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.
Riboflavin is involved in maintaining normal circulating levels of homocysteine; in riboflavin deficiency, homocysteine levels increase, elevating the risk of cardiovascular diseases. Redox reactions
Redox reactions are processes that involve the transfer of electrons. The flavin coenzymes support the function of roughly 70-80 flavoenzymes in humans (and hundreds more across all organisms, including those encoded by archeal, bacterial and fungal genomes) that are responsible for one- or two-electron redox reactions which capitalize on the ability of flavins to be converted between oxidized, half-reduced and fully reduced forms. FAD is also required for the activity of glutathione reductase, an essential enzyme in formation of the endogenous antioxidant, glutathione. Micronutrient metabolism
Riboflavin, FMN, and FAD are involved in the metabolism of niacin, vitamin B6, and folate. The synthesis of the niacin-containing coenzymes, NAD and NADP, from tryptophan involves the FAD-dependent enzyme, kynurenine 3-monooxygenase. Dietary deficiency of riboflavin can decrease the production of NAD and NADP, thereby promoting niacin deficiency. Conversion of vitamin B6 to its coenzyme, pyridoxal 5-phosphate synthase, involves the enzyme, pyridoxine 5-phosphate oxidase, which requires FMN. An enzyme involved in folate metabolism, 5,10-methylenetetrahydrofolate reductase, requires FAD to form the amino acid, methionine, from homocysteine.Riboflavin deficiency appears to impair the metabolism of the dietary mineral, iron, which is essential to the production of hemoglobin and red blood cells. Alleviating riboflavin deficiency in people who are deficient in both riboflavin and iron improves the effectiveness of iron supplementation for treating iron-deficiency anemia. Synthesis
Biosynthesis
Biosynthesis takes place in bacteria, fungi and plants, but not animals.