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. the majority of newly absorbed riboflavin is taken up by the liver on the first pass, indicating that postprandial appearance of riboflavin in blood plasma may underestimate absorption. three riboflavin transporter proteins have been identified: rfvt1 is present in the small intestine and also in the placenta; rfvt2 is highly expressed in brain and salivary glands; and rfvt3 is most highly expressed in the small intestine, testes, and prostate. infants with mutations in the genes encoding these transport proteins can be treated with riboflavin administered orally. riboflavin is reversibly converted to fmn and then fad. from riboflavin to fmn is the function of zinc-requiring riboflavin kinase; the reverse is accomplished by a phosphatase. from fmn to fad is the function of magnesium-requiring fad synthase; the reverse is accomplished by a pyrophosphatase. fad appears to be an inhibitory end-product that down-regulates its own formation. when excess riboflavin is absorbed by the small intestine, it is quickly removed from the blood and excreted in urine. urine color is used as a hydration status biomarker and, under normal conditions, correlates with urine specific gravity and urine osmolality. however, riboflavin supplementation in large excess of requirements causes urine to appear more yellow than normal. with normal dietary intake, about two-thirds of urinary output is riboflavin, the remainder having been partially metabolized to hydroxymethylriboflavin from oxidation within cells, and as other metabolites. when consumption exceeds the ability to absorb, riboflavin passes into the large intestine, where it is catabolized by bacteria to various metabolites that can be detected in feces. there is speculation that unabsorbed riboflavin could affect the large intestine microbiome. deficiency prevalence riboflavin deficiency is uncommon in the united states and in other countries with wheat flour or corn meal fortification programs. from data collected in biannual surveys of the u.s. population, for ages 20 and over, 22% of females and 19% of men reported consuming a supplement that contained riboflavin, typically a vitamin-mineral multi-supplement. for the non-supplement users, the dietary intake of adult women averaged 1.74 mg/day and men 2.44 mg/day. these amounts exceed the rdas for riboflavin of 1.1 and 1.3 mg/day respectively. for all age groups, on average, consumption from food exceeded the rdas. a 2001-02 u.s. survey reported that less than 3% of the population consumed less than the estimated average requirement of riboflavin. signs and symptoms riboflavin deficiency (also called ariboflavinosis) results in stomatitis, symptoms of which include chapped and fissured lips, inflammation of the corners of the mouth (angular stomatitis), sore throat, painful red tongue, and hair loss. the eyes can become itchy, watery, bloodshot, and sensitive to light. riboflavin deficiency is associated with anemia. prolonged riboflavin insufficiency may cause degeneration of the liver and nervous system. riboflavin deficiency may increase the risk of preeclampsia in pregnant women. deficiency of riboflavin during pregnancy can result in fetal birth defects, including heart and limb deformities. risk factors people at risk of having low riboflavin levels include alcoholics, vegetarian athletes, and practitioners of veganism. pregnant or lactating women and their infants may also be at risk, if the mother avoids meat and dairy products. anorexia and lactose intolerance increase the risk of riboflavin deficiency. people with physically demanding lives, such as athletes and laborers, may require higher riboflavin intake. the conversion of riboflavin into fad and fmn is impaired in people with hypothyroidism, adrenal insufficiency, and riboflavin transporter deficiency. causes riboflavin deficiency is usually found together with other nutrient deficiencies, particularly of other water-soluble vitamins. a deficiency of riboflavin can be primary (i.e. caused by poor vitamin sources in the regular diet) or secondary, which may be a result of conditions that affect absorption in the intestine. secondary deficiencies are typically caused by the body not being able to use the vitamin, or by an increased rate of excretion of the vitamin. diet patterns that increase risk of deficiency include veganism and low-dairy vegetarianism. diseases such as cancer, heart disease and diabetes may cause or exacerbate riboflavin deficiency. there are rare genetic defects that compromise riboflavin absorption, transport, metabolism or use by flavoproteins. one of these is riboflavin transporter deficiency, previously known as brown-vialetto-van laere syndrome. variants of the genes slc52a2 and slc52a3 which code for transporter proteins rdvt2 and rdvt3, respectively, are defective. infants and young children present with muscle weakness, cranial nerve deficits including hearing loss, sensory symptoms including sensory ataxia, feeding difficulties, and respiratory distress caused by a sensorimotor axonal neuropathy and cranial nerve pathology. when untreated, infants