There is general agreement that individuals engaged in strength and endurance exercise training require more dietary protein than the generally healthy population. As noted by Paddon-Jones, the RDA (0.8 g/kg bodyweight) “was never designed to provide prescriptive guidance for populations with extraordinary demands, be they clinical or athletic.” Just how much dietary protein is needed by exercisers is a matter of some debate, and (not surprisingly) will depend upon the type and intensity of the exercise. But in general, recommendations range from 1.2 to 2.0 g/kg, although a recent meta-analysis on protein supplementation involving resistance exercise trainees reported an upper 95% confidence interval of 2.2 g/kg/day.
While these recommendations are considerably higher than the RDA, they are compatible with guidelines from the Institute of Medicine to consume a diet that ranges in protein content of between 10 and 35% of calories. The upper end of that range represents a daily protein intake of 175 g for a person consuming 2,000 calories per day, which for a 160-pound person is about 2.4 g/kg body weight. Furthermore, according to NHANES data, the average American male consumes about 99 g of protein per day (about 1.4 g/kg body weight for a 160 pound person).6 Thus, it is not difficult to consume the amount of protein needed by exercisers-- especially when considering that caloric needs will increase as a result of exercise.
Over the years concerns have arisen about the potentially harmful effects of high-protein diets. One concern frequently mentioned is the adverse effect of protein intake on renal function. However, a very recently published meta-analysis of the clinical data found that high protein intakes do not adversely influence kidney function or glomerular filtration rate in healthy adults.7
In addition to total protein intake, questions have arisen about the impact of the timing of protein ingestion on performance, especially gains in strength and lean body mass in response to resistance exercise training. The anabolic window refers to the time period after resistance exercise during which dietary protein maximally stimulates muscle protein synthesis. A commonly held belief is that the anabolic window lasts for about 2 hours. However, more recent research refutes this belief and downplays the importance of timing.5,8 Enhanced protein synthesis following exercise may last for as long as 24 hours.
Another factor potentially influencing muscle mass is dietary protein distribution; that is, the distribution of protein intake throughout the day. Americans tend to consume most of their protein at dinner and via after-dinner snacking.9 However, evidence suggests that for optimal benefit, protein intake should be evenly distributed throughout the day. To this point, Schoenfeld and Aragon,10 propose that to maximize anabolism, one should consume protein at a target intake of 0.4 g/kg/meal across a minimum of 4 meals in order to reach a minimum of 1.6 g/kg/day. Using the upper daily intake of 2.2 g/kg/day reported in the literature and spreading intake over the same 4 meals would necessitate a maximum of 0.55 g/kg/meal. These recommendations mimic those made earlier by Paddon-Jones and Rasmussen11 to consume 25-30 g protein per meal.
Finally, there is the question of the influence of protein type on gains in muscle mass and strength. Because of its high leucine concentration and perhaps also because it is so quickly digested, whey protein is generally considered to be the optimal protein source for building muscle.12 Certainly, acute studies that measure muscle protein synthesis over a 4-hour period support this conclusion.13,14
However, a recent systematic review concluded that over a period of many weeks, protein source plays a minor (if any) role in determining gains in lean body tissue and strength in response to resistance exercise training.5 This point was recently confirmed by a meta-analysis that included 9 clinical trials that compared the effect of soy protein supplementation with whey protein supplementation and supplementation with other animal proteins on gains in strength and lean body mass in response to resistance exercise training programs lasting between 6 and 36 weeks.15 This analysis found no difference in gains between soy protein and whey and soy protein and the other animal proteins. Despite this finding, more work needs to be done to determine whether these findings apply to extremely highly-trained individuals. Still, overall, the evidence indicates that protein amount is more important than protein source.
Finally, there is speculative evidence to suggest that a blend of proteins may have subtle advantages over any single protein. For example, data from a 12-week trial by Reidy et al.16 found that a blend of casein (50%), whey (25%), and soy protein (25%) enhanced overall whole body lean mass in young men undergoing resistance exercise training more than whey protein alone. Also, Mobley et al.17 recently found that soy protein preferentially increased the size of type I (slow twitch) muscle fibers whereas whey protein supplementation preferentially increased the size of type II fibers (fast twitch).
In conclusion, exercise increases dietary protein requirements. With respect to gains in muscle mass and strength, data suggest protein type plays a much lesser role than protein amount. Although speculative, blends of proteins may have advantages over single proteins. For maximal benefit, dietary protein should be evenly distributed throughout the day.
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