pellagra, nutritional disorder caused by a dietary deficiency of niacin (also called nicotinic acid) or a failure of the body to absorb this vitamin or the amino acid tryptophan, which is converted to niacin in the body. Pellagra is characterized by skin lesions and by gastrointestinal and neurological disturbances; the so-called classical three Ds of pellagra are dermatitis, diarrhea, and dementia.

Skin lesions result from an abnormal sensitization of the skin to sunlight and tend to occur symmetrically on the exposed surfaces of the arms, legs, and neck. They may look at first like a severe sunburn, later becoming reddish brown, rough, and scaly. Gastrointestinal symptoms usually consist of diarrhea, with an accompanying inflammation of the mouth and the tongue and fissuring and dry scaling of the lips and corners of the mouth. Neurological signs appear later in most cases, when the skin and alimentary manifestations are prominent. The dementia, or mental aberrations, may include general nervousness, confusion, depression, apathy, and delirium.

In humans, pellagra is seldom a deficiency of niacin alone; response to niacin therapy tends to be partial, whereas the therapeutic administration of a well-balanced, high-protein diet and multivitamins commonly brings swift recovery. Mild or suspected instances of niacin deficiency can be effectively treated with a well-balanced diet alone.

Research by Joseph Goldberger of the United States Public Health Service and others showed that pellagra was the result of a nutritional deficiency. In 1937 it was shown that dogs with a disorder similar to pellagra known as black tongue could be cured by the administration of niacin. Pellagra is now seldom encountered in countries in which the population generally eats a well-balanced diet, but it still occurs in most countries in which people live on a diet that consists predominantly of corn (maize), which is low in tryptophan, and contains little or no protein-rich food. Such foods as milk and eggs, although low in niacin, will protect the body from pellagra because their proteins contain sufficient tryptophan for the synthesis of niacin. Pellagra can also be a side effect of chronic alcoholism. Symptoms closely resembling those of pellagra are seen in Hartnup disease.