This study compared the effects of whey and casein on plasma AA, nitrogen balance (NBAL), glutathione and performance in dieting athletes. Twenty cyclists consumed 40 g·d-1 whey (WHEY) or casein (CAS) for 3 wk. On d 18 – 21 subjects restricted intake to 20 kcal·kg-1·d-1 plus protein supplement. Apparent NBAL was estimated on d 18 – 21 while postabsorptive and 2 h postprandial plasma AA were measured on d 14 and 21. On d 1, 15 and 22 subjects performed an exercise performance test and provided blood for glutathione analysis. Both groups experienced similar negative NBAL (CAS = -19.7 ± 1.4 g, WHEY = -21.4 ± 2.7 g) during energy restriction. There were trends towards a reduction in performance during energy restriction (p = 0.073) and an interaction of group with day (p = 0.072). There were significant main effects of state (postabsorptive = 34.5 ± 2.4 µM, postprandial = 37.1 ± 3.0 µM; p = 0.038) and day (d 14 = 33.8 ± 2.2 µM, d 21 = 37.8 ± 3.2 µM; p = 0.008) on plasma cysteine. There was a significant interaction of state and day on glutamine (p = 0.002), as levels increased 1.3% from postabsorptive to postprandial measurements on d 14, but decreased 4.2% on d 21. The absolute change in postabsorptive cysteine from d 14 to d 21 was correlated with NBAL (r = 0.766, p = 0.01) in CAS but not in WHEY. Plasma glutamine did not correlate with NBAL in either group. / Master of Science