In addition, detoxification of foreign compounds by conjugation with GSH, sulfate or taurine requires utilization of Cys. Such use can impose a substantial SAA demand; for instance, metabolism of 4 doses of maximum strength acetaminophen, i.e., 4 g daily, will consume the equivalent of up to 250 mg Cys for GSH conjugation and 640 mg Cys for sulfate conjugation (13). This amount is very similar to the RDA for SAA. Thus, the combination of dietary and environmental factors affecting SAA homeostasis and the complex metabolic effects that can result from variation in SAA homeostasis emphasize the need for better means to evaluate adequacy of SAA nutrition. This need is present for otherwise healthy individuals and is accentuated in acute or chronic illness and during convalescence and metabolic/nutritional stabilization during recovery from illness. The lower intake level was selected because it is similar to the lower intake levels estimated from food frequency analysis (11). This level could occur with a vegetarian diet with a poor selection of foods, but would not be as extreme as found with prolonged fasting or starvation. The upper SAA intake level was selected because 3X RDA is about 20% above the average intake of SAA in this country, thus representing a somewhat high but otherwise normal SAA intake.