Why do we need vitamin A?
Originally Published: March 28, 1997 - Last Updated / Reviewed On: June 25, 2012
Why do you need vitamin A?
Vitamin A is an essential, fat-soluble vitamin that has many diverse benefits for humans. Vitamin A promotes eyesight and helps us see in the dark; aids in the differentiation of cells of the skin (lining the outside of the body) and mucous membranes (linings inside of the body); helps the body fight off infection and sustain the immune system; and, supports growth and remodeling of bone. In addition, dietary vitamin A, in the form of beta carotene (an antioxidant), may help reduce your risk for certain cancers.
Adequate vitamin A intake is essential to human health. Vitamin A deficiency can lead to night blindness (inability to see in the dark or to recover sight quickly after being exposed to a flash of bright light in the dark) and xerophthalmia (progressive blindness that becomes irreversible if not treated in time with vitamin A).
Vitamin A deficiency can also reduce the health and integrity of skin and other epithelial tissues. The effect on skin can result in dry skin and hyperkeratosis (the development of clumps of skin around hair follicles). The effect on epithelial tissues can negatively affect the digestion and absorption of nutrients and cause infections of major systems and their organs (i.e., gastrointestinal, nervous/muscular, respiratory, and urogenital). In addition, bone growth can stop and normal bone remodeling can become impaired, resulting in anemia and weakened immunity.
The Recommended Dietary Allowance (RDA) for vitamin A is measured in retinol equivalents (RE), retinol being the active form of vitamin A. For adult men, the RDA is 900 micrograms of RE per day and for adult women it is 700 micrograms of RE per day.
Despite its benefits, too much Vitamin A can cause toxicity, the effects of which can vary depending on its source. Excessive intake of vitamin A in dietary form is not harmful, but will cause one's skin to turn yellow in color. In contrast, large dose supplements (10 - 15 times the RDA) of vitamin A (as retinol) is harmful, and could result in the development of a fatty liver (hepatomegaly), dry skin, nausea, vomiting, fatigue, weakness, headaches, anorexia, and/or possibly increase the risk of birth defects among pregnant women. Symptoms depend upon whether or not vitamin A intake was taken over a long period of time (chronic) or a single excessive dose at one point in time (acute). In general, fat-soluble vitamins should not be consumed in excess of the recommendations because, unlike water-soluble vitamins in which the excess is excreted out of the body, an excess of fat-soluble vitamins will be stored and accumulated in the body.
It is highly recommended that vitamin A be consumed in the diet rather than from supplements. The richest sources of dietary vitamin A are liver, fish liver oils, milk, milk products, butter, and eggs. Liver is an especially rich source because vitamin A is primarily stored in the liver of animals and humans. Vitamin A is also found in a variety of dark green and deep orange fruits and vegetables, such as carrots, sweet potatoes, pumpkin, spinach, butternut squash, turnip greens, bok choy, mustard greens, and romaine lettuce. Beta carotene is the most active carotenoid (the red, orange, and yellow pigments) form of vitamin A. In addition, cooking (but not overcooking) increases the bioavailability of carotenoids in plant foods and absorption of dietary vitamin A is improved when consumed along with some fat in the same meal.
Hope this helps,