Introduction
High-protein diets have been shown to have many positive health benefits, including increased lean body mass (LBM), muscle, metabolic rate, and satiety1. Combined with repetitive resistance training, this is a potent combo to induce hypertrophy. The high-protein diet provides a constant supply of the building blocks needed to repair and build new tissue, and the resistance training forces adaptation. The supply (high protein diet) and demand (stimulus for adaptation) are there. However, it is common to underestimate the importance of the quality of the protein source. While a high-protein meal in and of itself is anabolic, the quality of that protein could be the “make-or-break” factor regarding muscle growth, especially if the meal contains a low amount of total protein.
Before diving into the difference between protein quality in plant and animal-based protein sources, let’s take a moment to understand how protein functions in our bodies. Proteins are simply chains of amino acids that are bound together. They can vary in size, from just a few amino acids to hundreds of thousands. The sequence of amino acids is what determines how the protein behaves in the body.
There are 20 different amino acids, each having varying characteristics and functions. Of these twenty, 9 are considered essential amino acids (EAA); these cannot be synthesized by the body and must be consumed through the diet. We can create the other 11 non-essential amino acids within our bodies. The EAAs are the essential building blocks for muscle tissue, but each EAA has a specified metabolic role in the body. For example, the amino acid, leucine, acts as a signaling molecule once a minimum threshold of plasma leucine is achieved. This stimulates mammalian target of rapamycin complex-1 (mTORC1), increasing muscle protein synthesis (MPS) after a protein-rich feeding.
One could start to imagine the importance of high-quality protein sources, as not all sources are created equal. Depending on the protein source used, the amino acid profile of the food will vary; the amino acid content found in steak will be much different than that of rice protein. Referring to our leucine example above, if your goal is to build as much muscle as possible, you will want your sources to be high in all essential amino acids, as they will be the building blocks used to build the muscle tissue itself. However, knowing that the amino acid leucine stimulates mTOR, you would also want the protein source to be particularly high in leucine so you can achieve the maximal anabolic response to that meal.
Besides the differences in amino acid profiles across food sources, some foods contain naturally occurring antinutrients that can interfere with protein digestion and decrease the amount absorbed into the blood. These factors can manipulate the anabolic response from protein.
With an understanding of the function of amino acids, let’s explore the differences in protein quality between animal and plant-based sources.
Cellulose
Plant-based protein sources are inferior to animal-based protein sources for various reasons. Starting from the structural component, plant cell walls primarily comprise cellulose. If you were to look at cellulose under a microscope, you would see hundreds to thousands of glucose molecules bound together by 1,4-β-glycosidic bonds. As a result, the glucose molecules become tightly packed together in a linear pattern, allowing strong hydrogen bonding2 shown below.
β-1,4-glycosidic linkages
Amylose in potatoes, for example, is also comprised of hundreds to thousands of glucose molecules bound together. However, they are linked by 1,4-α-glycosidic bonds. This results in the glucose chain forming a more spiral-like shape2. Compared to the β-glycosidic bonds, there is much less hydrogen bonding, and the molecules aren’t packed together nearly as tightly.
α-1,4-glycosidic bonds
The differences between the α and β-glycosidic bonds are the critical component here. Humans produce the enzyme α-amylase in our salivary glands and pancreas, which enables us to break these alpha bonds easily. However, we lack the appropriate enzyme (β-amylase) required to break the bonds in cellulose. Cellulose will bind up and shield proteins trapped inside the plant material and spare them from protein-digesting enzymes found in the stomach and small intestine. If the protein is undigested, it cannot be absorbed and will travel through the large intestines and be excreted.
