A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.  Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.  Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.
NAD, or nicotinamide adenine dinucleotide, is the basic coenzyme or molecule powering many cellular reactions. In cellular respiration, or the process of producing energy in the cells, this compound combines with two hydrogen atoms. Building up molecules in this way is called anabolism. The compound takes one hydrogen atom and the electron from the other hydrogen atom to turn into high-energy NADH. NADH then travels with the extra electrons and drops them off, releasing energy in a process called catabolism, when a molecule breaks down to produce energy.