A popular theory among fitness professionals is that taking short rest periods between sets maximizes muscular growth. The theory is primarily based on the hormone hypothesis, whereby limiting inter-set rest promotes greater elevations in post-exercise growth hormone, IFG-1 and testosteron...
July 23, 2016
A long-held belief in bodybuilding circles is that your body can only absorb a fairly small amount of protein in a single feeding. The exact dosage varies depending on who you listen to, but it’s generally purported to be somewhere around 20-30 grams of protein per meal.
While the claim is often taken as gospel, let’s take a close look at the research to draw evidence-based conclusions on the topic.
First and foremost, it’s important to note that from a nutritional standpoint the term “absorption” refers to the passage of nutrients from the gut into circulation – and in this context, there is virtually no limit to protein absorption. Once digested, the constituent amino acids of a given protein are transported through the intestinal cells (enterocytes) and then enter the bloodstream – pretty much all the amino acids consumed become available for use by tissues. The only potential issue with absorption is when you ingest individual free-form amino acids, as this can cause competition at the enterocytes whereby the amino acids present in the highest concentrations are absorbed at the expense of those that are less concentrated (6).
The more relevant question here is whether there’s in an upper limit to how much protein your body can use for muscle-building purposes. This question is a lot more complex and an evidence-based answer requires a good deal of extrapolation based on the limitations of current research.
Some researchers have proposed that muscle protein synthesis tops out at approximately 20-25 grams of protein per serving for young adults. Protein consumed above this dosage is thought to be oxidized for energy rather than used for tissue-building purposes – a phenomenon called the “muscle-full” effect (11). In what is often cited as the definitive support for this contention, Areta et al (1) investigated the effect of different protein boluses on resistance-trained men. All subjects performed a bout of resistance training and were then confined to rest where they consumed 80 grams of protein over a 12 hour recovery period in one of the following three conditions: 8 servings of 10 grams every 1.5 hours; 4 servings of 20 grams every 3 hours; or 2 servings of 40 grams every 6 hours. Over the course of the recovery period, the greatest effect on stimulation of muscle protein synthesis was seen in the group consuming 4 servings of 20 grams of protein. This would seem to indicate that there was no added benefit to consuming the higher dosage (40 grams), and that the additional amino acids were indeed oxidized for energy.
Case closed, right?
Not so fast.
Several variables influence the metabolism of protein and amino acids including the composition of the given protein source, the composition of the meal, and the dose of the protein or amino acids consumed (4). Individual factors such as age, training status, and the amount of lean body mass also come into play. The subjects in the Areta et al study consumed only whey protein during the post-workout period. Whey is a fast-acting protein, with an absorption rate estimated to be up to 10 grams an hour (4). A 20 gram whey bolus therefore would be completely absorbed in a two hour period. Although this rapid assimilation can transiently spike rates of muscle protein synthesis, it also causes a greater oxidation of the constituent amino acids and thus can result in a lower net protein accretion compared to a slow-absorbing protein source (5). On the other hand, cooked egg protein is absorbed at a rate of approximately 3 grams an hour (4). Thus, the same 20 gram protein bolus consumed as an omelet would take over 7 hours for full absorption, potentially allowing for a greater per-meal dosage without causing undue amino acid oxidation. Moreover, in real life you’ll generally be consuming whole foods that contain a combination of carbs and fats along with the protein component. This substantially slows down digestion, resulting in a much more time-released effect on amino acids into the body. In addition, the study only provided only 80 grams of protein over the course of the day to a group of resistance-trained men. This amounts to less than ½ gram per pound of body mass – well below the amount needed to maximize post-workout muscle protein synthesis (9).
A recent study by Kim et al (7) provides contrary evidence on the topic. Subjects came to the lab on two separate occasions: during one session they consumed 40 grams of beef protein and in the other session they consumed 70 grams of the same protein source. One group consumed the protein after a total-body resistance training bout while another did so in the absence of exercise. Results showed that while both conditions promoted increases in whole body nitrogen balance – a marker of anabolism – the higher protein intake resulted in a significantly greater anabolic response, which was largely attributed to a greater reduction in protein breakdown. A major difference between this study and that of Areta et al (1) is that subjects consumed mixed meals containing not only protein, but carbohydrates and dietary fats as well. Thus, the transit time of protein release would necessarily be much slower in this study, potentially accounting for dose-dependent differences in anabolism.
A limitation of the Kim et al study is that measures of anabolism were not specific to muscle but rather taken at the whole-body level. It is likely that much of the superior anabolic response noted with the higher protein intake was from tissues other than muscle, most notably the gut. However, protein turnover in the gut can allow these additional amino acids to be released into the bloodstream and subsequently used for muscle protein synthesis. The extent to which this phenomenon affects muscle-building is not clear, but it conceivably provides the potential for enhanced muscular gains.
