(See also Overview of Vitamins.)
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.
Good sources of vitamin A include fish liver oils, liver, egg yolks, butter, cream, and fortified milk.
Carotenoids, such as beta-carotene, are pigments in fruits and 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. Good sources of carotenoids are dark green and yellow-orange vegetables and yellow-orange fruits.
Drugs related to vitamin A (retinoids) are used to treat severe acne and psoriasis. Whether taking vitamin A, beta-carotene, and retinoids helps reduce the risk of certain types of skin cancer is being studied. However, the risk of certain cancers may be increased after taking beta-carotene supplements.
Vitamin A deficiency is usually caused by
This deficiency is common in areas of the world where people do not eat enough of foods that are good sources of vitamin A, such as
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. These disorders include chronic diarrhea, celiac disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts. Surgery on the intestine or pancreas can have the same effect.
Liver disorders can interfere with the storage of vitamin A. (Most of the body's vitamin A is stored in the liver.)
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 and thick—a condition called xerophthalmia. Xerophthalmia is particularly common among children who have a severe deficiency of calories (energy) or protein, which includes inadequate intake of vitamin A. Foamy deposits (Bitot spots) may appear in the whites of the eyes. The dry cornea may soften and deteriorate, 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.
The diagnosis of vitamin A deficiency is based on symptoms and a low level of vitamin A in the blood.
If people have problems seeing in the dark, eye tests, such as electroretinography, may be done to determine whether vitamin A deficiency is the cause.
To help confirm vitamin A deficiency, doctors may give people vitamin A supplements to see whether they relieve symptoms.
To help prevent vitamin A deficiency, people should eat dark green leafy vegetables, yellow and orange fruits (such as papayas and oranges), carrots, and yellow vegetables (such as squash and pumpkin). Other food sources include milk and cereals that are fortified with vitamin A, liver, egg yolks, and fish liver oils. The carotenoids that give fruits and vegetables their yellow, orange, or red color and that are converted to vitamin A in the body, are best absorbed from cooked or homogenized vegetables served with some fat or oil.
Children who live in developing countries and are at risk of vitamin A deficiency should take vitamin A supplements.
People who have vitamin A deficiency are given high doses of vitamin A for several days, followed by lower doses until vision and skin improve. 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 Toxicity)
Too much vitamin A can have harmful effects (toxicity). For example, taking daily doses 10 times the RDA (recommended daily allowance) or greater for a period of months can cause toxicity. Sometimes toxicity results from taking special formulations of high-dose vitamin A to treat severe acne or other skin disorders. A smaller dose can cause toxicity in infants, sometimes within a few weeks. If children accidentally take a very high dose, toxicity may develop quickly.
Consuming large amounts of carotenoids (which the body converts to vitamin A) in food does not cause toxicity because carotenoids are converted to vitamin A very slowly. Usually, no symptoms occur. However, when very large amounts of carotenoids are consumed, the skin may turn a deep yellow (called carotenosis), especially on the palms and soles.
High-dose supplements of beta-carotene may increase the risk of cancer, but carotenoids consumed in fruits and vegetables do not seem to increase this risk.
Most people with vitamin A toxicity have a headache and rash.
Consuming too much vitamin A over a long period of time can cause coarse hair, partial loss of hair (including the eyebrows), cracked lips, and dry, rough skin.
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. Their skin may itch. The liver and spleen may enlarge.
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 (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.