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 [2,3]. 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 [3,5].
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 [1,6,7]. 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 [1,6]. 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 [1,6]. This technique is less accurate for reflecting long-term riboflavin status than EGRAC [1,6]. 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 
Sources of Riboflavin
Foods that are particularly rich in riboflavin include eggs, organ meats (kidneys and liver), lean meats, and milk [2,4]. Green 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 [9,10]. 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 .
Several food sources of riboflavin are listed in Table 2.
|Table 2: Selected Food Sources of Riboflavin |
|Beef liver, pan fried, 3 ounces||2.9||171|
|Breakfast cereals, fortified with 100% of the DV for riboflavin, 1 serving||1.7||100|
|Oats, instant, fortified, cooked with water, 1 cup||1.1||65|
|Yogurt, plain, fat free, 1 cup||0.6||35|
|Milk, 2% fat, 1 cup||0.5||29|
|Beef, tenderloin steak, boneless, trimmed of fat, grilled, 3 ounces||0.4||24|
|Clams, mixed species, cooked, moist heat, 3 ounces||0.4||24|
|Mushrooms, portabella, sliced, grilled, ½ cup||0.3||18|
|Almonds, dry roasted, 1 ounce||0.3||18|
|Cheese, Swiss, 3 ounces||0.3||18|
|Rotisserie chicken, breast meat only, 3 ounces||0.2||12|
|Egg, whole, scrambled, 1 large||0.2||12|
|Quinoa, cooked, 1 cup||0.2||12|
|Bagel, plain, enriched, 1 medium (3½”–4” diameter)||0.2||12|
|Salmon, pink, canned, 3 ounces||0.2||12|
|Spinach, raw, 1 cup||0.1||6|
|Apple, with skin, 1 large||0.1||6|
|Kidney beans, canned, 1 cup||0.1||6|
|Macaroni, elbow shaped, whole wheat, cooked, 1 cup||0.1||6|
|Bread, whole wheat, 1 slice||0.1||6|
|Cod, Atlantic, cooked, dry heat, 3 ounces||0.1||6|
|Sunflower seeds, toasted, 1 ounce||0.1||6|
|Tomatoes, crushed, canned, ½ cup||0.1||6|
|Rice, white, enriched, long grain, cooked, ½ cup||0.1||6|
|Rice, brown, long grain, cooked, ½ cup||0||0|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for riboflavin is 1.7 mg for adults and children age 4 and older. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture’s (USDA’s) National Nutrient Database for Standard Reference website  lists the nutrient content of many foods and provides a comprehensive list of foods containing riboflavin arranged by nutrient content and food name.