Niacin or Vitamin B3 is an essential human vitamin, the lack of which causes pellagra. The body requires just less than 20mg per day to meet it's requirements. Too much niacin (over 35mg) per day, results in adverse side effects such as flushing of the skin. The body can make niacin from the amino acid tryptophan, but it is more efficient to derive it from dietary sources.
By itself, niacin is not important to body functions, although it does have a role in regulating the metabolism of fats in the liver. However, the body needs it to make certain enzymes that are critical to the functioning of the metabolism. Without it, the body loses its ability to deal with energy, fat and nucleic acids.
Niacin is found in most meats (a notable exception is pork), many fruits and vegetables, seeds, nuts, and even certain types of yeast.
The pandemic of pellagra in the U.S. south in the early 20th century was a result of a staple diet rich in corn and pork. Pork is one of the few meats that contains no niacin, and the niacin in corn is not available unless it is soaked in an alkali solution first.