EAAs and Leucine content
Beyond structural components, plant-based protein sources are naturally lower in the EAA leucine, as well as other EAAs, when compared to animal-based protein sources. Leucine is particularly important because it can directly activate the mammalian target of rapamycin complex-1 (mTORC1)3, which ultimately induces muscle protein synthesis (MPS).
mTOR
The body seems to specifically monitor leucine by the protein Sestrin2, which functions as a leucine-binding sensor for mTORC14. Once leucine and Sestrin2 are bound together, leucine inhibits Sestrin2, leading to the activation of mTORC1. It has been demonstrated that a minimum threshold of plasma leucine must be detected before MPS is stimulated. In healthy adults, 2-3g is generally enough to saturate the mTOR signaling pathway thoroughly and maximally stimulate MPS5. Leucine can therefore be thought of as the “trigger” for MPS. Aside from a minimum threshold, there is also a maximum ceiling. Once mTOR is maximally stimulated, adding additional protein or leucine will not further increase rates of MPS.
Regarding protein quality, Norton et al. 6 showed that when rats were fed a meal containing 10% protein from wheat protein, plasma leucine levels did not increase, and MPS did not occur. However, when rats were fed a meal of 10% protein from whey protein, plasma leucine nearly doubled, and MPS was stimulated. This study is one of many that supports the idea that a minimum threshold must be achieved to stimulate mTORC1 and increase MPS6, 7, 8. Additionally, it demonstrates that protein quality in a low-protein meal is an important determining factor regarding mTOR activation.
Brennan et al.
Plant-based protein sources have naturally lower levels of leucine, therefore, the anabolic response compared to that of animal sources is lower. Since low-quality protein sources may be a determining factor in MPS, one could begin to see why plant-based sources are inferior for muscle growth. This was demonstrated in a study conducted by Brennan et al. in 20199. This study assessed the bio-equivalence of blood EAAs response over 4 hours in response to drinking three plant-based protein powder blends (pea, pumpkin, coconut, and sunflower protein) compared to a control, which was whey protein isolate powder. This study was particularly unique because EAAs and leucine were equated across all blends. However, more total grams of protein had to be added to the plant-based blends (#1-3), 34g, 33g, and 34g, respectively, compared to whey protein’s 24g total. This is shown below in table 1.
Methods:
This study used healthy resistance-trained men aged 18-35 and was a randomized, double-blind, 4×4 cross-over study. Both the researchers and participants did not know which product they were consuming and each participant served as their own control, receiving each treatment throughout the experiment. Participants came in over four separate intervention periods and would consume one drink each visit. On the morning of each visit, the participants came in fasted and had an intravenous catheter inserted into their forearm vein. A fasting sample was collected, and each participant was given their respective drink and consumed it within 10 minutes. For the following 4 hours, blood samples were collected at 15 min, 30 min, 60 min, 120 min, 180 min, and 240 min to measure all nine EAAs at each respective time point9.
Results:
Even though all three plant-based protein blends had ~30% more total grams of protein than the whey protein isolate and equal amounts of EAAs and leucine, plant-based blends had a 30-40% reduction in blood EAA levels compared to whey protein. The researchers concluded that the plant-based protein drinks did not achieve bioequivalence to whey protein isolate. Data is shown below9
Antinutrients
Plant-based protein sources also contain naturally occurring antinutrients that decrease the digestibility of protein10. Some of these antinutrients include enzyme inhibitors, tannins, and phytic acid. Foods such as legumes, cereals, and potatoes contain antinutrients that interfere with protein-digesting enzymes, especially in the small intestine. This, in turn, decreases the total amount of protein able to be digested and absorbed. Digestive enzymes inhibited by these antinutrients include trypsin, chymotrypsin, carboxypeptidases, elastase, and amylase10, which will be highlighted for tracking purposes. Let’s briefly touch on the steps of protein digestion and highlight the enzymes discussed above.
Protein digestion in the stomach and small intestine:
Protein digestion begins in the stomach, where large, folded protein structures become denatured by HCl. This process causes the folded proteins to uncoil and expand, increasing the total surface area and exposing the proteins to the digestive enzyme, pepsin. Pepsin will chop these proteins into smaller fragments, referred to as polypeptides. These polypeptides are emptied into the small intestine, where further digestion occurs, followed by absorption.
