Complete proteins that contain high biological value (quality) amino acids (whey and casein) stimulate post-resistance exercise net protein anabolism without a difference between the two sources.108 CHO has an interactive effect with amino acids in that amino acids appear to increase protein synthesis, whereas CHO reduces protein breakdown.107 109 A combination of high-quality protein, amino acids, and carbohydrate clearly results in a more positive net protein balance following acute resistance exercise than does isoenergetic carbohydrate alone.110 One study found that the provision of CHO and essential amino acids before a bout of resistance exercise increased amino acid (phenylalanine) uptake to a greater extent than when provided immediately after, and that this was due to a greater delivery of amino acids.111 In a similar fashion, there is some evidence that the consumption of milk following resistance exercise may also increase the utilization of available amino acids for protein synthesis.112 This latter observation is of interest because it reflects the potentially beneficial role of a whole food product, as opposed to dietary supplements, in optimizing exercise adaptation. Other investigations of nutrient interactions have measured muscle FSR and whole-body protein turnover after resistance exercise in response. Compared with trials of CHO only, CHO + protein, and CHO + protein + free leucine, the CHO + protein + leucine trial resulted in the most positive protein balance through a reduction in breakdown and an increase in FSR.105 This latter study is the first to our knowledge supporting that the addition of an amino acid to a complete protein confers advantage in terms of post-exercise protein balance.
There has been an interest in the timing of nutrient delivery and the effects on glycogen synthesis in the recovery from endurance exercise.113,114 Studies have demonstrated that glycogen resynthesis is more rapid if the glucose is provided in the immediate post-exercise period vs. a 2-h delay,114 and that there may be a synergistic effect from the addition of protein to glucose drinks.113 However, in a recent study our group did not find evidence for a synergistic increase in postendurance exercise glycogen recovery with isoenergetic protein-glucose supplements vs. glucose alone.75 At higher levels of energy intake some have found that the addition of protein to carbohydrate may enhance the rate of post-exercise gly-cogen resynthesis compared with an isoenergetic carbohydrate only drink,115 while others have not,116 even with nonisoenergetic diets.117 Following resistance exercise, we found that isoenergetic glucose-protein supplements were similar in terms of glycogen resynthesis compared to glucose supplements alone.118 We also demonstrated that whole-body protein synthesis was greater for post-resistance exercise protein-glucose and glucose supplements than for placebo.119 Our group82 also found that the provision of a CHO + PRO + FAT defined formula diet given immediately post-endurance exercise in female athletes during a period of increased training volume enhanced performance, maintained weight, and tended (p = 0.06) to enhance nitrogen balance, compared to consumption of the same diet at a different time of the day. A series of studies by Burke and colleagues demonstrated that muscle glycogen content 24 h after endurance exercise was similarly restored whether subjects ate four large meals or many small snacks120 and whether protein and fat were consumed with the meals.121 These results suggest that the timing of postexercise CHO intake may not be critical if the next performance is not until 24 h later. However, if a sport requires several workouts or performances per day (e.g., a tournament), then a more rapid glycogen resynthesis may enhance performance. This phenomenon has also been shown for resistance exercise, where a CHO vs. placebo supplement given after one bout of exercise resulted in performance enhancement in a subsequent bout 4 h later.122
Taken together, the aforementioned observations suggest that the immediate post-exercise period, or at the onset or during exercise,111 is an important time to consume protein and CHO, particularly for the resistance athlete. This may have an impact on protein requirements and permit optimal muscle strength gains with any given protein intake. For the endurance athlete, the immediate provision of CHO is not as critical to glycogen resynthesis over the ensuing 24 h, provided that the daily CHO intake is high (~10 g/kg/day). Notwithstanding our current understanding, we still need to more fully examine the impact of immediate post-exercise intake of protein and CHO on 24-h whole-body protein retention. However, the acute response of protein turnover to exercise and amino acid consumption appears to reflect the balance over the ensuing 24 h.123 Together the findings in the aforementioned several paragraphs would predict that the timing (early post-exercise) and composition (protein + amino acids + carbohydrate) of the diet would positively influence (i.e., attenuate any possible increase) protein requirements.
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