with riboflavin transporter deficiency have labored breathing and are at risk of dying in the first decade of life. treatment with oral supplementation of high amounts of riboflavin is lifesaving. other inborn errors of metabolism include riboflavin-responsive multiple acyl-coa dehydrogenase deficiency, also known as a subset of glutaric acidemia type 2, and the c677t variant of the methylenetetrahydrofolate reductase enzyme, which in adults has been associated with risk of high blood pressure. diagnosis and assessment the assessment of riboflavin status is essential for confirming cases with non-specific symptoms whenever deficiency is suspected. total riboflavin excretion in healthy adults with normal riboflavin intake is about 120 micrograms per day, while excretion of less than 40 micrograms per day indicates deficiency. riboflavin excretion rates decrease as a person ages, but increase during periods of chronic stress and the use of some prescription drugs. indicators used in humans are erythrocyte glutathione reductase (egr), erythrocyte flavin concentration and urinary excretion. the erythrocyte glutathione reductase activity coefficient (egrac) provides a measure of tissue saturation and long-term riboflavin status. results are expressed as an activity coefficient ratio, determined by enzyme activity with and without the addition of fad to the culture medium. an egrac of 1.0 to 1.2 indicates that adequate amounts of riboflavin are present; 1.2 to 1.4 is considered low, greater than 1.4 indicates deficient. for the less sensitive "erythrocyte flavin method", values greater than 400 nmol/l are considered adequate and values below 270 nmol/l are considered deficient. urinary excretion is expressed as nmol of riboflavin per gram of creatinine. low is defined as in the range of 50 to 72 nmol/g. deficient is below 50 nmol/g. urinary excretion load tests have been used to determine dietary requirements. for adult men, as oral doses were increased from 0.5 mg to 1.1 mg, there was a modest linear increase in urinary riboflavin, reaching 100 micrograms for a subsequent 24-hour urine collection. beyond a load dose of 1.1 mg, urinary excretion increased rapidly, so that with a dose of 2.5 mg, urinary output was 800 micrograms for a 24-hour urine collection. history the name "riboflavin" comes from "ribose" (the sugar whose reduced form, ribitol, forms part of its structure) and "flavin", the ring-moiety which imparts the yellow color to the oxidized molecule (from latin flavus, "yellow"). the reduced form, which occurs in metabolism along with the oxidized form, appears as orange-yellow needles or crystals. the earliest reported identification, predating any concept of vitamins as essential nutrients, was by alexander wynter blyth. in 1879, blyth isolated a water-soluble component of cows' milk whey, which he named "lactochrome", that fluoresced yellow-green when exposed to light. in the early 1900s, several research laboratories were investigating constituents of foods, essential to maintain growth in rats. these constituents were initially divided into fat-soluble "vitamine" a and water-soluble "vitamine" b. (the "e" was dropped in 1920.) vitamin b was further thought to have two components, a heat-labile substance called b1 and a heat-stable substance called b2. vitamin b2 was tentatively identified to be the factor necessary for preventing pellagra, but that was later confirmed to be due to niacin (vitamin b3) deficiency. the confusion was due to the fact that riboflavin (b2) deficiency causes stomatitis symptoms similar to those seen in pellagra, but without the widespread peripheral skin lesions. for this reason, early in the history of identifying riboflavin deficiency in humans the condition was sometimes called "pellagra sine pellagra" (pellagra without pellagra). in 1935, paul gyorgy, in collaboration with chemist richard kuhn and physician t. wagner-jauregg, reported that rats kept on a b2-free diet were unable to gain weight. isolation of b2 from yeast revealed the presence of a bright yellow-green fluorescent product that restored normal growth when fed to rats. the growth restored was directly proportional to the intensity of the fluorescence. this observation enabled the researchers to develop a rapid chemical bioassay in 1933, and then isolate the factor from egg white, calling it ovoflavin. the same group then isolated the a similar preparation from whey and called it lactoflavin. in 1934, kuhn's group identified the chemical structure of these flavins as identical, settled on "riboflavin" as a name, and were also able to synthesize the vitamin. circa 1937, riboflavin was also referred to as "vitamin g". in 1938, richard kuhn was awarded the nobel prize in chemistry for his work on vitamins, which had included b2 and b6. in 1939, it was confirmed that riboflavin is essential for human health through a clinical trial conducted by william h. sebrell and roy e. butler. women fed a diet low in riboflavin developed stomatitis and other signs of deficiency, which were reversed when treated with synthetic riboflavin. the symptoms returned when the supplements were stopped.