Dietary riboflavin must be liberated as free riboflavin prior to absorption; therefore, covalently bound forms are unavailable for absorption. Most dietary riboflavin is in the form of noncovalently bound coenzymes, primarily flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), and free riboflavin is liberated following the action of the digestive enzymes. Riboflavin is absorbed in the upper ileum by an active, saturable carrier; although passive diffusion may also occur at high concentrations. The number of carriers appears to be regulated in direct proportion to the need for riboflavin. Following absorption, riboflavin is phosphorylated in the mucosal cell, which aids active transport through a trapping mechanism, followed by release of free riboflavin into the bloodstream.148,149
Riboflavin is also synthesized by the microflora of the large intestine, and small amounts are absorbed by an active carrier present in the colonic cells. Excess riboflavin is excreted by the kidney, primarily unchanged, although small amounts may appear as metabolites. Fecal levels are usually high because of synthesis by the microflora of the colon and are not an indicator of increased excretion or decreased absorption.
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