Once the polypeptides enter the small intestine, specific gut cells (mucosal enterocytes) are stimulated, and the hormone cholecystokinin (CCK) is released. CCK travels to the pancreas and stimulates the release of protein-digesting proteases (inactive forms of enzymes), including trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase into the small intestine. In the small intestine, trypsinogen is converted to its active form, Trypsin, by enzymes on the brush border membrane of enterocytes. Trypsin will then convert all other proteases to their activated state to do what they do best: chop up long polypeptides into short ones. The enzymes work in concert together and break down the polypeptides into free amino acids, dipeptides (2 peptides linked together), tripeptides (3 amino acids linked together), and some larger peptides called oligopeptides (3+ amino acids linked together), allowing them to be ready for absorption by the enterocytes (absorptive cells) that line the small intestine. Enterocytes can break down any remaining large oligopeptides with digestive enzymes aminopeptidase and carboxylase found on their outer membranes if needed.
Trypsin inhibitors
The most significant amount of trypsin inhibitors are found in cereals, beans, and legumes10. Common inhibitors include serpins, which mainly inhibit trypsin and chymotrypsin, and The Kunitz trypsin inhibitor and Bowman-Birk inhibitor11. As we learned above, trypsin is an enzyme integral in protein digestion. However, trypsin also activates the other proteases, chymotrypsinogen, procarboxypeptidase, and proelastase, to their active form. You could imagine how significant this would be as it is decreasing the amount of protein able to be digested and absorbed through the enterocyte cell and into the blood. This is just one antinutrient commonly found in plants. As mentioned earlier, other nutrients include tannins, phytic acid, and lectin, which affect protein digestibility.
Discussion
Plant-based protein sources are inferior to animal-based for various reasons, as we discussed. We first started by addressing the physical structure of plant material, trapping and shielding proteins from protein-digestive enzymes. Next, the composition of plant-based protein sources is particularly low in EAAs, including leucine, which is responsible for stimulating MPS. But worse, they don’t have the same anabolic response compared to animal-based protein sources, even if the total protein is increased by 30% and you standardize leucine and EAAs. Recall the study by Brennan et al.9 discussed above; if one wanted to use plant-based protein sources, one would have to ingest 33% more calories to achieve only 60-70% of the anabolic response from whey protein isolate. Meals containing low amounts of protein are particularly at risk for not achieving the minimum leucine threshold required for the initiation of mTOR. This is reinforced by the study conducted by Norton et al.6 as it was shown that MPS increased in all meals except the 10% wheat-protein group. Lastly, antinutrients, such as trypsin inhibitors, phytic acid, tannins, and lectins interfere with protein digestion and absorption.
References
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2. 5.1: Starch and Cellulose. Chemistry LibreTexts. Published April 24, 2015. Accessed October 3, 2022. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Smith)/05%3A_Stereochemistry/5.01%3A_Starch_and_Cellulose
3. Signaling pathways and molecular mechanisms through which branched-chain amino acids mediate translational control of protein synthesis – PubMed. Accessed October 3, 2022. https://pubmed.ncbi.nlm.nih.gov/16365087/
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8. Moore DR, Churchward-Venne TA, Witard O, et al. Protein Ingestion to Stimulate Myofibrillar Protein Synthesis Requires Greater Relative Protein Intakes in Healthy Older Versus Younger Men. The Journals of Gerontology: Series A. 2015;70(1):57-62. doi:10.1093/gerona/glu103
9. Brennan JL, Keerati-u-rai M, Yin H, et al. Differential Responses of Blood Essential Amino Acid Levels Following Ingestion of High-Quality Plant-Based Protein Blends Compared to Whey Protein—A Double-Blind Randomized, Cross-Over, Clinical Trial. Nutrients. 2019;11(12):2987. doi:10.3390/nu11122987
10. Gilani GS, Xiao CW, Cockell KA. Impact of Antinutritional Factors in Food Proteins on the Digestibility of Protein and the Bioavailability of Amino Acids and on Protein Quality. British Journal of Nutrition. 2012;108(S2):S315-S332. doi:10.1017/S0007114512002371
11. Samtiya M, Aluko RE, Dhewa T. Plant food anti-nutritional factors and their reduction strategies: an overview. Food Prod Process and Nutr. 2020;2(1):6. doi:10.1186/s43014-020-0020-5