Erin Simmons
Protein is often thought to be a workout necessity, the essential complement to every gym bag. Missing protein during the post-workout anabolic window is viewed as unfortunate, if not detrimental to one’s training goals. However, the scientific literature on this subject isn’t quite so black and white. Reviews of protein requirements have touted 1.8 g-1kg-1day-1 as the optimal protein intake for individuals undergoing training, when in fact the literature has proposed recommended intakes in a range from .8 g-1kg-1day-1 up to 3-4 g-1kg-1day-1.
There are two reasons for such variable results. First, untrained and trained individuals may have differing protein requirements. Untrained individuals exposed to exercise bouts must undergo physiological changes in response to this new stimulus, whereas trained individuals have adapted to the exercise stimulus and have achieved a new metabolic steady-state. Thus, protein requirements for untrained individuals are elevated while trained individuals have become more efficient in their protein utilization, potentially resulting in lower protein requirements. Many studies are performed on untrained subjects in order to maximize results, since trained individuals show a lesser magnitude of response to the exercise stimulus. It is therefore easy to see why protein recommendations for exercising individuals may fall in favor of higher intakes.
Secondly, the methods used to determine optimal protein intakes in past studies have assumptions and limitations that may have affected their results. Nitrogen balance methods, which compare the amount of nitrogen ingested in the form of protein with the amount of nitrogen excreted, can easily be measured over long time periods but with lesser accuracy. It is difficult to capture all excreted nitrogen, especially in exercise studies where nitrogen may be lost at accelerated rates through sweat. Additionally, nitrogen balance is unable to capture measures of efficiency of nitrogen use or determine which tissues are using nitrogen. The resolution of nitrogen balance methods is thus too low to accurately determine an optimal protein intake. More advanced methods involving radiolabeled amino acid tracers are the current standard of measuring protein synthesis. This method involves infusion of the tracer into the bloodstream to label specific amino acids in order to observe their uptake into the muscle. However, obtaining an accurate measure of uptake requires knowledge of the original pool of the labeled amino acid in the body and thus requires subjects to be fasted prior to measurement. This likely changes the response to both exercise and protein intake, resulting in seemingly high rates of protein synthesis immediately following exercise and protein consumption.
New methods of measuring protein synthesis have been developed that allow long-term measurements of protein synthesis during recovery from exercise with a high degree of accuracy. Such methods account for efficiency of amino acid use, resulting in determination of a true optimal protein intake. Additionally, fasting is not required, yielding an intake recommendation applicable to free-living individuals. It is likely that these new methods will challenge the high-protein paradigm that is currently accepted among active individuals and athletes, giving rise to a more optimal diet to enhance performance.