The argument has been made that regular exercise, particularly in elite athletes with highly demanding training regimens, increases protein requirements over those for sedentary individuals. This argument is often based on nitrogen balance. Several well-controlled studies have shown that nitrogen balance in athletes is greater than in inactive controls [1,3,4,19]. Increased protein needs may come from increased amino acid oxidation during exercise [20-23] or growth and repair of muscle tissue. Muscle protein synthesis (MPS) is increased after resistance [24-26] and endurance exercise [27,28], suggesting that additional protein would be necessary to provide amino acids for the increased protein synthesis. Increased synthesis is ostensibly necessary for production of new myofibrillar proteins for muscle growth during resistance training and for mitochondrial biogenesis during endurance training.
In contrast, it has been extensively argued that exercise, even extensive, prolonged, and intense exercise, does not increase the dietary requirement for protein [9,14,15,18,29-32]. The argument is often based on the fact that exercise has been shown to increase the efficiency of use of amino acids from ingested protein. Butterfield and others [29,30,33] demonstrated this concept in a series of classic experiments showing that even at relatively low protein intakes and negative energy balance, nitrogen balance was improved when exercise was performed. More recently, it has been shown that exercise training increases muscle protein balance [26,34], suggesting that the reuse of amino acids from muscle protein breakdown is more efficient. This notion was investigated in a prospective, longitudinal study on the whole-body protein level using stable isotopic tracers . Whole-body protein balance was reduced in novice weightlifters after training, suggesting that protein requirements would be less with regular exercise training.
A common criticism of the studies that show increased use of amino acids with exercise is that the intensity or duration of exercise is not as great as that practiced by top sport athletes, and the requirements would be underestimated [16-18]. Many studies have shown that amino acid oxidation is elevated during exercise [22,23,36,37]. Animal studies have shown that exercise of sufficient intensity and duration may result in a catabolic state after exercise. MPS is decreased after exercise at high intensities and long duration [38,39]. It also has been reported that low-intensity endurance and resistance exercise does not stimulate MPS [40,41]. These results, together with the data indicating that higher intensity exercise increases MPS [24-26], suggest that there may be a continuum of exercise intensity in which the response of muscle protein metabolism changes (Fig. 1). At lower intensities, there is no response, but as intensity increases, MPS is stimulated. At the highest levels of exercise intensity and duration, however, the impact of the exercise reduces the response of MPS. Protein requirements may be related to the intensity and duration of the exercise that is practiced.
Arguments against protein requirements often are based on difficulties showing increased muscle mass at higher levels of protein intake. At best, studies are equivocal. Although studies have shown gains in muscle mass at higher protein intakes [42,43], a meta-analysis concluded that protein supplements had no impact on lean body mass during training . When the apparent increases in nitrogen balance are extrapolated to gains in lean body mass, the calculations suggest gains that are physiologically impossible—on the order of 200 to 500 g/d [1,3,4]. These results show the tendency for nitrogen balance methods to overestimate nitrogen balance at high intakes, perhaps owing to increases in the urea pool size . Suffice to say that there are studies providing evidence
for increased protein requirements for athletes and the opposite. These arguments are described in detail in other articles [11-13,15,16,18].
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