Controversy exists about the maximum amount of protein that can be utilized for lean tissue-building purposes in a single meal for those involved in regimented resistance training. A long-held misperception in the lay public is that there is a limit to how much protein can be absorbed by the body. From a nutritional standpoint, the term “absorption” describes the passage of nutrients from the gut into systemic circulation. Based on this definition, the amount of protein that can be absorbed is virtually unlimited. Following digestion of a protein source, the constituent amino acids (AA) are transported through the enterocytes at the intestinal wall, enter the hepatic portal circulation, and the AA that are not utilized directly by the liver, then enter the bloodstream, after which almost all the AA ingested become available for use by tissues. While absorption is not a limiting factor with respect to whole proteins, there may be issues with consumption of individual free-form AA in this regard. Specifically, evidence shows the potential for competition at the intestinal wall, with AA that are present in the highest concentrations absorbed at the expense of those that are less concentrated [1].

It has been proposed that muscle protein synthesis (MPS) is maximized in young adults with an intake of ~ 20–25 g of a high-quality protein, consistent with the “muscle full” concept; anything above this amount is believed to be oxidized for energy or transaminated to form alternative bodily compounds [2]. The purpose of this paper is twofold: 1) to objectively review the literature in an effort to determine an upper anabolic threshold for per-meal protein intake; 2) draw relevant conclusions based on the current data so as to elucidate guidelines for per-meal daily protein distribution to optimize lean tissue accretion.

Although the previously discussed studies offer insight into how much protein the body can utilize in a given feeding, acute anabolic responses are not necessarily associated with long-term muscular gains [30]. The topic can only be answered by assessing the results of longitudinal studies that directly measure changes in lean mass with provision of varying protein dosages, as well as proteins of varying speeds of digestion/absorption.

Wilborn et al. [31], found no difference in lean mass gains after 8 weeks of pre- and post-resistance exercise supplementation with either whey or casein. Similarly, a lack of between-group differences in lean mass gain was found by Fabre et al. [32] when comparing the following whey/casein protein ratios consumed postexercise: 100/0, 50/50, 20/80.

In a 14-day study of elderly women, Arnal et al. [33] demonstrated that providing a majority of daily protein (79%) in a single meal (pulse pattern) resulted in a greater retention of fat-free mass compared to an evenly distributed intake partitioned over four daily meals (spread pattern). A follow-up study by the same lab in young women reported similar effects of pulse versus spread patterns of protein intake [34]. The combined findings of these studies indicate that muscle mass is not negatively affected by consuming the majority of daily protein as a large bolus. However, neither study employed regimented resistance training thereby limiting generalizability to individuals involved in intense exercise programs.

Insights into the effects of protein dosage can also be gleaned from studies on intermittent fasting (IF). Typical IF protocols require intake of daily nutrients, including protein, in a narrow time-frame – usually less than 8 h – followed by a prolonged fast. A recent systematic review concluded that IF has similar effects on fat-free mass compared with continuous eating protocols [35]. However, the studies reviewed in the analysis generally involved suboptimal protein intakes consumed as part of a low-energy diet without a resistance training component, again limiting the ability to extrapolate findings to resistance-trained individuals.

Helping to fill this literature gap is an 8-week trial by Tinsley et al. [36], comparing a time-restricted feeding (TRF) protocol of 20-h fasting/4-h feeding cycles done 4 days per week, with a normal-diet group (ND) in untrained subjects doing resistance training 3 days per week. The TRF group lost body weight via lower energy intake (667 kcal less on fasting vs. non-fasting days), but did not significantly lose lean mass (0.2 kg); ND gained lean mass (2.3 kg), but not to a statistically significant degree, although the magnitude of differences raises the possibility that these findings may be practically meaningful. Perhaps most interestingly, biceps brachii and rectus femoris cross sectional area showed similar increases in both groups despite the 20-h fasting cycles and concentrated feeding cycles in TRF, suggesting that the utilization of protein intake in the ad libitum 4-h feeding cycles was not hampered by an acute ceiling of anabolism. Unfortunately, protein and energy were not equated in this study. Subsequently, an 8-week trial by Moro et al. [37] using resistance-trained subjects on a 16-h fasting/8-h TRF cycle found significantly greater fat loss in TRF vs. ND (1.62 vs. 0.31 kg) while lean mass remained unchanged in both groups. These findings further call into question the concern for breaching a certain threshold of protein intake per meal for the goal of muscle retention.

