Riboflavin

Riboflavin operates through its two active coenzyme forms: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The parent vitamin (7,8-dimethyl-10-ribityl-isoalloxazine) is phosphorylated by cytoplasmic riboflavin kinase (RFK) to FMN, which can be further adenylated by FAD synthetase (FADS) to FAD. These flavoenzyme cofactors function as electron-transfer prosthetic groups, accepting and donating one or two electrons in redox reactions via the isoalloxazine ring system that can exist in oxidized (quinone), semi-reduced (semiquinone, radical), or fully reduced (hydroquinone) states — an unusual three-state electron carrier unique to flavins among biological cofactors. Approximately 90 human flavoproteins have been characterized, and the majority of cellular redox biology beyond NAD+/NADP+ passes through FMN or FAD at some point.

Mitochondrial electron transport chain. Complex I (NADH:ubiquinone oxidoreductase) contains one FMN prosthetic group per complex, which is the first electron acceptor from NADH at the matrix-facing surface of the inner mitochondrial membrane. Complex II (succinate:ubiquinone reductase / succinate dehydrogenase) contains one FAD prosthetic group, covalently attached to the flavoprotein subunit, which accepts electrons from succinate oxidation during the TCA cycle. Both complexes feed electrons into the ubiquinone (Q) pool — see the CoQ10 entry for the downstream electron carrier. Electron transfer flavoprotein (ETF), another FAD-containing complex, accepts electrons from several dehydrogenases including acyl-CoA dehydrogenase (see β-oxidation below) and transfers them to the Q pool via ETF:ubiquinone oxidoreductase. The mitochondrial dysfunction hypothesis of migraine, glutaric aciduria, and MADD all center on riboflavin''s role at Complex I, Complex II, and ETF.

Fatty acid β-oxidation. Four acyl-CoA dehydrogenases (very-long-chain, long-chain, medium-chain, short-chain: VLCAD, LCAD, MCAD, SCAD) use FAD to accept the first electron pair in the sequential β-oxidation of fatty acids. Deficiency of these enzymes (genetic) or of riboflavin cofactor (acquired) produces characteristic patterns of metabolic acidosis with elevated acylcarnitines that can be screened by tandem mass spectrometry. Multiple acyl-CoA dehydrogenase deficiency (MADD/GA2) can present as a riboflavin-responsive form in which the partial loss of function of multiple dehydrogenases is rescued by supersaturation with riboflavin cofactor.

Glutathione reductase and the folate-homocysteine cycle. Glutathione reductase (GSR) is an FAD-dependent enzyme that regenerates reduced glutathione (GSH) from oxidized glutathione (GSSG) using NADPH as the reducing equivalent. This is the critical regeneration step for the cellular glutathione antioxidant system, which handles hydrogen peroxide detoxification (glutathione peroxidase) and phase II drug metabolism (glutathione S-transferase). Methylenetetrahydrofolate reductase (MTHFR) is an FAD-dependent enzyme that catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate — the methyl donor for homocysteine remethylation to methionine. The common C677T polymorphism (rs1801133) produces an MTHFR variant (alanine → valine at codon 222) with reduced FAD binding affinity; 10-15% of European-ancestry individuals are homozygous (TT) and 40-50% are heterozygous (CT). TT homozygotes have ~30% of normal MTHFR enzymatic activity at standard physiological conditions, with resulting elevations in plasma homocysteine and sensitivity to folate and riboflavin status. The McNulty/Wilson/Horigan Irish trials have consistently shown that riboflavin supplementation stabilizes MTHFR activity in 677TT homozygotes, lowering blood pressure by 6-13 mmHg — a genotype-specific intervention that is among the cleanest examples of personalized vitamin therapy. Pyridoxine-5-phosphate oxidase (PNPO) is FMN-dependent and converts dietary pyridoxine/pyridoxamine/pyridoxine-phosphate/pyridoxamine-phosphate to the active PLP form — so severe riboflavin deficiency impairs B6 functional status as well.

Kynurenine pathway. Kynurenine monooxygenase (KMO) is FAD-dependent and catalyzes a key step in the tryptophan-to-NAD+ biosynthetic pathway (tryptophan → kynurenine → 3-hydroxykynurenine). Severe riboflavin deficiency can therefore impair endogenous niacin synthesis and contribute to pellagra-like presentations in mixed B-complex deficiency (see the Niacin entry).

