Section 12. Disorders of Nutrition and Metabolism

Chapter 154. Vitamins

Topics: Introduction | Vitamin A | Vitamin D | Vitamin E | Vitamin K | Vitamin B1 | Vitamin B2 | Niacin | Vitamin B6 | Vitamin B12 | Folic Acid | Vitamin C

Vitamin A

There are several forms of vitamin A. One form is a component of nerve cells that are sensitive to light (photoreceptors) in the eye's retina. Another form keeps the skin and the lining of the lungs, intestine, and urinary tract healthy. Carotenoids, such as beta-carotene, are slowly converted to vitamin A in the body. 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

Vitamin A deficiency is common in areas of the world where the intake of animal and fish liver, yellow and green leafy vegetables, eggs, and whole milk products is inadequate. 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.

Symptoms, Diagnosis, and Treatment

An early symptom of vitamin A deficiency is night blindness, which is caused by a disorder of the retina. The whites (sclera) 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 (see Section 12, Chapter 153), 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 diagnosis is based on symptoms and a low level of vitamin A in the blood. People who are at high risk of developing this deficiency and people who have it should take vitamin A supplements. If symptoms persist after 2 months, doctors usually evaluate the person for a disorder that causes malabsorption.

Vitamin A Excess

Too much vitamin A can cause toxicity. Taking daily doses 10 times the RDA or greater for a period of months can cause toxicity due to excess vitamin A. A smaller dose can cause toxicity in infants, sometimes within a few weeks.

Carotenoids can be consumed in foods without causing toxicity. 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

Very large amounts of vitamin A consumed all at once can cause drowsiness, irritability, headache, and vomiting within hours, followed by peeling of the skin. In children, pressure within the brain (intracranial pressure) is increased and vomiting occurs. Coma and death may occur unless vitamin A is discontinued.

When too much vitamin A is consumed over a period of time, symptoms include 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, and general weakness. Bone and joint pain are common, especially among children. The liver and spleen may enlarge.

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.

The diagnosis of vitamin A excess is based on symptoms and a high level of vitamin A in the blood. Treatment involves discontinuing vitamin A supplements.