Vitamin A is an essential fat-soluble vitamin 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.

As you can see, adequate vitamin A intake is essential to human health. If you don't get enough vitamin A, then you could become vitamin A-deficient (also known as Hypovitaminosis A). This 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; anemia can result; and, the immune response can become weaker. Vitamin A deficiency is one of the major nutrition problems in the developing world that is rarely seen in the developed world.

What happens if you take in too much vitamin A? You could develop vitamin A toxicity (a.k.a., Hypervitaminosis A), which is a more common nutrition problem in the developed world. Excessive intake of vitamin A in the form of beta carotene is not harmful, but will cause one's skin to turn yellow in color. This can be reversed by consuming less beta carotene.

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. Which symptoms you exhibit would depend upon whether or not vitamin A intake was regular 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 accumulate in the body.

How much vitamin A do we need? The Recommended Dietary Allowance (RDA) for vitamin A is measured in retinol equivalents (RE), retinol being the active form of vitamin A. For men, the RDA is 1000 micrograms of RE per day; and, for women, it is 800 micrograms of RE per day.

It is highly recommended that vitamin A be consumed from the diet rather than from supplements (particularly in the case of beta carotene). Alice emphasizes this point because vitamin A obtained from a varied diet offers the maximal potential of health benefits that supplements cannot. The richest sources of preformed 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, but it is inefficiently absorbed and converted to retinol in comparison to vitamin A from animal sources. FYI, cooking (but not overcooking) increases the bioavailability of carotenoids in plant foods; and, absorption of vitamin A from the diet is improved when consumed along with some fat in the same meal.

Alice

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