Niacin (nicotinic acid) is essential for the processing (metabolism) of carbohydrates, fats, and many other substances in the body. Foods rich in tryptophan (an amino acid), such as dairy products, can compensate for not consuming enough niacin in the diet because the body can convert tryptophan to niacin.
Niacin is sometimes used in two ways: as a synonym for nicotinic acid and as a broader term that includes nicotinamide and nicotinic acid, two forms of this B vitamin.
Niacin deficiency is uncommon in developed countries. It causes a disorder called pellagra, which affects the skin, digestive tract, and brain. Pellagra develops only if the diet is also deficient in tryptophan because the body can convert tryptophan to niacin. People who live in areas where maize (Indian corn) is the main food source are at risk of developing pellagra because maize is low in niacin and tryptophan. Furthermore, the niacinin maize cannot be absorbed in the intestine unless the maize is treated with alkali (as it is when tortillas are prepared). Pellagra may be a seasonal disorder, appearing each spring and lasting through the summer, when the diet consists mainly of maize products.
Pellagra also develops in people who have Hartnup disease, a rare hereditary disorder in which absorption of tryptophan is impaired (see Hartnup Disease).
Alcoholism, diarrhea, and cirrhosis can lead to a deficiency of niacin. Niacin deficiency may also occur when the antibiotic isoniazid is taken for a long time.
Many people with niacin deficiency also have deficiencies of iron, riboflavin, and vitamin B6.
Typically, people develop a symmetric, dark red rash that resembles a sunburn and becomes worse when it is exposed to sunlight (a condition called photosensitivity). The location of the rash is distinctive: on the hands (like gloves), on the feet and calves (like boots), around the neck (like a necklace), and on the face forming a butterfly shape. Skin abnormalities are persistent, and the affected areas may become brown and scaly.
The whole digestive tract is affected. The tongue and mouth may become inflamed and dark red. The tongue may swell, the mouth may burn, and sores may develop on both. The throat and esophagus may also burn. Other symptoms include nausea, vomiting, abdominal discomfort, constipation, and diarrhea (which may be bloody).
Later, fatigue, insomnia, and apathy develop. Malfunction of the brain (encephalopathy) usually follows. It is characterized by confusion, disorientation, hallucinations, and memory loss.
The diagnosis is based on the diet history and symptoms. Measuring a by-product of niacinin urine can help establish the diagnosis, but this test is not always available. The diagnosis is confirmed if niacin relieves symptoms.
Pellagra is treated with daily doses of niacin taken by mouth. Supplements of other B vitamins are also taken.
Nicotinic acid (but not nicotinamide) in high doses may be prescribed to lower high cholesterol and triglyceride (lipid) levels in the blood, but whether niacin reduces the risk of coronary artery disease and stroke is unclear.
Such high doses of niacin can cause flushing, itching, gout, and liver damage (rarely) and increase the level of sugar (glucose) in the blood. Most side effects can be minimized by starting with a relatively low dose and gradually increasing the dose. Taking aspirin before taking nicotinic acid and taking nicotinic acid after meals also help. If the side effects of nicotinic acid are intolerable, the dose may be decreased, other (especially extended-release) formulations may be tried, or niacin may be stopped and another lipid-lowering drug substituted (see Table: Lipid-Lowering Drugs).
Last full review/revision October 2014 by Larry E. Johnson, MD, PhD