Absorption by Human Subjects

Studies of riboflavin absorption by human subjects require a combination of a test dose, usually taken by mouth, and a sampling procedure to estimate the amount absorbed, and possibly also its subsequent fate. The sampling compartment is generally the

Food

Food

Covalently bound flavin, not used, excreted Riboflavin glucoside

Riboflavin

Active transport: FMN

Covalently bound flavin, not used, excreted Riboflavin glucoside

Riboflavin

Active transport: FMN

Passive diffusion

Gut wall

Pancreas

Riboflavin

Blood

Figure 1 Characteristics of the absorption process for riboflavin and its coenzymes.

urine, since plasma has proved generally unsatisfactory. Fecal sampling is also useless because of the synthesis of riboflavin by bacteria in the large bowel. Although the use of riboflavin labeled with radioactive or stable isotopes is theoretically possible, this has not yet been applied to human studies. The majority of reported studies have relied on relatively large 'bolus' oral doses of riboflavin, comprising at least several milligrams, with urinary monitoring over the subsequent few hours. Riboflavin can be quantitated in urine by its very characteristic fluorescence, or by microbiological assay, or more accurately by high performance liquid chromatography.

For the maximum absorption of a test dose of riboflavin, the duration of exposure within the upper ileum is critical, since this is the region of greatest absorptive efficiency. There is little evidence to suggest that slow-release forms of the vitamin can enhance its absorption, but there does appear to be some absorptive advantage for certain synthetic lipophilic esters, such as the tetrabutyrate ester, which is hydrolyzed to the free vitamin during or after absorption. These have been shown to possess beneficial (e.g., antioxidant) properties in some model systems, but their usefulness in human medicine is still at a very early stage of assessment. The concomitant presence of food can enhance absorption, possibly by increasing the transit time. There is little evidence that the efficiency of absorption varies markedly with age or sex in humans.

Measurements of the plasma pool of riboflavin following test doses is not a viable method of measuring absorption, because redistribution to other tissue sites plus urinary excretion takes place too rapidly for this pool to be representative of the amount absorbed. Although the urinary response to a test dose has been the most commonly used approach to studies of intestinal absorption in humans, it suffers from the potential disadvantage that physiological intakes, and especially low intakes of riboflavin from 'poor' food sources, cannot be measured by this technique. Such studies of small doses are however needed, in order to determine the factors that modulate riboflavin absorption in developing countries, where dietary sources of riboflavin are minimal and clinical signs of riboflavin deficiency are common. A much more sensitive biochemical marker of riboflavin status at low intakes is the index known as 'erythrocyte glutathione reductase activation coefficient' (EGRAC), which will be discussed in greater detail below. It is possible to achieve a graded response to graded intakes of riboflavin, and studies of absorption efficiency using this alternative marker as the outcome measure may become feasible (and useful) in the future.

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