While the results of the aforementioned studies provide a sound basis for speculation, it is important to note that measures of acute muscle protein synthesis do not necessarily correlate with muscular gains achieved from consistent lifting (10). To get a true grasp on the upper limit to protein intake in a single sitting, we need to look at long-term training studies that measure actual changes in lean mass.
Several studies have endeavored to investigate the effects of per-meal protein dosage on body composition over time. Arnal et al (2) found that feeding elderly women the bulk of their daily protein (79%) in a single meal (skewed condition) promoted greater retention of lean body mass versus spreading out consumption evenly over four daily meals (spread condition). A follow-up study by the same lab found no differences between skewed and spread protein feedings in a cohort of young women (3). The combined findings suggest that at the very least, consuming the majority of daily protein as a large bolus is not detrimental to lean mass accretion. Unfortunately, total protein intake in these studies was on the low side (~1 g/kg/day), and none employed a resistance training program. Thus, it is difficult to generalize findings to resistance-trained individuals seeking to maximize muscle mass.
Studies on intermittent fasting shed additional light on the topic. These protocols generally involve consumption of nutrients in a very limited time-frame – usually less than 8 hours – followed by a prolonged fast. A recent systematic review found that the majority of intermittent fasting protocols had similar effects on lean body mass compared to traditional eating patterns (12). But again, the studies involve suboptimal protein intakes without a resistance training component – and here the subjects were all in a caloric deficit. Not very applicable to the hard-training lifter.
Considering the limitations of the body of literature, here’s the take-home message based on current evidence: While certainly a threshold exists beyond which protein will be oxidized for energy rather than used for tissue-building purposes, the amount appears to be well above the often cited 20-30 gram limit provided that nutrients are obtained from whole-food based mixed meals. Given that the anabolic effect of a protein-rich meal lasts approximately 5-6 hours (8), a good rule-of-thumb for maximizing muscle growth would is to consume a minimum of 3-4 evenly distributed daily meals containing at least 30 grams of a high quality protein. Within these boundaries, it probably doesn’t matter how you allocate the rest of your protein consumption on a per-meal basis – just make sure you take in close to a gram per pound of body weight per day.
1. Areta, JL, Burke, LM, Ross, ML, Camera, DM, West, DW, Broad, EM, Jeacocke, NA, Moore, DR, Stellingwerff, T, Phillips, SM, Hawley, JA, and Coffey, VG. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J. Physiol. 591: 2319-2331, 2013.
2. Arnal, MA, Mosoni, L, Boirie, Y, Houlier, ML, Morin, L, Verdier, E, Ritz, P, Antoine, JM, Prugnaud, J, Beaufrere, B, and Mirand, PP. Protein pulse feeding improves protein retention in elderly women. Am. J. Clin. Nutr. 69: 1202-1208, 1999.
3. Arnal, MA, Mosoni, L, Boirie, Y, Houlier, ML, Morin, L, Verdier, E, Ritz, P, Antoine, JM, Prugnaud, J, Beaufrere, B, and Mirand, PP. Protein feeding pattern does not affect protein retention in young women. J. Nutr. 130: 1700-1704, 2000.
4. Bilsborough, S, and Mann, N. A review of issues of dietary protein intake in humans. Int. J. Sport Nutr. Exerc. Metab. 16: 129-152, 2006.
5. Dangin, M, Boirie, Y, Guillet, C, and Beaufrere, B. Influence of the protein digestion rate on protein turnover in young and elderly subjects. J. Nutr. 132: 3228S-33S, 2002.
6. Gropper, SS, Smith, JL, and Groff, JL. Advanced Nutrition and Human Metabolism. Belmont, CA; Wadsworth Cengage Learning, 2009.
7. Kim, IY, Schutzler, S, Schrader, A, Spencer, HJ, Azhar, G, Ferrando, AA, and Wolfe, RR. The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults. Am. J. Physiol. Endocrinol. Metab. 310: E73-80, 2016.
8. Layman, DK. Protein quantity and quality at levels above the RDA improves adult weight loss. J. Am. Coll. Nutr. 23: 631S-636S, 2004.
9. Lemon, PW, Tarnopolsky, MA, MacDougall, JD, and Atkinson, SA. Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. J. Appl. Physiol. 73: 767-775, 1992.
10. Mitchell, CJ, Churchward-Venne, TA, Parise, G, Bellamy, L, Baker, SK, Smith, K, Atherton, PJ, and Phillips, SM. Acute post-exercise myofibrillar protein synthesis is not correlated with resistance training-induced muscle hypertrophy in young men. PLoS One 9: e89431, 2014.
11. Morton, RW, McGlory, C, and Phillips, SM. Nutritional interventions to augment resistance training-induced skeletal muscle hypertrophy. Front. Physiol. 6: 245, 2015.
12. Seimon, RV, Roekenes, JA, Zibellini, J, Zhu, B, Gibson, AA, Hills, AP, Wood, RE, King, NA, Byrne, NM, and Sainsbury, A. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol. Cell. Endocrinol. 418 Pt 2: 153-172, 2015.
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