Vitamin A is actually a general term that refers to numerous related compounds that have an essential role in cell division, cell differentiation, reproduction, bone growth, and vision.
Cell differentiation pertains to the process wherein a cell becomes part of specialized tissues such as muscles, brain, blood, and lungs. Vitamin A aids in the regulation of the immune system and helps in preventing and fighting off infections by making white blood cells, which are responsible in destroying harmful viruses and bacteria.
This fat-soluble vitamin promotes a healthy surface lining of the eyes, as well as the intestinal, urinary, and respiratory tracts. Bacteria can easily enter and harm the body by causing infection, when such linings break down. Vitamin A also aids the mucous membrane and skin functions as shield against viruses and bacteria. There are also numerous uses of vitamin A for acne cures, see here also.
There are basically two categories of the vitamin, depending as to whether the vitamin A sources are plant or animal. The type of vitamin A that is found in fruits and vegetables is known as provitamin A carotenoid. The body can make these into retinol. Retinol is transported to the retina, as well as other parts through the circulation. The retina is found at the back of the eye and when light passes through the lens, the retina, upon sensing the light, will convert them into a nerve impulse that will interpreted by the brain.
In the United States, approximately 26% of vitamin A consumed by men is in the form of provitamin A carotenoids. Alpha-carotene, beta-carotene, and beta-cryptoxanthin are common provritamin A carotenoids that are found in plant foods. Among these, beta-carotene is the most common and efficiently converted into retinol and although both alpha-carotene and beta-cryptoxanthin are made into vitamin A, they are only half as efficient as beta-carotene.
The other of the types of vitamin A is called as the preformed vitamin A, which can be found in foods that come from animals. This type is absorbed in the form of retinol, one of the most active and useful forms of vitamin A, and will be converted into retinoic acid and retinal, which are other active forms of vitamin A in the body. Good preformed vitamin A sources include whole milk, liver, and some fortified food products.
Dietary Vitamin A Sources
AMOUNT OF VITAMIN A (IU)
|Margarine, fortified||1 tsp.||160|
|Skim milk, fortified||1 cup||330|
|Whole milk||1 cup||330|
|Low-fat milk||1 cup||210|
|American cheese||1 oz.||330|
|Swiss cheese||1 oz.||320|
|Apricots, canned||½ cup||2,260|
|Peaches, canned||½ cup||1,115|
|Winter squash||½ cup||1,200|
|Carrot, raw||1 medium||7,900|
|Collard greens, cooked||½ cup||6,030|
|Rice peppers||½ cup||2,225|
|Sweet potato||½ cup||7,840|
Vitamin A RDA
The Food and Nutrition Board (FNB) of the Institute of Medicine revised the recommended dietary allowance (RDA) for vitamin A in 2001.
FEMALE: mcg/day (IU/day)
MALE: mcg/day (IU/day)
|0 to 6 months||400 (1,333 IU)||400 (1,333 IU)|
|7 to 12 month||500 (1,667 IU)||500 (1,667 IU)|
|1 to 3 years||300 (1,000 IU)||300 (1,000 IU)|
|4 to 8 years||400 (1,333 IU)||400 (1,333 IU)|
|9 to 13 years||600 (2,000 IU)||600 (2,000 IU)|
|14 to 18 years||700 (2,333 IU)||900 (3,000 IU)|
|19 years and older||700 (2,333 IU)||900 (3,000 IU)|
|Pregnancy 18 years and younger||750 (2,500 IU)||–|
|Pregnancy 19 years and older||770 (2,567 IU)||–|
|Lactating 18 years and younger||1,200 (4,000 IU)||–|
|Lactating 19 years and older||1,300 (4,333 IU)||–|
Vitamin A Sources References and Further Reading
Linus Pauling Institute; University of Minnesota School of Public Health; MIT; University of Tennessee Extension; NIH; Idaho Central District Health Department.