niacin /nī″əsin/ , a white, crystalline, water-soluble vitamin of the B complex, usually occurring in various plant and animal tissues as nicotinamide. It functions as a coenzyme necessary for the breakdown and use of all major nutrients and is essential for healthy skin, normal functioning of the GI tract, maintenance of the nervous system, and synthesis of the sex hormones. It may be used therapeutically to help reduce high blood cholesterol levels. Rich dietary sources of both niacin and its precursor tryptophan are meats, poultry, fish, liver, kidney, eggs, nuts, peanut butter, brewer’s yeast, and wheat germ. Symptoms of deficiency include muscular weakness, general fatigue, loss of appetite, various skin eruptions, halitosis, stomatitis, insomnia, irritability, nausea, vomiting, recurring headaches, tender gums, tension, and depression. Severe deficiency results in pellagra. The vitamin is not stored in the body, and daily sources are needed. Niacin toxicity is associated with large doses of nicotinic acid (may occur with a dose as low as 50 to 100 mg). Symptoms include flushing, nausea, dizziness, diarrhea, abdominal pain, and alteration of glucose tolerance. Overdose can also exacerbate preexisting conditions such as cardiac arrhythmias and abnormal liver function. Also called nicotinic acid. See also pellagra.
