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Niacin deficiency causes pellagra, which is mostly found among people eating corn-based diets in parts of China, Africa, and India. Pellagra in North America is found mainly among alcoholics; in patients with congenital defects of intestinal and kidney absorption of tryptophan (Hartnup's disease); and in patients with carcinoid syndrome, in which there is increased conversion of tryptophan to serotonin. The early symptoms of pellagra include loss of appetite, generalized weakness and irritability, abdominal pain, and vomiting. Epithelial cell changes then ensue with stomatitis and bright-red glossitis, followed by a characteristic skin rash that is pigmented and scaling, particularly in skin areas exposed to sunlight. This rash is known as "Casal's necklace," when it rings the neck, and is seen in advanced cases. Vaginitis and esophagitis may also occur. Diarrhea (in part due to proctitis and in part due to malabsorption), depression, seizures, and dementia are also part of the pellagra syndrome - the four D's: dermatitis, diarrhea, and dementia leading to death.
The diagnosis of niacin deficiency is based on low levels of the urinary metabolites 2-methyl nicotinamide and 2-pyridone. Treatment of pellagra consists of oral supplementation of 100 to 200 mg of nicotinamide or nicotinic acid three times daily for 5 days. High doses of nicotinic acid (>3 g nicotinic acid per day) are used for the treatment of elevated cholesterol levels and in the treatment of types 2, 4, and 5 hyperlipidemias.
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