quantitated. Even less information is available on the levels of other water-soluble vitamins that may be desirable for performance enhancement. Certainly essential roles for vitamin B₆ in protein utilization and for riboflavin in general oxidative metabolism can be appreciated on the basis of known biochemical reactions, but the amounts of these and other vitamins that optimize performance with no compromise of long-term health in humans are not certain.

The gaps in information on the impact of stress or exertion on the ultimate disposition of natural derivatives of vitamins are especially great. There are few if any data on the efficiency of digestion of coenzymes within the tissues consumed as food in the face of shifting physical demands. Nothing is known about such effects on the release and recovery of vitamins from glycosides, esters, and peptides, which could account for a significant fraction of potential vitaminic material in many foods. The relationship of performance capability to micronutrient bioavailability has yet to be adequately researched.

What is known is that the metabolic pathways utilized in the molecular dissolution of nutrient compounds by E. coli and the human are generally similar, and among mammals even semiquantitative comparisons are usually valid. This is fortunate since it allows one to answer some questions at the cellular level with material obtained from animals (e.g., rats) that cannot ethically be accessed from humans. Cells that are freshly isolated after appropriate collagenase perfusion in situ of organs such as liver or kidney maintain viability for sufficient time in modified, oxygenated buffer to permit meaningful measurements of nutrient uptake and utilization. This technique allows separation of the cellular component in the overall fate of a nutrient that must undergo systemic absorption, circulatory transport, and organ dissemination. Moreover, there is no risk of cell transformation, which often occurs with the repeated transfers necessary in cell tissue culture techniques.

Considerable information has been gained regarding the mechanisms by which liver and kidney cells import and subsequently assimilate water-soluble vitamins such as riboflavin, B₆, biotin, and C (Bowman et al., 1989; McCormick and Zhang, 1993; Rose et al., 1986). Coupled with the extensive work of this laboratory on the metabolism of these vitamins and the cofactor lipoic acid (Chastain and McCormick, 1991; McCormick, 1975, 1979, 1989; McCormick and Wright, 1970), these investigators were poised to compare the means by which cells take in and differentially utilize certain natural derivatives. To illustrate this, isolated-cell and metabolic techniques recently have been used to augment understanding of the bioavailability of the glucosides of pyridoxine (Zhang et al., 1993) and of riboflavin (Joseph and McCormick, 1995).

The occurrence of natural glucosides of both vitamins B₂ (riboflavin) and B₆ has been known for some time, but their roles were less clear. The extent to