that this transport may depend on sodium ions, more recent work in humans (Said and Ma, 1994) indicates that uptake is independent of sodium ions. A small amount of riboflavin is absorbed in the large intestine (Sorrell et al., 1971).
In plasma some riboflavin is complexed with albumin; however, a large portion of riboflavin associates with other proteins, mainly immunoglobulins, for transport (Innis et al., 1985). Pregnancy increases the level of carrier proteins available for riboflavin (Natraj et al., 1988). This results in a higher rate of riboflavin uptake at the maternal surface of the placenta (Dancis et al., 1988).
At physiological concentrations the uptake of riboflavin into the cells of organs such as the liver is facilitated and may require specific carriers. At higher levels of intake, riboflavin can be absorbed by diffusion (Bowman et al., 1989; McCormick, 1989).
The metabolism of riboflavin is a tightly controlled process that depends on the riboflavin status of the individual (Lee and McCormick, 1983). Riboflavin is converted to coenzymes within the cellular cytoplasm of most tissues but mainly in the small intestine, liver, heart, and kidney (Brown, 1990; Darby, 1981). The metabolism of riboflavin begins with the adenosine triphosphate (ATP)-dependent phosphorylation of the vitamin to FMN. Flavokinase, the catalyst for this conversion, is under hormonal control. FMN can then be complexed with specific apoenzymes to form a variety of flavoproteins; however, most is converted to FAD by FAD synthetase. As a result, FAD is the predominant flavocoenzyme in body tissues (McCormick and Greene, 1994). Production of FAD is controlled by product inhibition such that an excess of FAD inhibits its further production (Yamada et al., 1990).
When riboflavin is absorbed in excess, very little is stored in the body tissues. The excess is excreted, primarily in the urine. A wide variety of flavin-related products have been identified in the urine of humans. In healthy adults consuming well-balanced diets, riboflavin accounts for 60 to 70 percent of the excreted urinary flavins (McCormick, 1989). Urinary excretion of riboflavin varies with intake, metabolic events, and age (McCormick, 1994). In newborns, urinary excretion is slow (Jusko and Levy, 1975; Jusko et al., 1970);