Vitamin A is a fat-soluble antioxidant that’s essential for vision, healthy skin, bone growth, fetal development and gene expression. But vitamin A is available in several forms, and retinol is one of those forms.
Definition
What people call “vitamin A” is really a general term for a group of related compounds. The type of vitamin A people eat depends on the source. Plants contain vitamin A in the form of carotenoids, such as lycopene, lutein and zeaxanthin. Animal sources provide a form called retinyl palmitate, which is easily converted to retinol. Retinol is one of the most active forms of vitamin A in the body.
Function
Retinol is the form of vitamin A specifically needed to maintain eye health. It travels to the retina, where it begins a chain of chemical reactions that result in vision. The Linus Pauling Institute notes that it is anti-infective because retinol maintains healthy cells in the skin and in the tissues lining the nose, and they are the body’s first defense against invading germs and viruses. Retinol is also needed to ensure that a fetus develops properly.
Measurement
The various forms of vitamin A are not equally potent. They’re absorbed and used by the body at different rates. The plant form of vitamin A called beta-carotene is not absorbed as easily as retinol, and once digested, it must go through a more complicated process to become retinol. For this reason, foods are given a “retinol activity equivalent” that represents the amount of retinol that will ultimately be available to the body no matter what the source. You will also see the older standard of measurement called the international unit, or IU. One IU equals 0.3mcg of retinol.
Sources
The recommended daily intake for vitamin A is 2,300 IU/day for women and 3,000 IU/day for men. The best animal source of vitamin A is beef liver. A 3 oz. serving has 22,175 IU. One chicken liver has 2,612 IU, 1 cup of milk contains 478, one egg has 335 and ½ fillet of fish provides 285 IU. Supplements provide retinol in the form of retinyl palmitate or retinyl acetate.
Warnings
Too much retinol may cause a build-up of vitamin A because it's quickly absorbed and stored in the body. Toxicity is rare and seldom occurs from plant sources of vitamin A, but your risk increases if you're getting enough retinol from animal sources and also taking supplements. Toxicity usually only occurs after long-term consumption of vitamin A in excess of 10 times the RDA, according to the Linus Pauling Institute. Do not consume more than 3,000mcg, or 10,000 IU, daily according to standards determined by the Institute of Medicine. Symptoms of toxicity include nausea, headache, dizziness, loss of appetite, dry skin and fatigue.
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