Is There an Upper Limit to How Much Protein You Can Absorb?
Updated: Dec 31, 2023
There’s a new protein study that seemingly blows the top off what researchers have known regarding how much dietary protein we can absorb and utilize after a weight lifting session. Although researchers have known the original myth hasn’t held true for a few years, the new upper limit is a much higher value than previously realized.
However, what I’ve noticed on social media is an incomplete interpretation of this paper—especially how it translates into what you as the consumer would actually do.
So, in this post, I’m going to review the following:
How dietary protein moves from mouth to circulation.
The origins of the initial upper limit of protein absorption.
A review of what the new paper outlines.
What the paper missed.
The reality of applying the results into your daily life.
Before we begin, click here to access your copy of the article. Below is the full citation (1).
Trommelen, J., van Lieshout, G.A.A., Nyakayiru, J., Holwerda, A.M., Smeets, J.S.J., ... & van Loon, L.J.C. (2023). The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Reports Medicine,4(12):101324. https://doi.org/10.1016/j.xcrm.2023.101324
Protein Digestion and Absorption: What Happens After We Eat?
Depending on the macronutrient consumed, whether it’s carbohydrate, fat, or protein, the body has a different way to break it down (digestion) and move it from the gut and into circulation (absorption).
For starters, we don’t absorb whole proteins. Whole proteins must first be broken down into smaller pieces.
For instance, if you were renovating a house and had a brick wall you wanted to remove, the construction workers you hire can’t pick up the wall and move it. The workers would first need to break it down brick-by-brick, with the individual bricks being easier to then transport. Think of the entire wall as a whole protein and the individual bricks the amino acids (a.k.a., the building blocks of protein). What the body can absorb are amino acids and smaller chains of linked amino acids. How the body goes from brick wall to individual bricks is accomplished, in part, by enzymes (site-specific scissors).
Protein Digestion in the Stomach and Small Intestine
No protein digestion occurs earlier on in the mouth or esophagus (2).
Within the stomach, hydrochloric acid (HCl) is released from the parietal cells to create an acidic environment of a pH ~2.0 (or that of lemon juice) (3). The pre-enzyme pepsinogen is also released from stomach cells (the chief cells). Pepsinogen becomes immediately activated to the enzyme pepsin upon interacting with either the HCl or other pepsin enzymes (2,3).
The acidic environment of the stomach also denatures—or unfolds—proteins, exposing those long strands of linked amino acids to further breakdown from pepsin (2,3).
Once the original whole protein has been chopped into smaller pieces via the enzyme scissors, they’re ready to be released from the stomach and into the small intestine.
Protein Digestion and Absorption in the Small Intestine
The small intestine is separated into three regions. The first, closest region to the stomach is called the duodenum. When the liquid-nutrient mixture (a.k.a., chyme) is released into the duodenum, another set of construction workers arrive.
The workers needed to further digest protein include:
The proenzymes trypsinogen, prochymotrypsinogen, procarboxypeptidases A and B, and proelastase, which are released from the pancreas and into the intestine.
The hormones cholecystokinin (CCK) and secretin (2,3).
Enteropeptidase is released from the enterocytes (cells lining the walls of the small intestine) and activates trypsinogen into its active enzyme (scissor) form, trypsin. Similar to pepsin, trypsin activates prochymotrypsinogen and the procarboxypeptidases into their active forms (chymotrypsin and carboxypeptidase A and B) (2).
CCK is released from the intestine in response to food intake, especially those containing protein and fat (2). Secretin is released by S cells in the duodenum in response to acid accompanying the stomach’s release of chyme into the intestine. The presence of secretin stimulates a release of juices from the pancreas, including enzymes and bicarbonate (helps increase the pH of the incoming stomach juice from an acid to a base, which helps those newly-released enzymes function) (4).
Ultimately, once whole proteins have been broken down into either individual amino acids or smaller linked chains of them, they’re ready to be absorbed into circulation. Most of this is done in the upper two regions of the small intestine (the duodenum and jejunum) and mainly done via transport carriers, being helpers in moving proteins through the intestinal walls (2).
