Whole-body protein turnover response to short-term high-protein diets during weight loss: a randomized controlled trial

To determine whole-body protein turnover responses to high-protein diets during weight loss, 39 adults (age, 21+/-1 years; VO2peak, 48+/-1 ml kg(-1) min(-1); body mass index, 25+/-1 kg m(2)) were randomized to diets providing protein at the recommend dietary allowance (RDA), 2 x -RDA or 3 x -RDA. A 10-day weight maintenance period preceded a 21-day, 40% energy deficit. Postabsorptive (FASTED) and postprandial (FED) whole-body protein turnover was determined during weight maintenance (day 10) and energy deficit (day 31) using [1-C-13] leucine. FASTED flux, synthesis and breakdown were lower (P<0.05) for energy deficit than weight maintenance. Protein flux and synthesis were higher (P<0.05) for FED than FASTED. Feeding attenuated (P<0.05) breakdown during weight maintenance but not energy deficit. Oxidation increased (P<0.05) between dietary protein levels and feeding stimulated oxidation, although oxidative responses to feeding were higher (P<0.05) for energy deficit than weight maintenance. FASTED net balance decreased between dietary protein levels, but in the FED state, net balance was lower for 3 x -RDA as compared with RDA and 2 x -RDA (diet-by-state, P<0.05). Consuming dietary protein at levels above the RDA, particularly 3 x -RDA, during short-term weight loss increases protein oxidation with concomitant reductions in net protein balance.