Riboflavin Vit. B-2

Riboflavin (vitamin B-2) is chemically known as 7,8-dimethyl-10 (1'-D-ribityl)isoalloxazine. In tissues there is a broad distribution of flavin but little is present as free riboflavin. The majority is found in flavocoenzymes [mainly flavin adenine dinucleotide (FAD) and lesser amounts are in mononucleotide (riboflavin-5'-phosphate)(FMN). Levels of flavin range from a few µg/g wet weight in skeletal muscle and intestine, and up to 30 to 35 µg/g in liver and kidney. Flavocoenzymes are largely non-covalently associated within diverse flavoproteins, but some exist as 8a-linked FAD.

As the catalytically operating moiety within bound flavocoenzymes, riboflavin participates in oxidation-reduction reactions in numerous metabolic pathways and in energy production via the respiratory chain. Flavoproteins participate in both one- and two-electron transfers. They operate in pyridine nucleotide-dependent and independent and independent dehydrogenations, reactions with sulfur-containing compounds, hydroxylation, oxidative decarboxylations, deoxygenations, and reduction of O2 to hydrogen peroxide following abstraction of hydrogen from substrates.

Deficiencies: Ariboflavinosis is characterized by weakness, sore throat, hyperemia and edema of the pharyngeal and oral mucous membranes, cheilosis, angular stomatitis, glossitis, seborrheic dermatitis, corneal vascularization, and normochromic, normocytic anemia associated with pure red cell hypoplasia of the bone marrow.

Clinical uses: Riboflavin is used therapeutically to ameliorate ariboflavinosis resulting from diverse causes such as inadequate dietary intake, decreased assimilation, rare genetic defects in the formation of specific flavoproteins, hormonal disorders and after use of certain drugs. Also, riboflavin supplements are provided during treatment of neonatal jaundice with phototherapy. The side chain of the vitamin is photochemically destroyed as it is involved in the photosensitized oxidation of bilirubin to more polar excretable compounds.

Diet recommendations: The Recommended Dietary Allowances range from 0.4 mg/d for early infants to 1.3 mg/d for women and 1.7 mg/d for men. An additional 0.3 mg/d is recommended during pregnancy and 0.5 mg/d for lactation.

Food sources: Small amounts of riboflavin, occurring largely as digestible coenzymes, are present in most plant and animal tissues. Especially good sources are milk, eggs, enriched cereals and grains, ice cream, liver, some lean meats, and green vegetables such as broccoli. As much as a third of North American intake is attributed to milk and other dairy products.

Toxicity: The limited capacity to absorb orally administered riboflavin precludes its potential for harm. Riboflavin intake of many times the RDA is without demonstrable toxicity. Nevertheless, the photosensitizing properties of riboflavin raise the possibility of some theoretical, potential risks.

Recent research: A comprehensive assessment of the human bio-kinetics for riboflavin absorption, transport, metabolism, and excretion has just been completed. Ongoing studies of a more biochemical nature are leading to detailed understanding of structures and mechanisms of numerous flavo-proteins from diverse organisms.

For further information:

McCormick, D.B. (1994) Riboflavin. In: Modern Nutrition in Health and Disease (Shils, M.S., Olson, J.A. & Shike, M., eds.), 8th ed., pp. 366-375. Lea & Febiger, Philadelphia, PA

Yagi, K., ed. (1994) Flavins and Flavoproteins. Walter de Gruyter, New York, NY; Rivlin, R.S. (1996) Riboflavin. In: Present Knowledge in Nutrition (Ziegler, E.K. & Filer, L.J., eds.), 7th ed. ILSI Press, Washington, DC.

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