Fig 1. Study scheme for dietary sulfur amino acid (SAA) effects on plasma cysteine, cystine and cysteine/cystine redox potential.

The 13 healthy adult study subjects were each admitted into the Emory General Clinical Research Center (GCRC) for a 13-day inpatient study period. The first 3 days were an equilibration phase, during which conventional food items approximating the RDA for SAA (18.7 mg·kg⁻¹) were given. The subjects were given chemically defined oral diets for a subsequent 10-day study period providing maintenance energy intake and 1.0 g/kg body weight/day protein equivalents. During the study period without SAA, all 13 subjects were fed a chemically defined diet devoid of Met and Cys (SAA insufficiency phase). This was followed by a SAA repletion phase during which the chemically defined diet providing either 56 mg SAA per kg body weight per day (n=8), similar to average SAA intake based on U.S. consumption data, or 117 mg·kg⁻¹·d⁻¹ SAA (n=5), similar to 99 percentile intake of Americans, was given. Protein equivalents were provided as an L-amino acid mixture containing 9 essential amino acids, without or with methionine (Met) and 9 non-essential amino acids, without or with cysteine (Cys). The SAA repletion diets were made isocaloric and isonitrogenous to the SAA insufficient diet by proportional adjustment of the 9 non-essential amino acids in the L-amino acid mixture. Arrows designate time points for overnight fasting plasma sample collection for free Cys, free CySS and E_hCySS, which were obtained at 0830 AM.