Sulfur amino acid intake in humans is variable and optimal SAA intake has not been adequately defined. The Recommended Dietary Allowance for SAA is based upon nitrogen and sulfur amino acid balance studies and for an adult male is about 1 g (210 mg Met plus 800 mg Met or Cys) (10,11). The average American diet contains about 100 g/d of protein (2); the mean SAA intake is about 2.4 g, but there is a large variation, ranging from < 0.3 g to > 5 g (12). Most of the SAA is derived from animal protein, which constitutes about 2/3 of the protein intake. Except for legumes and some nuts, a gram of plant-derived protein contains only 10-20% of the SAA content of animal protein (13,14). Because plant-derived foods are low in total protein, individuals who do not consume animal protein are at risk of SAA deficiency. The number of Americans with low dietary intake of SAA is relatively small, but most people undergo intermittent periods of SAA deficiency due to food selection, dieting, fasting and illness. The non-protein content of Cys in human liver is about 1 g, largely present in GSH, and approximates the daily SAA requirement (15).