Additional flavoproteins. Xanthine oxidase (FAD-dependent, purine catabolism), monoamine oxidase (FAD-dependent, neurotransmitter degradation — the target of MAOI antidepressants), D-amino acid oxidase, NADPH oxidase (NOX family), nitric oxide synthase (NOS, with FAD + FMN both), the complex II succinate dehydrogenase, and many more complete the flavoenzyme landscape. This breadth explains why pharmacologic riboflavin has system-wide effects distinct from most single-enzyme-targeted drugs.

Riboflavin absorption and transport. Dietary riboflavin exists as free riboflavin plus FMN and FAD forms. Intestinal phosphatases hydrolyze FMN and FAD to free riboflavin prior to absorption. Uptake across the enterocyte apical membrane uses riboflavin transporter 3 (RFVT3, SLC52A3), a saturable sodium-independent facilitated transporter. At physiologic intakes (1-2 mg), absorption is near-complete; at pharmacologic oral doses (100-400 mg), transport is saturated and fractional absorption declines sharply — the maximum absorbed from a single oral dose is approximately 20-27 mg, with the remainder passing into the colon or being excreted. This saturable absorption has the practical implication that high daily doses should be divided into multiple administrations to maximize systemic delivery. Basolateral efflux uses RFVT1 (SLC52A1). Cellular uptake in peripheral tissues predominantly uses RFVT2 (SLC52A2). The genetic disorders Brown-Vialetto-Van Laere (SLC52A2 and SLC52A3 mutations) and Fazio-Londe (SLC52A3 mutations) result from loss of function of these transporters, producing tissue-specific riboflavin deficiency despite adequate intake; high-dose oral riboflavin can partly overcome the transport defect via passive diffusion at supersaturating concentrations. Intracellular riboflavin is phosphorylated by RFK to FMN, which can be further adenylated by FADS to FAD. FMN and FAD are retained in cells (tight protein binding), while free riboflavin is exported in urine. The characteristic bright yellow-orange fluorescent urine at pharmacologic doses reflects unmetabolized riboflavin excretion and is harmless.

Half-life and kinetics. Riboflavin has a plasma half-life of 66-84 minutes and tissue half-life considerably longer. There is no substantial body store of riboflavin; a moderate intake supports continuous need. Deficiency can manifest within weeks of inadequate intake.

Drug-riboflavin interactions. Phenothiazines (chlorpromazine), tricyclic antidepressants, and doxorubicin chemotherapeutics competitively inhibit riboflavin phosphorylation and transport, increasing riboflavin requirement. Probenecid reduces renal clearance of riboflavin. Oral contraceptives modestly reduce circulating riboflavin. Anticonvulsants (phenobarbital, phenytoin) increase riboflavin catabolism. Chronic alcohol use impairs absorption, phosphorylation, and storage through multiple mechanisms. UV light and phototherapy destroy riboflavin in plasma and in parenteral nutrition solutions, requiring shielding of TPN bags during infusion. Sulfonamides, tetracyclines, and certain antibiotics also interact. Boric acid toxicity (rare) depletes riboflavin.

