V i t a m i n B 3
Vitamin B3 (Niacin) is required for the activation of many enzymes in the body, is involved in both DNA repair and the production of steroid hormones in the adrenal gland. Like all B vitamins, niacin plays part in the energy metabolism.
Vitamin B3 has a critical role in the chemical processing of fats in the body and helps to synthesize starch to be stored in the body's muscles and liver for eventual use as an energy source. High energy requirements (brain) or high turnover rate organs (gut, skin) are usually the most sensitive to a deficiency.
Vitamin B3 is used within the adrenal glands to produce sex hormones as well as stress hormones. Sex hormones are used throughout the body including their vital function of producing viable reproductive cells in the testes and ovaries. Stress hormones are used to regulate everything from breathing, to heart rate and energy consumption.
Vitamin B3 is also important in cellular growth and division not only because of the energy required for these processes, but because it is required to be present inside the cell during DNA replication. It even has an important function in repairing damaged DNA like that found from overexposure to ultraviolet radiation from the sun.
Niacin is needed for proper functioning of the digestive system as well, by playing a role in the production of hydrochloric acid, which is needed for good digestive function. It also acts to guard pancreas health.
Vitamin B3 has been found to decrease the risk of cardiovascular events in a number of controlled human trials. Niacin also works in conjunction with chromium to help regulate and stabilize blood sugar levels by promoting proper insulin function.
Although niacin is required for production of cholesterol by the liver, the vitamin has repeatedly been used to successfully lower total blood cholesterol in individuals with elevated cholesterol levels. This cholesterol-lowering effect of vitamin B3 only occurs at high doses that must be obtained through nutrient supplementation, and most likely involves a chemical feature of vitamin B3 that is not directly related to fat or fat processing.
16 mg RDA - 450 mg UL
RDA (recommanded dietary allowances) per day 
UL (upper intake level) per day 
 Dietary Reference Intakes: Vitamins.
The National Academies, 2001
 Risk Factor: Vitamin Deficiency
Andreas Jopp, 2010