As coenzymes, the B vitamins are essential components in most major metabolic reactions. As water-soluble substances, B vitamins are not generally stored in the body in any appreciable amounts (with the exception of vitamin B-12). Therefore, the body needs an adequate supply of B vitamins on a daily basis.
Vitamin B-1 (thiamin), vitamin B-2 (riboflavin), and niacinamide are all essential coenzymes in energy production. Thiamin is required for the energetics of the glycolytic and Citric Acid Cycle reactions. Thiamin is also related to nerve impulse transmission. Riboflavin is a component of coenzymes FAD and FMN, which are intermediates in many redox reactions, including energy production and cellular respiration reactions. Niacin is a component of the coenzymes NAD and NADP, which are also integral components of energy production reactions.
Vitamin B-6 dependent enzymes are required for the biosynthesis of many neurotransmitters, including serotonin, epinephrine, and norepinephrine. Vitamin B-6, a coenzyme in amino acid metabolism, is also necessary for the processing of homocysteine and the conversion of tryptophan into niacin.
Folic acid together with vitamin B-12 serves as a methyl donor for biosynthetic reactions, including the conversion of homocysteine to methionine.
Optimum metabolism of proteins, carbohydrates, and fats depends upon adequate levels of biotin and pantothenic acid. Biotin is essential for many metabolic carboxylation reactions, while pantothenic acid, as part of Coenzyme A, is essential to energy production via the Citric Acid Cycle.
While not truly vitamins, choline, inositol, and paraaminobenzoic acid are important, related nutrients to B vitamins. Choline serves not only as a methyl donor for homocysteine metabolism following conversion to betaine, but also as a structural component of cellular membranes as phosphatidylcholine and of the neurotransmitter acetylcholine. Inositol aids in the cellular response to hormonal signals, serves as a source of arachidonic acid, and is active in cellular membranes as phoshatidylinositol. Finally, para-aminobenzoic acid is an integral component of folic acid as well as having antioxidant properties.