That movement, or absorption, is the point where the current debate begins.
A Review of What Researchers Knew
Transport carriers are like elevators: There are only so many elevators in an office building and they can only carry so much at one time. However, amino acids are known to use multiple elevators (transport carriers), so they aren’t tied to a specific one. Less than 1% of amino acids are found in the feces—meaning they’re being absorbed and removed from the small intestine (which eventually becomes the large intestine, the transporter of bowel movements) (3).
The “muscle full effect” proposed that there is a certain amount of dietary protein needed to stimulate muscle protein synthesis (MPS), or muscle building, and that any intake above that dose is not used for MPS, but rather oxidized and removed from the body via urine. This quantity was originally thought to be in the range of 25-30 grams of protein (and one study showed that upwards of 90 grams protein was as effective as 30 grams at stimulating MPS), but given humans come in various sizes, researchers extrapolated a value based on body weight, landing in a range of 0.325-0.4 grams protein per kilogram body weight per day (g/kg/day) (3, 5-9).
However, those earlier studies were done in smaller muscle groups. But what about when larger muscle groups were involved?
Macnaughton et al (2016) ran this study, finding that 40 grams of protein reduced MPS rates compared to the previous research. The assumption was that the consumed protein was now diluted across larger muscles (10). So maybe there wasn’t a hard cut off as previously suggested.
And as science does, it continues to build from previous studies and the knowledge acquired.
A Review of What the New Paper Outlines
A Brief Outline of the Study's Design
Who was enrolled in the study? Thirty-six healthy, recreationally active young men (see the below table). The men were aged 25±6 years. To be allowed in this study, the men had to have regularly exercised 1-3 times per week—neither less nor more frequent.
How were the men split up? They were randomly split into three groups of 12.
What type of study was this? A randomized controlled study, meaning the participants didn’t know what protein group they were in.
What were the protein quantities tested? Zero grams protein (the control group), 25 grams protein, and 100 grams protein. The participants received their protein mixed into water in a non-transparent plastic container.
What was the protein source? Protein derived from cow’s milk, processed into milk protein concentrate.
When was the protein provided to the men? After a 60-minute whole-body resistance exercise session conducted in the morning. The inclusion of exercise is important, as it helps stimulate the movement of amino acids into the muscles.
Did the men eat anything before the exercise session? No. Their last meal, provided by the researchers to ensure everyone consumed an identical meal, was finished by 10pm the night before. They exercised in the fasted state.
How did the researchers know what was happening within the men’s bodies to come to their study conclusions? The proteins were flagged with tracers so that the researchers knew where it was in the body and what it was doing (i.e., protein GPS trackers). Multiple plasma (blood) and muscle tissue samples were collected over a period of 12 hours. In total, four muscle biopsies and 15 blood samples were extracted from each male participant.
The Study's Main Takeaways
Postprandial protein anabolism remains elevated during prolonged hyperaminoacidemia. Translation:
Protein utilization continues well after someone has consumed it, and utilization remains high for a long period of time. In this study, the timeline measured was 12 hours. Earlier research measured a much shorter timeframe.
The higher 100-gram protein dose induced a greater response compared to the 25-gram protein group.
A saturation level was not found (i.e., no muscle full effect).
The authors noted that "the ingestion of larger amounts of protein requires a more prolonged period to allow full digestion, amino acid absorption, and subsequent amino acid release into the circulation" (1).
Protein ingestion has a negligible impact on whole-body amino acid oxidation. Translation:
Previous researchers thought that excess protein beyond a certain threshold was oxidized and excreted from the body. That is no longer thought to be true.
And then what I found fascinating as a takeaway: Meal frequency (and protein dosing throughout the day) may not be as important as once thought. It's been known that intermittent fasting doesn't seem to negatively affect muscle mass maintenance, assuming total protein goals for the day are met. (1) This study, and its prolonged protein finding, could be why.
