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.
Here's welcome news: You may have inherited your mom's slow-mo metabolism, but you’re not stuck with it. New research shows you can trick your body into burning calories more efficiently, especially if you hit the gym. By strength-training just a couple of times a week, for example, you’ll reverse 50% of the seemingly inevitable metabolism slow-down that comes with age, says Gary Hunter, PhD, a professor of human studies at the University of Alabama at Birmingham. So take control of your metabolism by making these boosters part of your routine—and (finally) stop sweating every cookie.