A fat soluble vitamin (can be stored for later use), vitamin A is split into two groups: retinol (the active form of vitamin A) and carotenoids (present in plants and can be turned into a form of vitamin A). These carotenoids are beta-carotene, alpha-carotene, and beta-cryptoxanthin.
Vitamin A benefits
Vitamin A is primarily responsible for maintaining healthy skin, teeth, bone growth, soft tissue, mucus membranes, various cellular functions, and reproduction. It is also essential in aiding the body with cell differentiation (the process in which the body decides if a cell should become part of the brain, muscles, lungs, etc.).
Vitamin A plays a substantial role in vision and the immune system. It helps the eye to adjust in low light situations and the beta-carotene present is an antioxidant which protects cells from free radicals. These free radicals are believed to contribute to diseases as well as the aging process.
Sources of vitamin A
There are two primary sources of vitamin A: animals and plants.
When it is found in animals, it is known as preformed vitamin A. This is absorbed by the body in the form of retinol, the active form of vitamin A and most widely used by the body. Animal products that include vitamin A are eggs, meat (beef, chicken, fish, liver), milk (skim milk must be fortified), and cheese. It should be noted that most of these sources can be high in saturated fat and cholesterol with the exception of skim fat.
Plant sources of vitamin A contain provitamin A carotenoid (beta-carotene) which is then made into retinol by the body. This process is not as efficient as directly absorbing the retinol via animal sources. The general rule of thumb is to look at the color of the fruit or vegetable. If it is bright yellow or orange like cantaloupe, apricots, mango, papaya, carrots, pumpkin, or sweet potatoes then it should have vitamin A. Dark green, leafy vegetables like broccoli and spinach are also strong sources of beta-carotene.
Studies conducted by U.S. Department of Health and Human sources have shown that most American receive adequate amounts of vitamin A throughout their daily food consumption. The charts below outline the recommended amounts in various age groups.
|Table 1: Recommended Dietary Allowances (RDAs) for vitamin A1|
|1-3||300 (1,000 IU)|
|4-8||400 (1,320 IU)|
|9-13||600 (2,000 IU)|
|14-18||900 (3,000 IU)||700 (2,310 IU)||750 (2,500 IU)||1,200 (4,000 IU)|
|19+||900 (3,000 IU)||700 (2,310 IU)||770 (2,565 IU)||1,300 (4,300 IU)|
mcg = micrograms
RAE = Retinol Activity Equivalents
|Table 2: Adequate Intakes (AIs) for vitamin A for infants1|
|Age (months)||Males and females (mcg RAE)|
|0-6||400 (1,320 IU)|
|7-12||500 (1,650 IU)|
These values have been established based upon the amount consumed by a healthy infant via breast milk.
It is possible to consume too much vitamin A (retinol). Sustained high storage levels of vitamin A is called hypervitaminosis and may result in various symptoms such as birth defects, liver problems, lowered bone density (osteoporosis), and issues with the central nervous system. Below is a chart defining the upper level limits one should not exceed for vitamin A (retinol). Studies have shown that large amounts of beta-carotene will not make you sick but could lead to the skin turning yellow or orange.
|Table 3: Tolerable Upper Intake Levels (ULs) for retinol1|
|Age (years)||Children (mcg RAE)||Males (mcg RAE)||Females (mcg RAE)||Pregnancy (mcg RAE)||Lactation (mcg RAE)|
|0-1||600 (2,000 IU)|
|1-3||600 (2,000 IU)|
|4-8||900 (3,000 IU)|
|9-13||1,700 (5,610 IU)|
|14-18||2,800 (9,240 IU)||2,800 (9,240 IU)||2,800 (9,240 IU)||2,800 (9,240 IU)|
|19+||3,000 (10,000 IU)||3,000 (10,000 IU)||3,000 (10,000 IU)||3,000 (10,000 IU)|
Vitamin A deficiency
A deficiency in vitamin A is more prominent in developing countries where adequate food sources can be scarce. The most common symptom of a vitamin A deficiency is night blindness where it is difficult to see in dark settings. Another symptom is a lowered ability to fight infection.
Aside from a poor intake of vitamin A, deficiencies may also occur in those who experience chronic diarrhea as well as high levels of alcohol consumption. One should consult a physician if they are prone to these two situations.
1Vitamin A and Carotenoids - http://ods.od.nih.gov/factsheets/vitamina/
Vitamin A - http://www.nlm.nih.gov/medlineplus/ency/article/002400.htm