Niacin

Niacin is essential for the metabolism of carbohydrates, fats, and many other substances in the body.

Niacin Deficiency

Niacin (nicotinamide or nicotinic acid) deficiency causes pellagra only if tryptophan, an amino acid, is also deficient. Tryptophan can be converted to niacin in the body. 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 niacin in 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.

Alcoholics and other undernourished people are at risk of developing pellagra. Inadequate intake of iron and vitamins B₂ and B₆ increase the risk of niacin deficiency. Pellagra develops in people who have Hartnup disease, a rare hereditary disorder in which tryptophan absorption is impaired (see Section 11, Chapter 146).

Symptoms, Diagnosis, and Treatment

Pellagra affects the skin, digestive tract, and brain. A symmetric, reddened rash that resembles a sunburn and becomes worse when exposed to sunlight (a condition called photosensitivity) may occur. 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 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, constipation, and diarrhea.

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, symptoms, and circumstances of the person. Tests to confirm the diagnosis are not readily available.

Pellagra is treated with high daily doses of nicotinamide, a form of niacin, taken by mouth. Supplements of other B vitamins are also taken.

Niacin Excess

Niacin (but not nicotinamide) in high doses may be prescribed to lower high fat (lipid) levels in the blood. Such doses can cause flushing, itching, gout, liver damage, and an increase in the levels 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 niacin also helps. If the side effects of niacin are intolerable, the dose may be decreased, other (especially slow-release) formulations may be tried, or niacin may be discontinued and another lipid-lowering drug substituted (see Section 11, Chapter 157).