Definition
Riboflavin, also known as vitamin B2, is a water-soluble vitamin and is one of the B vitamins. Unlike folate and vitamin B6, which occur in several chemically related forms known as vitamers, riboflavin is only one chemical compound. It is a starting compound in the synthesis of the coenzymes flavin mononucleotide (FMN, also known as riboflavin-5-phosphate) and flavin adenine dinucleotide (FAD). FAD is the more abundant form of flavin, reported to bind to 75% of the number of flavin-dependent protein encoded genes in the all-species genome (the flavoproteome) and serves as a co-enzyme for 84% of human-encoded flavoproteins.In its purified, solid form, riboflavin is a yellow-orange crystalline powder with a slight odor and bitter taste. It is soluble in polar solvents, such as water and aqueous sodium chloride solutions, and slightly soluble in alcohols. It is not soluble in non-polar or weakly polar organic solvents such as chloroform, benzene or acetone. In solution or during dry storage as a powder, riboflavin is heat stable if not exposed to light. When heated to decompose, it releases toxic fumes containing nitric oxide. Functions
Riboflavin is essential to the formation of two major coenzymes, FMN and FAD. These coenzymes are involved in energy metabolism, cell respiration, antibody production, growth and development. Riboflavin is essential for the metabolism of carbohydrates, protein and fats. FAD contributes to conversion of tryptophan to niacin (vitamin B3) and the conversion of vitamin B6 to the coenzyme pyridoxal 5-phosphate requires FMN.
It is difficult to incorporate riboflavin into liquid products because it has poor solubility in water, hence the requirement for riboflavin-5-phosphate (FMN, also called E101 when used as colorant), a more soluble form of riboflavin. The enrichment of bread and ready-to-eat breakfast cereals contributes significantly to the dietary supply of the vitamin. Free riboflavin is naturally present in animal-sourced foods along with protein-bound FMN and FAD. Cows milk contains mainly free riboflavin, but both FMN and FAD are present at low concentrations. Fortification
Some countries require or recommend fortification of grain foods. As of 2021, 56 countries, mostly in North and South America and southeast Africa, require food fortification of wheat flour or maize (corn) flour with riboflavin or riboflavin-5-phosphate sodium. The amounts stipulated range from 1.3 to 5.75 mg/kg. An additional 16 countries have a voluntary fortification program. For example, the Indian government recommends 4.0 mg/kg for "maida" (white) and "atta" (whole wheat) flour. Absorption, metabolism, excretion
More than 90% of riboflavin in the diet is in the form of protein-bound FMN and FAD. Exposure to gastric acid in the stomach releases the coenzymes, which are subsequently enzymatically hydrolyzed in the proximal small intestine to release free riboflavin.Absorption occurs via a rapid active transport system, with some additional passive diffusion occurring at high concentrations. Bile salts facilitate uptake, so absorption is improved when the vitamin is consumed with a meal. One small clinical trial in adults reported that the maximum amount of riboflavin that can be absorbed from a single dose is 27 mg.