Plant vs. Animal Protein Sources. And the winner is…
Andrea Crane MS HNFM, BS – InAllYourWays.com
In this article I have accumulated data on both plant and animal sources of protein for the purpose of muscle building and maintenance. For the sake of argument let’s assume that I am speaking of a plant-based diet that is done ‘well’ (this is a very generous assumption considering most individuals’ execution of a plant-based diet).
Before we get started, it must be understood that proteins are made up of building blocks called amino acids (‘aminos’). The body digests these proteins into single aminos or groups of two or three for further absorption.
So who really cares where the proteins come from…plant OR animal? Are these diets at parity when it comes to muscle health, building and maintenance? Let’s look at the data.
Muscle Protein Synthesis (aka ‘anabolic activity’ aka ‘muscle building’)
The ability of a protein source to stimulate muscle synthesis (building but also maintaining the current mass) is dependent upon several factors: dietary protein digestibility and subsequent availability, the kinetics of amino acid absorption and the ratios and amounts of essential amino acids present in the protein (especially leucine).
Even if a protein is present, if it cannot be adequately digested, it cannot be used metabolically. Animal proteins like dairy and eggs are 100% digestible and beef is at 92%. This means that 100% and 92%, respectively, of the available amino acids can be broken down from the original protein, making them potentially available to be used by the body to make muscle. Because of anti-nutrient compounds (that interfere with digestion and absorption) that are present in whole, plant foods, the digestibility of plant proteins is generally, considerably lower.
…hydrolyzed vegetable protein isolates (powders like pea and soy) do begin to approach the digestibility of animal proteins.
- Whole Wheat: 45% Oat: 57%
- Pea: 67%
- Soy (highest): 91%
It needs to be mentioned that highly processed, hydrolyzed vegetable protein isolates (powders like pea and soy) do begin to approach the digestibility of animal proteins.
Once the protein is digested (broken into smaller pieces), the amino acids must then be available for the body to use and this involves both absorption and the fate of those amino acids once absorbed.
Once proteins are broken down into their amino acid pieces, those aminos compete for absorption into your blood stream! So even though you may be eating a ‘complete’ protein from a combination of plants, the ratios of aminos critical to the initiation of muscle synthesis simply is not high enough for them to be adequately absorbed. To put it simply, the critical aminos can be ‘crowded out’ by other, more concentrated aminos. This leads us to a discussion of the importance of the amino acid composition and their biochemical fate, once absorbed.
Amino Acid Fates
Plant proteins, when digested, lead to greater ‘free amino acids’in the gut than animal source which ‘clump together’ a little bit. So what? The increase in free amino acid concentration in the gut leads to more of these free amino acids getting shunted to the liver, increasing the free amino acid concentration in the
liver. Increased levels in the liver stimulates urea-genesis which means that the aminos are metabolized to urea and excreted as waste. Thus they are not able to be used to make proteins. Studies involving the plant proteins from soy and wheat found that these proteins are more readily converted to urea (a nitrogen-based waste product) instead of contributing to the body’s amino acid pool and helping to make proteins.
Studies involving the plant proteins from soy and wheat found that these proteins are more readily converted to urea (a nitrogen-based waste product) instead of contributing to the body’s amino acid pool and helping to make proteins.
Animal proteins do not convert so readily to urea and do end up contributing to the amino acid pool from which the body draws to make it’s own proteins. The reasons why the aminos that result from plant proteins are shunted to the liver are poorly understood, however, it is surmised that it is due to a relative lack of specific essential amino acids found in plant proteins. In other words, it is likely that animal proteins possess favorable ratios of essential aminos for skeletal muscle building.
Essential Amino Acids (EAA’s)
Essential amino acids are those that the body cannot create for itself. There are 20 amino acids that make up the vast array of proteins in the body. Eleven of these can be created ‘denovo’ (the body can make them as needed…no supplementation through diet is necessary). The other nine, however, must be consumed in the diet. Please note there are sources that continue to say there are only eight essential amino acids; this is incorrect and does not reflect our current understanding.
This likely means that skeletal muscle is used as a storage unit essential amino acids, much like bones are used as a calcium storage unit.
In regards to muscle protein synthesis, the body is in a constant balancing act. Every tissue in the body is in need of these essential amino acids, not just skeletal muscle. When you EAT essential amino acids (in the form of protein), they will stimulate muscle building and supply EAA for the entire body. This stimulation will last for 4-5 hours. So what happens when you are not ‘eating’ EAA? When 4-5 hours has passed, the body is still in need of EAA for normal, cellular house-keeping activities. The body begins to catabolize (break down) skeletal muscle protein to provide EAA for other body tissues. This likely means that skeletal muscle is used as a storage unit for EAA, much like bones are used as a calcium storage unit.
The Amino Acid ‘Pool’
…essential aminos must be constantly replenished from either dietary sources or the breakdown of existing skeletal muscle.
