What is the function of nicotinic acid in the human body?

""Niacin was first described by Hugo Weidel in 1873 in his studies of nicotine.[4] The original preparation remains useful: the oxidation of nicotine using nitric acid.[5] Niacin was extracted from livers by Conrad Elvehjem who later identified the active ingredient, then referred to as the "pellagra-preventing factor" and the "anti-blacktongue factor."[6] When the biological significance of nicotinic acid was realized, it was thought appropriate to choose a name to dissociate it from nicotine, in order to avoid the perception that vitamins or niacin-rich food contains nicotine, or that cigarettes contain vitamins. The resulting name 'niacin' was derived from nicotinic acid + vitamin.""