Riboflavin (vitamin B2) is a water-soluble vitamin that serves as the precursor to two universal flavoprotein cofactors — flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) — which together serve as electron-carrying prosthetic groups in more than 90 human enzymes including Complex I and Complex II of the mitochondrial electron transport chain, the acyl-CoA dehydrogenases of fatty acid β-oxidation, glutathione reductase (the enzyme that regenerates reduced glutathione for antioxidant defense), methylenetetrahydrofolate reductase (MTHFR, the critical folate cycle enzyme), pyridoxine-5-phosphate oxidase (PNPO, which converts dietary B6 vitamers to active PLP), and kynurenine monooxygenase in the tryptophan-to-NAD+ pathway. The breadth of FMN/FAD dependency makes riboflavin a biochemically central vitamin despite its clinical understatement — most vitamin discussions focus on B1 (beriberi/Wernicke''s), B3 (pellagra), B6 (neuropathy), B9 (NTDs), and B12 (pernicious anemia), while B2 operates quietly in the background. The adult RDA is 1.3 mg/day for men, 1.1 mg/day for women, 1.4 mg/day in pregnancy, 1.6 mg/day in lactation. There is no formally established tolerable upper intake level because riboflavin has an exceptionally wide therapeutic window — pharmacologic doses up to 400 mg/day are used clinically for migraine prophylaxis without known organ toxicity, and doses up to several grams per day have been used for Brown-Vialetto-Van Laere syndrome without dose-limiting adverse effects other than the well-known bright yellow-orange urinary fluorescence that indicates excess is being excreted intact. Primary riboflavin deficiency (ariboflavinosis) is uncommon in isolated form in Western populations but recurs in chronic alcoholism, severe malabsorption, chronic phenothiazine or tricyclic antidepressant therapy (these drugs compete with riboflavin for intestinal transport and cellular phosphorylation), prolonged phototherapy (UV destroys riboflavin — a historical concern with phototherapy for neonatal jaundice), and specific genetic disorders of riboflavin metabolism. Symptomatic deficiency produces a characteristic picture of angular cheilitis (painful fissured corners of the mouth), glossitis (red, inflamed tongue), seborrheic dermatitis of the nasolabial folds and scrotum, photophobia, corneal vascularization, and normocytic normochromic anemia with reticulocytopenia. Two monogenic disorders of riboflavin transport produce severe clinical syndromes responsive to pharmacologic riboflavin: Brown-Vialetto-Van Laere syndrome (BVVL) and Fazio-Londe syndrome, caused by SLC52A2 (RFVT2) and SLC52A3 (RFVT3) mutations, present with childhood-onset cranial neuropathies, sensorineural deafness, bulbar palsy, and respiratory failure; aggressive high-dose riboflavin (10-80 mg/kg/day, sometimes up to 1500 mg/day) can halt disease progression and partially reverse neurological deficits, especially when initiated early. Multiple acyl-CoA dehydrogenase deficiency (MADD, also called glutaric aciduria type II) can present as a riboflavin-responsive form (RR-MADD) in which pharmacologic riboflavin 100-400 mg/day produces biochemical and clinical response. The supplement and clinical uses of riboflavin cluster in several distinct domains. Migraine prophylaxis is the best-established non-deficiency indication: Schoenen''s 1998 randomized trial of 400 mg/day riboflavin for 3 months reduced migraine attack frequency by 50%+ and established riboflavin as an evidence-based migraine preventive (PMID 9484373), adopted by the American Headache Society and American Academy of Neurology as a Level B preventive option for episodic migraine. The mechanism involves mitochondrial support through Complex I and Complex II of the electron transport chain, addressing the mitochondrial dysfunction hypothesis of migraine pathophysiology. Blood pressure in MTHFR 677TT homozygotes is a distinctive evidence-based use: McNulty, Wilson, and Horigan''s Irish trials have shown that riboflavin 1.6 mg/day reduces systolic blood pressure by 6-13 mmHg in the 10-15% of European-ancestry individuals homozygous for the MTHFR C677T polymorphism, whose MTHFR enzyme has reduced FAD binding and responds to riboflavin cofactor stabilization. Corneal crosslinking with riboflavin is a standard ophthalmological procedure for progressive keratoconus and corneal ectasia: topical riboflavin 0.1% solution plus UVA 370 nm exposure (Dresden protocol from Spoerl and Wollensak 2003) generates reactive oxygen species that form covalent crosslinks between collagen fibrils, stabilizing the cornea and halting ectasia progression. Glutaric aciduria type I (GA1) treatment includes riboflavin 50-300 mg/day alongside L-carnitine and protein restriction under pediatric metabolic supervision. Anemia with deficiency responds to repletion. Cataracts and photoprotection: riboflavin''s antioxidant role in the lens has generated interest for cataract prevention, with observational but not randomized evidence. Food sources concentrate in milk and dairy products (the original name "lactoflavin" referred to the vitamin''s isolation from milk), eggs, organ meats (liver, kidney), lean beef and pork, almonds, leafy greens, mushrooms, and fortified grains and cereals. Riboflavin is destroyed by UV light — the historical concern with milk stored in clear glass bottles exposed to sunlight — and by alkaline cooking conditions. See also Thiamine, Niacin, Vitamin B6, Folate, and Vitamin B12 for the broader B-complex, Alpha-Lipoic Acid for the shared mitochondrial cofactor role, CoQ10 for the electron transport chain partnership (Complex I/II feed CoQ), Iron for the overlapping role in kynurenine pathway and cytochrome biology, and Magnesium for the MTHFR cofactor context. This overview is educational only and is not medical advice — high-dose riboflavin for migraine or specific metabolic conditions should be physician-directed despite the excellent safety profile.

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