Vitamin B2 (also known as Riboflavin) is all about energy. And it does its job as a coenzyme that catalyzes redox—short for reduction/oxidation—reactions. They move electrons between different molecules during a chemical reaction. All redox reactions require a molecule with extra electrons. You may be familiar with the redox reactions that happen between antioxidants and free radicals. The same mechanics of electron transfer are at work here, for a different purpose—energy.
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 [1-3]. 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 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 riboflavin deficiency. The risk of riboflavin loss from exposure to light is the reason why milk is not typically stored in glass containers.
Riboflavin status is not routinely measured in healthy people. A stable and sensitive measure of riboflavin deficiency is the erythrocyte glutathione reductase activity coefficient (EGRAC), which is based on the ratio between this enzyme’s in vitro activity in the presence of FAD to that without added FAD. The most appropriate EGRAC thresholds for indicating normal or abnormal riboflavin status are uncertain . An EGRAC of 1.2 or less is usually used to indicate adequate riboflavin status, 1.2–1.4 to indicate marginal deficiency, and greater than 1.4 to indicate riboflavin deficiency. However, a higher EGRAC does not necessarily correlate with degree of riboflavin deficiency. Furthermore, the EGRAC cannot be used in people with glucose-6-phosphate dehydrogenase deficiency, which is present in about 10% of African Americans.
Another widely used measure of riboflavin status is fluorometric measurement of urinary excretion over 24 hours (expressed as total amount of riboflavin excreted or in relation to the amount of creatinine excreted). Because the body can store only small amounts of riboflavin, urinary excretion reflects dietary intake until tissues are saturated. Total riboflavin excretion in healthy, riboflavin-replete adults is at least 120 mcg/day; a rate of less than 40 mcg/day indicates deficiency. This technique is less accurate for reflecting long-term riboflavin status than EGRAC. Also, urinary excretion levels can decrease with age and increase with exposure to stress and certain drugs, and the amount excreted strongly reflects recent intake.
Riboflavin is part of two energy-catalyzing coenzymes: flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). Memorizing the names aren’t as important as knowing the coenzymes’ ability to donate an electron in reactions help your body produce energy from your diet.
As your body breaks down food, it breaks the chemical bonds. One result of breaking these bonds is the release of electrons. One of riboflavin’s jobs is to capture these electrons and help squeeze every last bit of energy out of it so your body can put it to work.
Vitamin B2 doesn’t just aid in the metabolism of glucose, amino acids, and fatty acids. Riboflavin also helps your body metabolize drugs and steroids, and helps convert tryptophan to niacin.
Riboflavin deficiency shows up alongside deficiencies in other B vitamins, particularly niacin and pyridoxine. Athletes, alcoholics, and pregnant women are at higher risk for deficiency. But riboflavin is widely available in the diet, with no oral toxicity reported.