Vitamin B3 Niacin
Niacin (vitamin B3) is widely distributed in plant and animal foods. Good sources include yeast, meat, liver, cereals, legumes, and seeds. The major dietary forms of niacin are nicotinamide adenine dinucleotide (NAD) and NAD phosphate (NADP). NAD and NADP are hydrolyzed intestinal enzymes into nicotinamide, which is converted by intestinal flora to nicotinic acid. Both forms of niacin are then absorbed, released into the circulation and rapidly taken up by the liver, kidneys, and erythrocytes. Intracellular nicotinamide and nicotinic acid are converted back to NAD and NADP. Many enzymatic reactions depend upon NAD and NADP, which function as coenzymes.
Niacin deficiency predisposes to pellagra. The most characteristic finding is the presence of a symmetric hyperpigmented rash on sun exposed areas of skin.Other clinical findings include red tongue, diarrhea and vomiting. Neurologic symptoms include insomnia, anxiety, disorientation, delusions, dementia, and encephalopathy.
Pellagra is now extremely uncommon in the western world except as a complication of alcoholism, anorexia nervosa, or malabsorption. Pellagra due to dietary deficiency still occurs in some regions of India, China, and Africa. Niacin deficiency can accompany carcinoid syndrome because tryptophan is metabolized to 5 OH-tryptophan instead of nicotinic acid. Several drugs can lead to niacin deficiency including isoniazid, 5-fluorouracil, pyrazinamide, 6-mercaptopurine, hydantoin, ethionamide, phenobarbital, azathioprine, and chloramphenicol.
Niacin is measured by high performance liquid chromatography. Reference range at Cambridge Biomedical Research Group is 0.50 - 8.45 ug/mL for patients 10 years and older and 0.5-8.91 ug/mL for younger patients.
Specimen requirement is a lavender-top (EDTA) tube of blood. Plasma should be separated within 30 minutes of collection, protected from light and immediately frozen.