What is Not Reflected in This Study? A Few Thoughts.
Similar to the earlier studies, high-quality complete protein sources were used—meaning all nine essential amino acids were present in optimal amounts. But what about the protein quantity needed for those who consume plant proteins (e.g., vegan)?* This study does not support that any source of protein will result in the outcome—only milk protein, which contains the essential amino acid leucine, known for its triggering of MPS (3).
This study was conducted in men who typically exercise 1-3 times per week (neither more nor less). This is neither a college student-athlete nor an elite athlete.
Women were not included in this study.
The men studied had slightly lower protein intakes before the exercise-protein intervention occurred than what would be encouraged of an athlete. Their intake was on average 1.3±0.3 g/kg/day. Depending on the sport and competition cycle an athlete is in, dietary intake can be as low as 1.2 g/kg/day and well above 2.0 g/kg/day (11). This is interesting, as the study is not attempting to tell its readers to eat a ton of protein in the day, yet is simply evaluating a one-time protein dose and its absorptive abilities.
People don’t eat only protein at a meal—they eat a mixture of macronutrients (key word here is "eat", not "drink" like this study measured). Would the results of this study be the same if the 100 grams protein (even as a beverage) was accompanied with carbohydrates and fiber (whole grain bread), and fats (avocado)? An actual meal could affect the plasma amino acid rise seen in the current study.
And what about the long-term impact on muscle? Does one or multiple 100-gram protein doses result in a performance benefit? Larger muscles? Stronger humans?
These are a few examples of what this study did not measure, and where we as providers need to be cautious in how we’re messaging the results to the public (i.e., it’s not a blanket statement to everyone, of every age, athletic level, dietary pattern, and sex).
*Often, it's claimed that plant proteins are lower quality or are missing essential amino acids. They aren't. Plant proteins contain all EAAs, but often have lower values of select EAAs. For instance, beans are lower (not lacking) in methionine and cysteine.
Lastly, How Does This All Apply to You as the Eater?
Remember the hormone above called CCK? It has other functions, one of which is appetite regulation. CCK stimulates the brain’s awareness of fullness in two ways: binding to areas in the brain stem and an area of the stomach (the pylorus) that signals the brain to reduce hunger and increase satiety signals (2). Protein makes you feel full. It’s no accident that weight-loss diets tend to be high in protein: Protein is satisfying, protein tells the brain the body is no longer hungry, and protein's satiety helps mask symptoms of a calorie deficit needed to reduce one’s body weight.
Here’s what I think many consumers of this paper will do, whether it’s providing recommendations to their social media followers or those watching their videos: Vastly increase one’s protein intake without thought to any other nutrient or health or performance goal. For instance:
Those who need to gain weight will likely struggle if their hunger signals are greatly suppressed, since protein is highly satisfying, suppresses appetite, and has the highest thermic effect (i.e., it takes more energy to process protein in the body) (11).
Immune, heart, and gut health all depend on fibre, which comes from plants (i.e., not animal protein sources) (12). If satisfying protein edges out other foods on the plate, fibre intake could decrease.
The source of protein matters from a heart health perspective: Diets high in saturated fat are linked with a greater risk of heart disease and saturated fat is mainly found in animal foods (plus in high levels in palm and coconut oils). Thus, if protein intake increases, the likelihood that saturated fat intake will likely follow suite (13).
In the study, 100 grams milk protein (in the form of a concentrate) was used. To consume 100 grams protein in one sitting, this would equate to:
~12.5 8-ounce glasses of cow or soy milk (or ~0.8 gallons) (1% cow’s milk: ~1,280 calories and 19.2 grams saturated fat).
~16 large eggs (~1,145 kcal and 25 g SF).
~320 grams skinless cooked chicken breasts (or ~2 large) (~530 kcal and 3.3 g SF).
~7 cups cooked chickpeas (~1,880 kcal and 3 g SF).
~5 scoops whey protein isolate powder (~500 kcal and 0 g SF).
