On the Relative Efficacy of Nicotinamide and Nicotinic Acid as Precursors of Nicotinamide Adenine Dinucleotide

Nicotinamide has a much longer half-life than nicotinic acid, both in liver and in blood. These observations, considered together with the known inhibitory action of high levels of nicotinic acid on nicotinamide adenine dinucleotide synthesis, explain the greater efficacy of nicotinamide than of nicotinic acid as a precursor of NAD in vivo, despite the fact that nicotinic acid is an intermediate in the synthesis of NAD from nicotinamide. At high dose levels, the half-life both of nicotinamide and of nicotinic acid is determined primarily by the rate of urinary excretion of the unchanged compound, and not by metabolic transformation. At low dose levels, both compounds are excreted primarily as metabolites rather than as unchanged compounds.