Diseases & Conditions
Vitamin A (Retinol)
Vitamin A (retinol) is necessary for the function of light-sensitive nerve cells (photoreceptors) in the eye's retina. It also helps keep the skin and the lining of the lungs, intestine, and urinary tract healthy and protects against infections. Carotenoids, such as beta-carotene, are pigments in vegetables that give them their yellow, orange or red color. Once consumed, carotenoids are slowly converted to vitamin A in the body. Carotenoids are best absorbed from cooked or homogenized vegetables served with some fat or oil.
Drugs related to vitamin A (retinoids) are used to treat severe acne and psoriasis and are being investigated for the treatment of certain types of cancer.
Vitamin A Deficiency
Night blindness is an early symptom. Blindness can eventually develop. The eyes, skin, and other tissues become dry and damaged, and infections develop more often. The diagnosis is based on symptoms and blood tests. Taking high doses of vitamin A for several days corrects the deficiency.
Vitamin A deficiency is common in areas of the world where people do not eat enough of certain foods: Animal and fish liver Orange, yellow, and green vegetables Eggs Fortified milk products
For example, the deficiency occurs in southern and eastern Asia, where rice is the main food. Disorders that impair the intestine's absorption of fats can reduce the absorption of vitamin A and increase the risk of vitamin A deficiency. Surgery on the intestine or pancreas can have the same effect. Liver disorders can interfere with the storage of vitamin A. Most multiple vitamins contain little or no vitamin A.
An early symptom of vitamin A deficiency is night blindness, which is caused by a disorder of the retina. Soon thereafter, the whites (conjunctiva) and corneas of the eyes may become dry—a condition called xerophthalmia. Xerophthalmia is particularly common among children who have a severe deficiency of calories (energy) or protein which includes an inadequate intake of vitamin A. Foamy deposits (Bitot's spots) may appear in the whites of the eyes. The dry cornea may soften and ulcerate, and blindness may result. Vitamin A deficiency is a common cause of blindness in developing countries.
The skin becomes dry and scaly, and the lining of the lungs, intestine, and urinary tract thicken and stiffen. The immune system does not function normally, making infections more likely, particularly in infants and children.
Children's growth and development may be slowed.
Did You Know...
Many multiple vitamins contain little or no vitamin A.
Diagnosis and Treatment
The diagnosis is based on symptoms and a low level of vitamin A in the blood.
If people have conditions that put them at risk of developing this deficiency, they should take vitamin A supplements.
People who have the deficiency are given high doses of vitamin A for several days. Infants should not be given high doses repeatedly because such doses can be toxic. If symptoms persist after 2 months, doctors usually check for a disorder that impairs fat absorption.
Vitamin A Excess
Consuming too much vitamin A causes hair loss, cracked lips, dry skin, weakened bones, headaches, and increased pressure in the brain. The diagnosis is based on symptoms and blood tests. Most people recover completely when they stop taking vitamin A supplements.
Too much vitamin A can cause toxicity. For example, taking daily doses 10 times the RDA (recommended daily allowance) or greater for a period of months can cause toxicity. Special formulations of high dose vitamin A may be taken to treat severe acne or other skin disorders. A smaller dose can cause toxicity in infants, sometimes within a few weeks. Sometimes children accidentally take a very high dose, and toxicity occurs quickly.
Carotenoids can be consumed in foods without causing toxicity because their conversion to vitamin A is very slow. However, when large amounts are consumed, the skin turns a deep yellow (carotenosis), especially on the palms and soles. High-dose supplements of beta-carotene may increase the risk of cancer.
Symptoms, Diagnosis, and Treatment
Consuming too much vitamin A over a period of time can cause coarse hair, partial loss of hair (including the eyebrows), cracked lips, and dry, rough skin, which may peel. Later symptoms include severe headaches, increased pressure within the brain (intracranial pressure), and general weakness. Bone and joint pain are common, especially among children. Fractures may occur easily, especially in older people. Children may lose their appetite and not grow and develop normally. The liver and spleen may enlarge.
Did You Know...
In infants or children, very high doses of vitamin A can have harmful effects. Taking very high doses of vitamin A or isotretinoin Some Trade Names ACCUTANE (a drug derived from vitamin A) during pregnancy can cause birth defects.
Consuming very large amounts of vitamin A all at once can cause drowsiness, irritability, headache, nausea, and vomiting within hours, followed by peeling of the skin. Pressure within the brain is increased, particularly in children, and vomiting occurs. Coma and death may occur unless vitamin A consumption is stopped.
Taking isotretinoin Some Trade Names ACCUTANE (a vitamin A derivative used to treat severe acne) during pregnancy may cause birth defects. Women who are or who may become pregnant should not consume vitamin A in amounts above the safe upper limit (3,000 micrograms) because birth defects are a risk.
The diagnosis of vitamin A excess is based on symptoms and a high level of vitamin A in the blood.
Treatment involves stopping vitamin A supplements. Most people recover completely.
Last full review/revision August 2007 by Larry E. Johnson, MD, PhD
Source: The Merck Manual Home Edition