The vitamin niacin (nicotinic acid, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}$ ) can be isolated from a variety of natural sources such as liver, yeast, milk, and whole grain. It also can be synthesized from commercially available materials. From a nutritional point of view, which source of nicotinic acid is best for use in a multivitamin tablet? Why?