Niacin is an important nutrient - deficiencies are mostly seen in locations where there is a lot of poverty, malnutrition and alcoholism, and in countries where corn, the only grain low in niacin, is a major food source.
Niacin deficiency can manifest in a variety of ways. Mild deficiency can result in an intolerance to cold (due to slow metabolism). More severe deficiency results in pellagra (diarrhea, dermatitis, and dementia).
Niacin is found in a number of foods - animal products, fruits and vegetables, nuts and seeds, mushrooms and brewer's yeast - so it is not generally an issue when people have a balanced diet.