Flavins are a group of organic compounds built around a specific three-ring structure called an isoalloxazine. The term comes from the Latin word for yellow, flavus, which describes their color when oxidized. These compounds are necessary for a wide range of metabolic reactions in most living things. Though their names sound similar, flavins are structurally and functionally distinct from flavonoids, which are a different class of compounds found in plants. The foundational flavin is riboflavin, also known as vitamin B2.
The Main Forms of Flavin
The journey of flavins in the body begins with riboflavin, a nutrient we must obtain from our diet. Once absorbed, the body does not use riboflavin directly. Instead, it serves as the precursor for two more complex and functionally active molecules.
The first active form is flavin mononucleotide, commonly known as FMN. The creation of FMN occurs when an enzyme called riboflavin kinase attaches a phosphate group to the riboflavin molecule. FMN is the main form of riboflavin found within cells and tissues, and it is more soluble, making it easier to transport and use.
The second coenzyme form is flavin adenine dinucleotide, or FAD. This molecule is synthesized by adding an adenosine monophosphate group to FMN, which enhances its chemical capabilities. Both FMN and FAD are the active forms of flavin in the body that participate in biochemical reactions.
How Flavins Function in the Body
The primary biological role of FMN and FAD is to serve as coenzymes, which are helper molecules for enzymes. They specifically work with a class of enzymes called flavoproteins. These flavoproteins catalyze many oxidation-reduction (redox) reactions that are part of metabolism. Redox reactions involve the transfer of electrons from one molecule to another.
In this process, flavins act like rechargeable batteries for the cell. They can accept electrons, becoming reduced, and then donate those electrons elsewhere, becoming oxidized. This ability to shuttle electrons is important for converting food into usable cellular energy. Flavins can handle the transfer of either one or two electrons at a time, making them versatile for metabolic processes.
This electron-carrying capacity is important in cellular respiration. FAD is a direct participant in the Krebs cycle, where it accepts electrons during the breakdown of metabolic byproducts. Both FMN and FAD are components of the electron transport chain. Here, they donate the electrons they carry, driving the process that produces most of the cell’s energy in the form of ATP.
Dietary Sources and Daily Needs
Since humans cannot synthesize flavins, they must be consumed through the diet in the form of riboflavin. This vitamin is water-soluble, meaning the body does not store it in large amounts, so a consistent dietary intake is necessary. A well-rounded diet provides sufficient amounts of riboflavin.
Excellent sources of riboflavin include:
- Dairy products like milk, cheese, and yogurt
- Lean meats such as poultry, beef, and fish
- Eggs
- Green leafy vegetables like spinach
- Legumes, including lentils and beans
- Nuts and fortified cereals
The Recommended Dietary Allowance (RDA) for riboflavin varies by age, sex, and life stage. For adult men, the RDA is 1.3 milligrams (mg) per day, while for adult women, it is 1.1 mg per day. These needs increase during pregnancy to 1.4 mg per day and during lactation to 1.6 mg per day.
Consequences of Flavin Deficiency
A lack of sufficient riboflavin leads to a deficiency known as ariboflavinosis. This condition is uncommon in developed nations where fortified foods are widespread, but it can affect individuals with poor diets or conditions that impair nutrient absorption. The symptoms primarily impact the skin and mucous membranes.
Common signs include a sore throat and inflammation of the mouth and tongue. Cracks and sores can develop on the outside of the lips (cheilosis) and at the corners of the mouth (angular stomatitis). The tongue may also take on a purplish-red hue.
Since riboflavin is a water-soluble vitamin, the body excretes any excess. This makes toxicity from high doses of riboflavin from food or supplements exceedingly rare.