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Niacin is a water-soluble vitamin available as either nicotinic acid (Figure N.1) or niacinamide (nicotinamide). Niacin has previously been known as vitamin B3, although that title is rarely used today. Niacin typically functions in its coenzyme forms that included the oxidized and reduced forms of nicotinamide adenine dinucleotide (NAD), which are also needed for certain functions in their phosphorylated forms. Roles of niacin include the production of energy from macronutrients through various metabolic pathways including glycolysis, pyruvate dehydrogenase, Krebs cycle, beta-oxidation, and the electron transport system. Niacin is also needed for the synthesis of fat and glycogen.