It has been known for many years that CHO intake has a significant sparing effect upon amino acid oxidation and protein balance.8190 The dietary interaction between protein and CHO may have implications for those athletes who habitually consume fad diets that stress a very low CHO intake. Given that CHO is the predominant fuel utilized during endurance exercise,40,41 and that this substrate can become depleted during prolonged endurance exercise,40 it is important for amino acid metabolism to be considered in light of CHO intake and storage (i.e., glycogen) status of the athlete. CHO loading has been shown to attenuate plasma and sweat urea excretion following endurance exercise.91 Furthermore, CHO supplementation increases whole-body protein synthesis92 and attenuates proteolysis.93 We have reported that both men and women show attenuated total amino acid oxidation (serial urinary urea excretion) during endurance exercise when CHO supplements are consumed during exercise.55 These findings have been confirmed in well-trained cyclists.94 These latter two studies55,94 emphasize the fact that CHO consumption during exercise is an effective strategy to attenuate potential exercise-induced increases in amino acid oxidation. Furthermore, we have also found that urea excretion in urine was lower during a period of intensive endurance exercise training when women consumed a post-exercise supplement containing CHO with a small amount of protein, compared to consuming the same supplement at a time separated from the exercise bout by more than 4 h.8 This finding was replicated in men performing resistance exercise,95 where a post-exercise CHO supplement resulted in a more positive nitrogen balance and an attenuation of 3-MH excretion (myo-fibrillar proteolysis).95 It is important to note that these post-exercise nutritional strategies net positive effects on nitrogen balance over a 24-h period and not just in the immediate post-exercise period.82,95 Finally, glucose consumption during endurance exercise appears to reduce leucine oxidation (~20%), but only when dietary protein intakes are rather high (1.8 g/kg/day) and not when they are low (0.7 g/kg/day — below any Recommended Dietary Allowance [RDA] for any country).52 The positive effects of CHO on net protein balance are probably due to an insulinmediated stimulation of protein synthesis and an attenuation of protein breakdown (see below). The convenience and relative inexpense of CHO supplementation makes this an attractive strategy to favorably alter net protein balance in resistance sports.
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