Recent Trends in Protein Requirements
A number of recent studies have found varying results from any given protein intake when factors such as the timing of protein ingestion, the pattern of digestion, and energy intake of the participant were varied [18]. In this context it may be too simplistic to administer a 'general' protein requirement. What is known however is that amino acids stimulate potent anabolic effects. Further, it appears that a relatively higher protein intake can positively effect indexes of performance, increase positive nitrogen balance, and enhance lean body mass. Wolfe summarizes by stating that:
"our results, along with virtually all data in the literature on the topic, suggest that increasing amino acid intake will increase muscle mass, with all other variables remaining constant. Whereas this concept runs counter to popular perception, real-life examples abound in obese individuals who have significantly elevated muscle mass despite living sedentary lives. Thus, it is likely that increasing amino acid intake...will promote muscle anabolism, whether in depleted individuals such as the elderly or in active athletes trying to increase muscle mass. The exact nutritional approach will determine the extent of anabolic response (emphasis added) [18]."
Summary of protein requirements
In summary protein requirements appear to be elevated for strength training athletes. This increased need is attributed to enhanced oxidation rates of endogenous amino acids during exercise [27], the need for increased substrate to repair damaged muscle tissue [12], and the capacity to maintain elevated protein synthesis for greater amounts of muscle tissue [18]. Techniques utilized to measure protein requirements include nitrogen balance methods [15-17], tracer techniques [13], and performance and body composition techniques [28,29]. Nitrogen Balance techniques suggest that the protein requirements to attain zero nitrogen balance range from 1.2–2.2 grams of protein per kg of bodyweight [1,7,14,23,24,31-33]. Further, there is evidence that nitrogen retention increases as nitrogen uptake increases [16]. Hegsted [16] presented a series of studies which suggested that 20% of the nitrogen above maintenance is retained. However, these results have not directly translated to enhanced lean body mass [13,15,16]. There are two rationales for these findings. The first is that nitrogen retention is inherently overestimated [16], largely because nitrogen losses are underestimated [16]. A second rationale is that the overall length of most nitrogen balance studies do not allow for a statistically significant measure of LBM increases [1].
Tracer techniques have been recently employed and generally support the protein requirements obtained through nitrogen balance studies [13,24]. In fact Tarnopolsky et al. [13] found that protein synthesis increased from low (0.8 g/kg) to moderate (1.4 g/kg) intakes. While there was an 8.6% increase from moderate to high (2.4) protein intakes, these results did not reach significance. The authors suggested that this non significant trend appears to support the suggestion that the real protein requirements of athletes were closer to the 1.8 grams of protein per kg of bodyweight daily. Further, oxidation rates of leucine increased greatly from moderate to high protein intakes. This was taken as evidence that protein intakes above maintenance are oxidized. Unfortunately, these results are slightly confounded by the fact that the higher protein intake occurred through supplementation of whey protein, which enhanced the protein quality of the diet. This may cause an under estimation of the protein needs of athletes had they simply expanded their normal protein sources. Further, whey is a fast digesting protein and is known to naturally increase leucine oxidation rates [5,6]. Therefore, the increased oxidation rates may have been related to both an increase in protein intake, as well as a higher proportion of that intake coming from whey protein. Future studies will need to delineate between these variables.
Recently there have been a number of studies which have investigated higher protein intakes on indexes of performance and body composition [28-30]. A number of these studies have supported the efficacy of higher protein intakes, and yielded greater indexes of strength, and enhanced lean body mass [28-30]. This led Wolfe to conclude that increasing protein intake "will increase muscle mass, with all other variables remaining constant [40]."
It is further known that a number of other variables affect protein intake. Perhaps the most critical of these is energy intake. When an individual is in a caloric deficit, protein needs are greater than when the individual is in maintenance or a caloric surplus [34]. Finally it has been recently postulated that a true general protein requirement may be impossible to find considering that studies strongly suggest that different results will be obtained with the same protein intake when a number of variables are manipulated [1]. The remainder of the paper will provide an in depth analysis of a number of these variables.
So much to say about such matters.