In contrast to the above findings showing neutral-to-positive effects of a temporally concentrated meal intake, Arciero et al. [38] compared 3 diets: 2 high-protein (35% of total energy) diets consisting of 3 (HP3) and 6 meals/day (HP6), and a traditional protein intake (15% of total energy) consumed in 3 meals/day (TD3). During the initial 28-day eucaloric phase, HP3 and HP6 consumed protein at 2.27 & 2.15 g/kg, respectively, while TD3 consumed 0.9 g/kg. HP6 was the only goup that significantly gained lean mass. During the subsequent 28-day eucaloric phase, HP3 and HP6 consumed protein at 1.71 & 1.65 g/kg, respectively, while TD3 consumed 0.75 g/kg. HP6 maintained its lean mass gain, outperforming the other 2 treatments in this respect (HP actually showed a significant loss of lean mass compared to the control). The discrepancy between the latter findings and those in the IF/TRF trials remains to be reconciled. In any case, it is notable that comparisons in this vein specifically geared toward the goal of muscle gain, hypercaloric comparisons in particular, are lacking.

An important distinction needs to be made between acute meal challenges comparing different protein amounts (including serial feedings in the acute phase following resistance training) and chronic meal feedings comparing different protein distributions through the day, over the course of several weeks or months. Longitudinal studies examining body composition have not consistently corroborated the results of acute studies examining muscle protein flux. Quantifying a maximum amount of protein per meal that can be utilized for muscle anabolism has been a challenging pursuit due to the multitude of variables open for investigation. Perhaps the most comprehensive synthesis of findings in this area has been done by Morton et al. [2], who concluded that 0.4 g/kg/meal would optimally stimulate MPS. This was based on the addition of two standard deviations to their finding that 0.25 g/kg/meal maximally stimulates MPS in young men. In line with this hypothesis, Moore et al. [39] mentioned the caveat that their findings were estimated means for maximizing MPS, and that the dosing ceilings can be as high as ~ 0.60 g/kg for some older men and ~ 0.40 g/kg for some younger men. Importantly, these estimates are based on the sole provision of a rapidly digesting protein source that would conceivably increase potential for oxidation of AA when consumed in larger boluses. It seems logical that a slower-acting protein source, particularly when consumed in combination with other macronutrients, would delay absorption and thus enhance the utilization of the constituent AA. However, the practical implications of this phenomenon remain speculative and questionable [21].

The collective body of evidence indicates that total daily protein intake for the goal of maximizing resistance training-induced gains in muscle mass and strength is approximately 1.6 g/kg, at least in non-dieting (eucaloric or hypercaloric) conditions [6]. However, 1.6 g/kg/day should not be viewed as an ironclad or universal limit beyond which protein intake will be either wasted or used for physiological demands aside from muscle growth. A recent meta-analysis on protein supplementation involving resistance trainees reported an upper 95% confidence interval (CI) of 2.2 g/kg/day [6]. Bandegan et al. [7] also showed an upper CI of 2.2 g/kg/day in a cohort of young male bodybuilders, although the method of assessment (indicator amino acid oxidation technique) used in this study has not received universal acceptance for determining optimal protein requirements. This reinforces the practical need to individualize dietary programming, and remain open to exceeding estimated averages. It is therefore a relatively simple and elegant solution to consume protein at a target intake of 0.4 g/kg/meal across a minimum of four meals in order to reach a minimum of 1.6 g/kg/day – if indeed the primary goal is to build muscle. Using the upper CI daily intake of 2.2 g/kg/day over the same four meals would necessitate a maximum of 0.55 g/kg/meal. This tactic would apply what is currently known to maximize acute anabolic responses as well as chronic anabolic adaptations. While research shows that consumption of higher protein doses (> 20 g) results in greater AA oxidation [40], evidence indicates that this is not the fate for all the additional ingested AAs as some are utilized for tissue-building purposes. Further research is nevertheless needed to quantify a specific upper threshold for per-meal protein intake.