Differences in quantity and timing of SAA intake can result in substantial effects on Met, Cys and GSH homeostasis. Animal studies show that there is a diurnal variation in hepatic GSH in which the level increases following consumption of foods high in Cys and decreases in the post-absorptive period (16-17). The total amount of Cys present in GSH in the human body is about 3 g, roughly equal to the SAA requirement for 3 d (13) so that the Cys supply/utilization pattern can be considered in 4 phases. In the first phase, systemic Cys supply is from dietary Cys and metabolism of dietary Met. During this phase, excess Cys signals enhanced catabolism (18). Under post-absorptive conditions, probably for a period of about a day, systemic supply of Cys occurs principally from hepatic release of GSH (14). Under these conditions, Cys metabolism continues to support biosynthetic needs for taurine and sulfate. Although not explicitly established for humans, diurnal variation in hepatic GSH in mice ceases after 24 h of fasting so that systemic Cys supply probably enters a third phase after about 24 h. In this phase, an increased fraction of total systemic supply is likely to be derived from non-hepatic tissues (e.g., skeletal muscle; 15). Because sulfate and taurine production normally occurs in the intestines and liver (19), a decline in utilization of Cys for these functions is expected during this phase. A fourth phase appears likely to occur after about 3 days in which conservation of Met and Cys would be maximal.