The form of vitamin A found in foods of animal origin is called retinol, or pre-formed vitamin A. It's the most active form, and your body can convert it into retinal or retinoic acid, two additional active forms of vitamin A, if needed. Sources of retinol include dairy products, eggs, liver and fortified foods.
The forms of vitamin A found in plants are called provitamin A carotenoids. The best known carotenoids are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Your body converts these carotenoids to retinol, and they are also thought to be antioxidants that may protect your body from free-radical damage. The best sources of carotenoids are brightly colored and dark green vegetables including carrots, spinach, and kale, and fruits like apricots, papaya and mango.
Because vitamin A is an important component for normal vision, vitamin A deficiency will cause night-blindness, or a decreased ability to see in dim light. Another symptom of vitamin A deficiency is diminished immune system function, which means your body will have difficulty fighting infections.
Vitamin A deficiency due to poor diet is rare in developed countries and is more likely to result from inflammatory diseases that damage the digestive tract and prevent absorption of vitamin A, such as Crohn's disease and celiac disease. Alcoholism, zinc deficiency and pancreatic diseases can also affect the amount of vitamin A in the body.
If you have vitamin A deficiency symptoms, you need to see a health care provider who can order blood tests to determine if a vitamin A deficiency is the problem or if there are other causes.
Getting Too Much Vitamin AYou can get too much vitamin A if you're not careful. Vitamin A is one of the fat-soluble vitamins, which your body can store, so supplemental vitamin A is rarely needed and can actually be dangerous. The tolerable upper limit of vitamin A in adults in 10,000 micrograms. Consuming very large amounts of vitamin A supplements can result in headaches, dizziness, nausea, vomiting and visual problems. Hypervitaminosis A is a condition that occurs when your body builds up too much vitamin A over time and can lead to liver problems, weakened bones and birth defects.
The carotenoids are sold as dietary supplements with the notion they will function as antioxidants, and they are considered to be safer than preformed vitamin A supplements because the body will slow down the conversion from carotenoid to vitamin A as the body stores fill. However, large doses of carotenoids will give the skin an orange color. Research has shown mixed results on the safety and efficacy of carotenoid supplements, so it's probably best to get your carotenoids from a healthy diet instead.
Office of Dietary Supplements, National Institutes of Health. "Dietary Supplement Fact Sheet: Vitamin A and Carotenoids." Accessed December 14, 2010. http://ods.od.nih.gov/factsheets/vitamina.