Riboflavin (vitamin B-2) plays an important role in energy metabolism as part of the flavin mononucleotide (FMN) and flavine adenine dinucleotide (FAD) molecules. Both FAD and FNM are important oxidising agents that participate in aerobic respiration. The main sources of riboflavin in Western diets are milk and dairy products which make up 51% of intake in preschool children, 35% in school children, 27% in adults and 36% in the elderly. This reduced intake from dairy products in adults and the elderly reflects the decreasing milk consumption with age as demonstrated in the National Food Consumption Survey in the United Kingdom. The remaining intakes are derived from meat, fortified cereals, fatty fish, dark-green vegetables and some fruits. As a result, riboflavin deficiency tends to be found in populations whose inhabitants eat little or no dairy or meat products, such as areas of Guatemala.
Breakfast cereals are fortified with riboflavin which increases intake in individuals who do not consume large quantities of meat or diary products. Excretion of riboflavin increases sharply as intakes exceed 1 mg, with the inflection point in the excretion rate being considered the tissue saturation level. Urinary excretion is not considered an effective marker of riboflavin levels. Assessment of levels tends to be performed with an FAD stimulation test, which is considered an accurate index of riboflavin deficiency in adults. The difference between intakes of riboflavin that cause tissue saturation (>1 mg) and deficiency (<0.5 mg) is small. Some riboflavin is present in food as free riboflavin (an isoalloxazine ring bound to a ribitol side chain) and eggs and milk contain high amounts of free riboflavin bound to specific binding proteins. However most is present as FAD and to a lesser extent the FMN molecules.
Absorption of riboflavin in the FAD and FNM forms is achieved by hydrolysis to free riboflavin in the gut, a reaction catalysed by phosphatase enzymes. Absorption occurs in the duodenum via an active transport mechanism that shows linear absorption up to around 25 mg of free riboflavin. Above these values little absorption occurs. Absorption of free riboflavin to the enterocytes of the gut occurs across the apical membrane, after which the riboflavin is metabolised to FNM by the enzymes cytosolic flavokinase. Intracellular FNM can be hydrolysed to free riboflavin and can then cross the basolateral membrane and enter the portal system where it is converted to FAD in the liver. Colonic microflora can synthesise riboflavin which is absorbed by an active transport system. Research suggests that the contribution by bacteria to total riboflavin intakes might be quantitatively important to human health
Riboflavin deficiency can cause neurodegeneraton in some animals with deficiency during pregnancy being linked to deformation of the foetus including soft tissue and bone abnormalities in some species. In human, evidence suggests that riboflavin deficiency may cause improper handling of iron which can result in haematological problems and the development of riboflavin responsive anaemia, through impaired iron absorption. No clear deficiency signs exist in humans but reports suggest that mouth lesions and inflammation of the tongue may be symptomatic. Riboflavin is required to form the oxidising co-factors FAD and FNM and deficiency of riboflavin may impair energy metabolism. In rats, riboflavin deficiency causes a dose related reduction in succinate dehydrogenase, an enzyme involved in the citric acid cycle. Riboflavin deficient rats have altered hepatic lipid profiles thought to be a result of altered β-oxidation of fatty acids because of alterations to the necessary flavin electron acceptors.
RdB
