A method for increasing the bioavailability of cysteine within an animal by reducing extracellular cysteine disulfide molecules to produce extracellular cysteine molecules. A membrane permeable thiol molecule that forms a membrane permeable disulfide molecule after becoming oxidized participates in thiol-disulfide exchange reactions on both sides of the cellular membrane, crossing the membrane randomly via simple diffusion. The membrane permeable disulfide, within or upon entering the highly reductive environment of a cell, becomes reduced to form a membrane permeable thiol molecule. the membrane permeable thiol can then diffuse to the exterior of the cell, where it is available to reduce another cystine molecule. This process can continue cycling, producing net cumulative increase in the concentration of extracellular cysteine. This is beneficial for providing cysteine as a nutrient to cells that require cysteine for nourishment.