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Vitamin A
Description:
Vitamin A is a fat-soluble vitamin with four major functions in the body: (1) It helps cells reproduce normally—a process called differentiation (cells that have not properly differentiated are more likely to undergo pre-cancerous changes). (2) It is required for vision; vitamin A maintains healthy cells in various structures of the eye and is required for the transduction of light into nerve signals in the retina. (3) It is required for normal growth and development of the embryo and fetus, influencing genes that determine the sequential development of organs in embryonic development. (4) It may be required for normal reproductive function, with influences on the function and development of sperm, ovaries and placenta.
Method of Action:
Vitamin A occurs in several forms; retinol, retinal (or retinene), and retinoic acid. It can be metabolically converted to retinal ester. Sources of vitamin A are the carotenes, yellow-red crystalline compounds synthesized by plants.
Vitamin A is split by various enzymes in the pancreatic juice to free retinol. The retinol form is then absorbed by the mucosal cells of the intestine, where it is incorporated in small transport particles of fat called chylomicrons. These particles are released into the lymphatic system and eventually are carried to and stored in the liver. Factors that affect fat absorption also affect vitamin A absorption because vitamin A is a fat-soluble vitamin.
Vitamin A is mobilized from the liver bound to the retinol-binding protein (RBP) which serves to transport vitamin A into the circulatory system for subsequent use.
Vitamin A is an essential component of photoreception. As a visual pigment, it is combined with a protein opsin and is itself what is called the II-cis form. This form changes to the II-trans retinaldehyde upon light exposure, releasing the protein opsin and invoking visual excitation response. Vitamin A also plays an important role in the development of bone and maintenance of epithelial cells.
Beta-carotene, a vitamin A precursor that is yellow in color and a constituent pigment of green, orange, and yellow vegetables, is one of the most important sources of vitamin to humans. Beta-carotene yields two molecules of vitamin A per molecule of beta-carotene metabolized, but this conversion is only about 50% efficient in the body.
Properties & Uses:
One form of vitamin A, retinoic acid, has proven to be effective in some cases for the treatment of acne and various skin disorders when applied topically.
Vitamin A has been shown to be of value in protection against cancer. This has been related to the function of retinoids in promoting normal differentiation of epithelial cells during premalignant periods.
Vitamin A supplementation has proved effective in reversing symptoms of night blindness and corneal damage. It has been considered an anti-infective vitamin because deficiency is directly linked to high incidence of respiratory ailments. Adequate vitamin A intake will decrease the added risk of respiratory infection by maintaining the body's stores at a protective level.
Vitamin A seems to stimulate the immune system and the production of white blood cells. Often when some kind of infection like the flu or a cold hits an intake of 100,000 to 150,000 IU for not more than three days will effectively turn on the immune system to the extent that the infection is stopped. The implications of this for other health problems should be obvious adequate supplementation of vitamin A on a daily basis will keep the immune system up and ready to fight back, although it is certainly no miracle cure for all the ills and abuses of the body.
Toxicity Levels:
Hypervitaminosis has been observed in individuals ingesting 50,000 IU of vitamin A daily for several years. Symptoms of vitamin A toxicity include: bone pain and fragility, coarsening or loss of hair, dry and fissured skin, irritability, fatigue, severe headaches, anorexia, abnormal bone growth, liver damage and abdominal discomfort.
Toxicity in children can result from prolonged intake of vitamin A 25,000 IU daily. Some symptoms include: drowsiness, vomiting, loss of hair, double vision, skin rashes, and skin cracking.
Toxicity symptoms, in most instances, will subside within a few days after curtailed ingestion of vitamin A.
The above figures are several years old, and were considered safe by all. However, new figures show lower doses involving toxicity of Vitamin A. The new figures consider that toxicity may occur from animal foods and/or supplements with intakes of over 50,000 IU/day in adults and 15,00 IU per day in infants and young children. Intakes of as little as 3,000 IU/day may increase the risk of birth defects in pregnant women.
Recommended Dietary Allowances:
For most people, up to 25,000 IU (7,500 mcg) of vitamin A per day is considered safe. However, people over age 65 and those with liver disease should probably not supplement with more than 15,000 IU per day, unless supervised by a doctor. In women who could become pregnant, the maximum safe intake is being re-evaluated. However, less than 10,000 IU (3,000 mcg) per day is generally accepted as safe. There is concern that larger intakes could cause birth defects. Whether the average person would benefit from vitamin A supplementation remains unclear.
Food Sources:
Liver, dairy products, and cod liver oil are good sources of vitamin A. Vitamin A is also available in supplement form.
Deficiency Symptoms:
People who limit their consumption of liver, dairy foods, and beta-carotene-containing vegetables can develop a vitamin A deficiency. Extremely low birth weight babies (2.2 pounds or less) are at high risk of being born with a deficiency, and vitamin A shots given to these infants have been reported in double-blind research to reduce the risk of lung disease.1 The earliest deficiency sign is poor night vision. Deficiency symptoms can also include dry skin, increased risk of infections, and metaplasia (a precancerous condition). Severe deficiencies causing blindness are extremely rare in Western societies.
Less severe deficiencies are more likely to occur with a variety of conditions causing malabsorption. A high incidence of vitamin A deficiency in people infected with HIV has also been reported. People with hypothyroidism have an impaired ability to convert beta-carotene to vitamin A.2 3 For this reason, some doctors suggest taking supplemental vitamin A (perhaps 5,000–10,000 IU per day) if they are not consuming adequate amounts in their diet.
Very old people with type 2 diabetes have shown a significant age-related decline in blood levels of vitamin A, irrespective of their dietary intake.
Side Effects & Interactions:
If you don't get enough vitamin A, you are more susceptible to infectious diseases and vision problems.
If you get too much vitamin A, you can become sick. Large doses of vitamin A can also cause birth defects. Acute vitamin A poisoning usually occurs when an adult takes several hundred thousand IU. Symptoms of chronic vitamin A poisoning may occur in adults who regularly take more than 25,000 IU a day. Babies and children are more sensitive and can become sick after taking smaller doses of vitamin A or vitamin A-containing products such as retinol (found in skin creams).
See also: Hypervitaminosis A.
Increased amounts of beta-carotene can turn the color of skin to yellow or orange. The skin color returns to normal once the increased intake of beta-carotene is reduced.