Vitamin A, a fat-soluble vitamin, plays an important role in bone growth, reproduction, immune function, hormone synthesis and regulation, and vision. Your eyes need vitamin A to help them convert light into brain signals that allow you to perceive images. Vitamin A works to protect you against infection by helping create healthy white blood cells and by promoting healthy skin. Vitamin A helps cells divide and develop into specialized cells, like blood cells, lung cells, brain cells and other distinct tissues.
Preformed vitamin A (a.k.a. retinol) is found in foods of animal origin, such as liver, eggs, milk fortified with vitamin A, fortified cereals and fish. Green leafy vegetables and orange vegetables and fruits supply vitamin A in the form of carotenoids. To easily meet your recommended intakes, the Dietary Guidelines for Americans suggests 3 cups per week of dark leafy green vegetables and 2 cups per week of orange vegetables.
One of the first detectable signs of vitamin A deficiency is night blindness or the inability to see in dim light. Common in developing countries but rare in the United States, night blindness is a reversible condition that responds rapidly to treatment. However, if the deficiency becomes more severe, it develops into an irreversible condition termed xerophthalmia, which is a drying of parts of the eye that will eventually result in blindness. Other effects of vitamin A deficiency include decreased immune function; dry, scaly skin; problems with reproduction; and cessation of bone growth.
Since vitamin A is a fat-soluble vitamin, it can be stored in the body. Thus, a healthy adult who stopped eating good food sources of the vitamin might not see deficiency symptoms until these stores were depleted—possibly for several months.
High intakes of beta carotene from foods have not been shown to be toxic in humans, but may cause an unwanted side effect: yellow (or orange) skin. Fortunately, the beta-carotene tinge is harmless and only temporary. High doses of preformed vitamin A from animal foods or supplements can cause both acute (short-term) or chronic toxicity. Signs of short-term vitamin toxicity include nausea, vomiting, headache, dizziness and blurred vision. Chronic toxicity (due to ingestion of large doses for months or years) is often characterized by dry itchy skin, loss of appetite, headache, birth defects, liver abnormalities, central nervous system disorders, and bone and joint pain. Getting too much preformed vitamin A may also result in reduced bone mineral density, which can lead to osteoporosis.
First, you need to understand that you can get vitamin A from animal or plant sources and that these sources are treated differently by the body. Vitamin A from animal foods, supplements and fortified foods is called “preformed vitamin A,” or retinol. Preformed vitamin A is in a form that is highly available to your body. Vitamin A from plant sources comes in the form of carotenoids, primarily beta carotene. To account for the absorption of different forms of vitamin A, scientists created a unit called retinol activity equivalent (RAE). For example, 12 micrograms (mcg) of beta carotene equals 1 mcg of retinol. To complicate matters, nutrition and supplement labels don’t use RAEs; instead they use International Units (IU). We’ve done the conversions for you in the chart below.
The following table lists the recommended intake for healthy people based on current scientific information and accounts for the fact that sources of vitamin A are absorbed by the body differently.