Introduction
Riboflavin (also known as vitamin B2) is one of the B vitamins, which are all water soluble. Riboflavin is naturally present in some foods, added to some food products, and available as a dietary supplement. This vitamin is an essential component of two major coenzymes, flavin mononucleotide (FMN; also known as riboflavin-5’-phosphate) and flavin adenine dinucleotide (FAD). These coenzymes play major roles in energy production; cellular function, growth, and development; and metabolism of fats, drugs, and steroids. The conversion of the amino acid tryptophan to niacin (sometimes referred to as vitamin B3) requires FAD. Similarly, the conversion of vitamin B6 to the coenzyme pyridoxal 5’-phosphate needs FMN. In addition, riboflavin helps maintain normal levels of homocysteine, an amino acid in the blood.
More than 90% of dietary riboflavin is in the form of FAD or FMN; the remaining 10% is comprised of the free form and glycosides or esters. Most riboflavin is absorbed in the proximal small intestine. The body absorbs little riboflavin from single doses beyond 27 mg and stores only small amounts of what is riboflavin in the liver, heart, and kidneys. When excess amounts are consumed, they are either not absorbed or the small amount that is absorbed is excreted in urine. Bacteria in the large intestine produce free riboflavin that can be absorbed by the large intestine in amounts that depend on the diet. More riboflavin is produced after ingestion of vegetable-based than meat-based foods.
Riboflavin is yellow and naturally fluorescent when exposed to ultraviolet light. Moreover, ultraviolet and visible light can rapidly inactivate riboflavin and its derivatives. Because of this sensitivity, lengthy light therapy to treat jaundice in newborns or skin disorders can lead to what is riboflavin deficiency. The risk of riboflavin loss from exposure to light is the reason why milk is not typically stored in glass containers.

Sources of Riboflavin
Foods that are particularly rich in riboflavin include eggs, organ meats (kidneys and liver), lean meats, and milk. Some vegetables also contain riboflavin. Grains and cereals are fortified with riboflavin in the United States and many other countries. The largest dietary contributors of total riboflavin intake in U.S. men and women are milk and milk drinks, bread and bread products, mixed foods whose main ingredient is meat, ready-to-eat cereals, and mixed foods whose main ingredient is grain. The riboflavin in most foods is in the form of FAD, although the main form in eggs and milk is free riboflavin.
About 95% of riboflavin in the form of FAD or FMN from food is bioavailable up to a maximum of about 27 mg of riboflavin per meal or dose.The bioavailability of free riboflavin is similar to that of FAD and FMN. Because riboflavin is soluble in water, about twice as much riboflavin content is lost in cooking water when foods are boiled as when they are prepared in other ways, such as by steaming or microwaving.

Riboflavin Deficiency
Riboflavin deficiency is extremely rare in the United States. In addition to inadequate intake, causes of what is riboflavin deficiency can include endocrine abnormalities (such as thyroid hormone insufficiency) and some diseases. The signs and symptoms of riboflavin deficiency (also known as ariboflavinosis) include skin disorders, hyperemia (excess blood) and edema of the mouth and throat, angular stomatitis (lesions at the corners of the mouth), cheilosis (swollen, cracked lips), hair loss, reproductive problems, sore throat, itchy and red eyes, and degeneration of the liver and nervous system. People with riboflavin deficiency typically have deficiencies of other nutrients, so some of these signs and symptoms might reflect these other deficiencies. Severe riboflavin deficiency can impair the metabolism of other nutrients, especially other B vitamins, through diminished levels of flavin coenzymes. Anemia and cataracts can develop if riboflavin deficiency is severe and prolonged.
The earlier changes associated with what is riboflavin deficiency are easily reversed. However, riboflavin supplements rarely reverse later anatomical changes (such as formation of cataracts).

Groups at Risk of Riboflavin Inadequacy
The following groups are among those most likely to have inadequate riboflavin status.
1.Vegetarian athletes
Exercise produces stress in the metabolic pathways that use riboflavin. The Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine state that vegetarian athletes are at risk of riboflavin deficiency because of their increased need for this nutrient and because some vegetarians exclude all animal products (including milk, yogurt, cheese, and eggs), which tend to be good sources of riboflavin, from their diets. These associations recommend that vegetarian athletes consult a sports dietitian to avoid this potential problem.
2.Pregnant and lactating women and their infants
Pregnant or lactating women who rarely consume meats or dairy products (such as those living in developing countries and some vegetarians in the United States) are at risk of riboflavin deficiency, which can have adverse effects on the health of both mothers and their infants. Riboflavin deficiency during pregnancy, for example, can increase the risk of preeclampsia. The limited evidence on the benefits of riboflavin supplements during pregnancy in both developed and developing countries is mixed.
Riboflavin intakes during pregnancy have a positive association with infant birth weight and length. Infants of mothers with riboflavin deficiency or low dietary intakes (less than 1.2 mg/day) during pregnancy have a higher risk of deficiency and of certain birth defects (such as outflow tract defects of the heart). However, maternal riboflavin intake has no association with the risk of orofacial clefts in infants.