(See also Overview of Vitamins.)
Niacin (nicotinic acid), a B vitamin, is essential for the processing (metabolism) of carbohydrates, fats, and many other substances in the body and for the normal functioning of cells.
Good sources of niacin include dried yeast, liver, meat, fish, legumes, and whole-grain or enriched cereal products. 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.
A distinctive dark red rash appears on the hands, feet, calves, neck, and face, and the tongue and mouth turn dark red.
People have digestive tract problems, fatigue, insomnia, apathy, and later confusion and memory loss.
The diagnosis is based on the diet history, symptoms, and sometimes urine tests.
High doses of nicotinamide or nicotinic acid, taken by mouth, can correct the deficiency.
Niacin deficiency is uncommon in developed countries.
Many people with niacin deficiency also have deficiencies of protein, iron, riboflavin (a B vitamin), and vitamin B6.
One form of niacin deficiency results from a deficiency of niacin and tryptophan (an amino acid). This form causes a disorder called pellagra, which affects the skin, digestive tract, and brain. Pellagra develops only if the diet is deficient in niacin and 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 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.
Pellagra also develops in people who have one of the following:
The following can lead to a deficiency of niacin:
Typically, people with pellagra develop a symmetric, dark red rash that resembles a sunburn and becomes worse when it is exposed to sunlight (a condition called photosensitivity). The rash occurs in areas of sun exposure, and its location is distinctive:
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. Saliva production may increase or decrease. 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. People may be overexcited, depressed, extremely elated (manic), delirious, or paranoid (thinking that people intend to harm them).
The diagnosis of niacin deficiency is based on the diet history and symptoms. Measuring a by-product of niacin in urine can help establish the diagnosis, but this test is not always available.
The diagnosis is confirmed if niacin relieves symptoms.
There are two types of niacin supplements: nicotinamide and nicotinic acid. Pellagra is treated with daily doses of nicotinamide taken by mouth. Nicotinamide, unlike nicotinic acid, does not cause flushing, itching, burning, or tingling sensations.
Because people with niacin deficiency often have other nutritional deficiencies, eating a balanced diet is important. Supplements of other B vitamins are also taken.
Nicotinic acid (but not nicotinamide) in high doses may be prescribed to improve cholesterol and triglyceride (lipid) levels in the blood. Nicotinic acid can have the following beneficial effects:
However, whether nicotinic acid reduces the risk of coronary artery disease and stroke is unclear.
Such high doses of nicotinic acid can cause flushing, itching, gout, and liver damage (rarely) and increase the level of sugar (glucose) in the blood. Flushing may be worse after drinking alcohol, being physically active, being in the sun, and eating spicy foods.
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 nicotinic acid may be stopped and another lipid-lowering drug substituted (see Table: Lipid-Lowering Drugs).