Sulfur amino acids are involved in central metabolic processes. The biologically useful sulfur is derived almost exclusively from the amino acids methionine and cysteine. Methionine (Met) is an essential amino acid and is metabolized in individuals by the transulfuration pathway to form cysteine (Cys) (1). In addition to use in the primary sequence of most proteins, both Met and Cys are required for other metabolic functions. Met is converted to S-adenosylmethionine, which is used for methylation reactions (2) for structural and functional modifications of proteins, RNA and DNA, as well as synthesis of phospholipids and signaling molecules. The carbon skeleton of Met is also used for biosynthesis of polyamines, which are required for cell division and cell growth (3). Cysteine is used for biosynthesis of glutathione (GSH), coenzyme A, taurine and sulfate (4). GSH functions in redox regulation (5) and detoxification of oxidants and reactive electrophiles (6). Coenzyme A is central to fatty acid metabolism and the citric acid cycle; taurine is utilized for bile acid synthesis and osmotic regulation (7); sulfate is used as a structural component of oligosaccharides (8), transport of steroid hormones (9) and detoxification of foreign compounds (10). Thus, there is a requirement for adequate sulfur amino acid intake that extends beyond the need for adequate amounts to maintain normal protein synthesis and turnover.