The level of dietary protein intake influences protein metabolism in response to exercise. During and after endurance exercise, the provision of extra protein (beyond requirement) resulted in an increase in leucine oxidation68,96,97 and in one study appeared to attenuate muscle FSR.10 The latter study measured mixed-muscle FSR in response to three different protein intakes (0.8, 1.8, 3.6 g/kg/day) in endurance-trained athletes and found that FSR was lowest at the highest protein intake level,10 yet whole-body leucine oxidation was greatest (a nutrient excess).97 Our group has shown that the provision of dietary protein at levels above requirement (e.g., 2.8 vs. 1.8 g/kg/day) resulted in an exponential increase in amino acid oxidation with no further increase in protein synthesis in male strength athletes.53 In addition, the provision of dietary protein at 2.6 g/kg/day during resistance exercise training in young males doing weight training did not confer any strength or mass benefits compared to a diet supplying 1.35 g/kg/day.98 Taken together, these data indicate that protein consumed in excess of need is oxidized as energy and does not have a net anabolic effect per se. However, there is a lower limit of protein intake where a further reduction in protein intake will have a negative impact on protein synthesis. It is the determination of these inflection/plateau points that ultimately will determine the optimal protein intake for a given type of exercise (exponential increase in oxidation and plateau in synthesis).
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