One hundred grams of protein is a lot to consume in one sitting, which is likely one of many reasons why the researchers chose an isolated protein. In reality, someone can use protein powders and a protein-containing liquid when making a smoothie or a shaker bottle to gain higher protein levels. Yet how practical is this to do for every snack and meal, every day?
Remember: This study was designed to support a research question and NOT to provide real world nutrition recommendations.
Take-Home Messages
Realistically, the take-home message from this paper is that if you are a non-competitive athlete, young and healthy male, exercises 1-3 times per week, and consumes a single dose of 100 grams of milk protein concentrate after a fasted 60-minute intense resistance training session, then the protein will be utilized by the muscles for an extended period of time, longer than once considered. If you’re someone who purposefully limits their protein doses throughout the day to ~25-30 grams of protein because of the earlier research studies discussed above, you likely don’t need to.
That’s literally it.
This is a great study, as it’s going to help the research community continue to build upon this body of work. But it’s too early to take the conclusions and run with them.
References
(1) Trommelen, J., van Lieshout, G.A.A., Nyakayiru, J., Holwerda, A.M., Smeets, J.S.J., ... & van Loon, L.J.C. (2023). The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Reports Medicine,4(12):101324. doi.org/10.1016/j.xcrm.2023.101324
(2) Gropper, S.S., Smith, J.L., & Carr, T.P. (2022). Advanced nutrition and human metabolism, eighth edition. Boston, MA: Cengage.
(3) Jeukendrup, A., & Gleeson, M. (2019). Sport nutrition, third edition. Champaign, IL: Human Kinetics.
(4) DiGregorio N, Sharma S. Physiology, Secretin. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537116/
(5) Moore, D.R., Robinson, M.J., Fry, J.L., Tang, J.E., Glover, E.I., ... & Phillips, S.M. (2009). Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. Am J Clin Nutr,89(1):161-8. doi: 10.3945/ajcn.2008.26401
(6) Witard, O.C., Jackman, S.R., Breen, L., Smith, K., Selby, A., & Tipton, K.D., (2014). Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise. Am J Clin Nutr,99(1):86-95. doi: 10.3945/ajcn.112.055517
(7) Symons, T.B., Sheffield-Moore, M., Wolfe, R.R., & Paddon-Jones, D. (2009). A moderated serving of high-quality protein maximally stimulates skeletal muscle protein synthesis in young and elderly subjects. J Am Diet Assoc,109(9):1582-6. doi: 10.1016/j.jada.2009.06.369
(8) Atherton, P.J., Etheridge, T., Watt, P.W., Wilkinson, D., Selby, A., ... & Rennie, M.J. (2010). Muscle full effect after oral protein: time-dependent concordance and discordance between human muscle protein synthesis and mTORC1 signaling. Am J Clin Nutr,92(5):1080-8. doi: 10.3945/ajcn.2010.29819
(9) Moore, D.R., Churchward-Venne, T.A., Witard, O., Breen, L., Burd, N.A., ... & Phillips, S.M. (2015). Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men. J Gerontol A Biol Sci Med Sci,70(1):57-62. doi: 10.1093/gerona/glu103
(10) Macnaughton, L.S., Wardle, S.L., Witard, O.C., McGlory, C., Hamilton, D.L., ... & Tipton, K.D. (2016). The response of muscle protein synthesis following whole-body resistance exercise is greater following 40 g than 20 g of ingested whey protein. Physiol Rep,4(15):e12893. doi: 10.14814/phy2.12893
(11) Karpinski, C., & Rosenbloom, C. (2007). Sports nutrition: A handbook for professionals, sixth edition. Chicago, IL: Academy of Nutrition and Dietetics.
(12) Solan, M. (2023, February 1). Healthy gut, healthy heart. Harvard Health Publishing, Harvard Medical School. https://www.health.harvard.edu/staying-healthy/healthy-gut-healthy-heart
(13) American Heart Association. (2021, November 1). Saturated Fat. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/saturated-fats
