retinoic acid, for use in the body. But it is also stored in the liver, for later use, as a retinyl ester. Animal sources of Vitamin A are generally the retinyl ester.
Beta-carotene and related compounds (called carotenoids) can be converted into Vitamin A by the body. This process is efficient, so a microgram of any carotenoids in the body is essentially equivalent to a microgram of Vitamin A in the body. However, the process of absorption of carotenoids from sources in the diet or from supplements is inefficient. And so a much larger quantity of beta-carotene (and/or other carotenoids) is needed in the diet to give the same healthful effect as retinol. However, carotenoids also have a much lower toxicity when taken in excess than retinol and its related forms.
The recommended amount of Vitamin A can be met by any of the above sources of Vitamin A (retinal, or beta-carotene, or any of the related compounds). However, a given amount of beta-carotene is not absorbed by the body as well as the same quantity of retinol. So a system has been devised to express the amount of each compound needed to equal an amount of retinal: RAE, which stands for retinol activity equivalents.
1 microgram (one millionth of a gram) of retinol is equal to 1 RAE. But for the carotenoids, either 12 times or 24 times as much is needed to have the same healthful effect on the body as retinol.
To obtain one RAE (retinol activity equivalent), you need:
2 mcg of beta-carotene in a supplement
12 mcg of Beta-carotene in the diet
24 mcg of Alpha-carotene in the diet
24 mcg of Beta-cryptoxanthin in the diet
All of the above compounds are types of carotenoids. Supplements (pills for providing beta-carotene) are absorbed by the body more effectively than the same carotenoids as found in foods.
The term IU for International Units is an older standard for measuring the effectiveness of various forms of Vitamin A.
1 IU retinol = 0.3 mcg RAE
1 IU beta-carotene from dietary supplements = 0.15 mcg RAE
1 IU beta-carotene from food = 0.05 mcg RAE
1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE
The U.S. Recommended Daily Allowance of Vitamin A can be met by retinol or beta-carotene or by any of the other forms of those compounds, singly or in combination. This searchable USDA Nutrient Database allows you to easily look up the total RAE of all the retinol and/or carotenoids in many different foods. The value listed as “Vitamin A, RAE” is the total for all of the Vitamin A related compounds in the food. The value listed as “Vitamin A, IU” is the same value in International Units (so it is 20 times the value listed for RAE).
For infants from birth to one year, there is no U.S. RDA. Instead, a value is listed for ‘Adequate Intake’ based on the average intake of Vitamin A in breast milk for healthy mothers and their infants. Amounts of Vitamin A are listed as mcg of RAE per day.
birth to 6 months: 400
6 months to 12 months: 500
1 through 3 years old: 300
4 through 8 years old: 400
9 through 13 years old: 600
14 through 18 years old: males, 900; females, 700
adults over 18 years old: males, 900; females, 700
For pregnant women –
14 through 18 years old: 750
19 through 50 years old: 770
For lactating women –
14 through 18 years old: 1200
19 through 50 years old: 1300
Institute of Medicine, Dietary Reference Intakes for Vitamin A;
Institute of Medicine, Dietary Reference Intakes: The Essential Guide to Nutrient Requirements;
Wikipedia, Vitamin A.