The Effect of Whey Protein Isolate on Plasma Amino Acids, Nitrogen Balance, Glutathione and Performance during Energy Restriction in Athletes

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.