Absorption, metabolism, and excretion of riboflavin-5'-phosphate in man.

Evidence is presented which indicates that riboflavin-5′-phosphate (FMN) is absorbed from the gastrointestinal tract of man by specialized transport rather than by passive diffusion. Oral administration of the vitamin after a meal results in more extensive absorption which appears to be primarily due to a decrease in intestinal transit rate resulting in longer retention of FMN at absorption sites in the small intestine. Both FMN and riboflavin are excreted in the urine primarily, if not solely, as free riboflavin. The urinary recovery of riboflavin is the same when equimolar amounts of either form of the vitamin are administered orally in solution. The time course of urinary excretion of riboflavin after administration of high doses of FMN, together with data available in the literature, suggest that the vitamin is subject to enterohepatic cycling.