Studies show that riboflavin is not required in intakes above around 2 mg per day for a typical adult (here) and excess riboflavin is excreted in the urine. Anyone who has taken B vitamins will know that they can turn the urine a bright yellow colour. This colour is the unmetabolised riboflavin in the urine, and is at first glance may seem suggestive of saturation of the metabolic pathways of the body with riboflavin. Researchers have used feeding studies to assess the effects of supplemental riboflavin, and in one such study1, secondary school boys from nigeria had their riboflavin statuses assessed before and after supplementation with 5 mg of riboflavin for 7 days using urinary excretion and riboflavin and erythrocyte glutathione reductase as biomarkers. The study was conducted in nigeria and the energy intake of some of the subjects was lower than recommended. Dietary riboflavin varied from 0.99 to 1.34 mg per day, mainly from milk and dairy products, and generally intakes were ~80 % of the recommended level.
Urinary excretion of riboflavin did not indicate that subjects were deficient in riboflavin. However, supplemental riboflavin significantly increased urinary excretion within a few days from 0.3 to 3.5 mg/g creatinine. None of the subjects had clinical ariboflavinosis (riboflavin deficiency), but biochemical measures of riboflavin status were low in some individuals and 38 % of the subjects could be said to have poor (subclinical) riboflavin status. Supplementation improved the erythrocyte glutathione reductase status of some of the individuals, but this was not reflected in changes in urinary excretion. Urinary measured of riboflavin may therefore be a poor indicator of riboflavin status because they do correlate well with glutathione reductase activity. Therefore metabolic assessment of glutathione reductase is a better measure of actual riboflavin status, and yellow urine does not confirm saturation of metabolic pathways with riboflavin.
RdB