What about the body’s amino acid ‘pool’? Don’t we have a storage unit full of essential aminos to draw from? We often (including myself) speak of this illusory pool of amino acids. However, it is a rather nebulous construct as there is no physical site of a protein reservoir where they are just floating in some contained space, waiting to be conscripted for protein synthesis! There does exist, however, a fraction of blood volume that is ‘free amino acids’. These amino acids are floating in the blood stream and will be pulled into cells as they are needed. This ‘reservoir’ is quite small and transient and the fraction of essential aminos must be constantly replenished from either dietary sources or the breakdown of existing skeletal muscle. Simply put, you either re-fill the pool from outside proteins or from the breakdown of your existing muscle. It must be understood that muscle-building will never surpass the rate of muscle breakdown without eating protein. Said a final way, to build muscle, you must eat protein.
Leucine is an essential amino acid but also in the class of ‘branched-chain amino acids’ (isoleucine and valine are the other two). Leucine works as a potent, cellular signaling molecule. Not only signaling to build more muscle but also to increase the speed at which that muscle is created. Leucine is roughly 10 times more anabolic (muscle-building) than any other amino acid. Therefore, it’s importance in a diet cannot be understated.
Where does leucine come from? It is heavily represented in animal proteins, especially dairy. Whey is the best, single source of leucine as well as a full complement of the other essential amino acids necessary to build muscle. Leucine also exists in plant protein, although considerably less than animal. The one interesting exception is maize (corn) which contains a higher percentage of leucine than any animal protein, save whey. (Interesting aside…92% of the corn grown in the US in 2016 was genetically modified – another topic for another day!) However, as would be expected, maize is lower in other essential amino acids than animal-based proteins. In order to get a 3 gram dose of leucine (the leucine found in 23 g of whey protein) from maize, one would have to consume over half a pound of corn.
…to get a 3 g dose of leucine (found in 23 g of whey protein) from maize, one would have to consume over half a pound of corn.
One question that comes to mind is, “Can I eat a vegetarian diet or a low protein diet and simply supplement leucine (or branched-chain aminos)?” Unfortunately, it’s not as simple as that. Leucine is not the grail of muscle-building, rather you must have an abundant, circulating pool of EAA or all the leucine in the world will not help you build or maintain muscle (see above ‘Essential Amino Acids’). A lack of EAA becomes a rate-limiting step in protein synthesis and thus are ultimately as important as the branched-chain aminos themselves.
What’s the Bottom Line?
It has been established in the literature that the ingestion of animal protein (beef, whey or milk) leads to higher immediate rates of muscle protein synthesis than does the ingestion of soy protein. As well, in longer-term studies, it has been shown that ingestion of whey or milk protein leads to significant gains in lean body mass when compared to the same amount of soy protein.
In longer-term studies, it has been shown that ingestion of whey or milk protein leads to significant gains in lean body mass when compared to the same amount of soy protein.
That said, there was a VERY interesting study done on men aged 65 and up. They were randomized into either a ‘beef-containing’ diet or a ‘lacto-ovo vegetarian’ diet supplemented with soy protein. The men averaged about 1.15 g protein/kg of body weight. Each group was given supervised resistance training three times weekly. At the end of the study it was found that there was no appreciable difference in the amount of muscle that was built. At first glance, it might appear that this confirms that soy protein is equivalent to beef protein when it comes to building and maintaining muscle. However, even the men who were given soy protein were still eating eggs and dairy protein.
Increasingly, it appears that the amount of muscle building/maintaining that takes place has everything to do with ‘quality’ and ‘quantity’ and it appears both must be there to optimize the physiology.
When looking at muscle protein synthesis, is there any way to optimize plant protein to match the efficacy of animal protein? It turns out, there might be.
When looking at muscle protein synthesis, is there any way to optimize plant protein to match the efficacy of animal protein? It turns out, there might be.
1. The missing or low levels of EAA’s in plant-based proteins.
If the branched-chain aminos are present in amounts that are required for muscle synthesis, we are still typically missing lysine and methionine. What about supplementing these aminos? Unfortunately, there just are not studies that have been done to test this theory. While it would make sense that supplementing these poorly represented aminos would result in protein synthesis that would match animal proteins, we do not have that data at this time. However, I think a prudent individual could experiment with this. That said, because we have no data, we do not know how much to supplement. There do exist studies in children where wheat was supplemented with lysine (in an amount that was 5.5% of the total wheat protein). Extremely positive results were gained with increases in height and weight. The weight gains were determined to be comprised of lean body mass (muscle) rather than fat tissue. But we have no similar studies in adults or with any other amino acid.
2. Can one simply eat more plant protein to make up for the shortfalls?
Well… yes…to a degree. A lot of the literature that shows parity in animal and vegetable sources when the protein amounts were increased are muddied by the fact that the ‘vegetarians’ they were looking at were ‘ovo-lacto-vegetarians’ and thus were receiving a portion of their daily protein intake from animal sources (egg and dairy), while being supplemented with additional soy or wheat protein. However, even these studies still needed to push the protein amounts to higher than what is commonly recommended to see result equality.
When older women were studied (over 65), those who ate an omnivorous diet had an average of almost 10 lbs more lean muscle than those who were long term vegetarians.
In studies looking at older individuals and their intake of plant vs. animal protein, the animal protein was a clear winner. Researchers found over and over again that long-term vegetarianism vs. an omnivorous diet let to significant losses in muscle mass. In fact, when older women were studied (over 65), those who ate an omnivorous diet had an average of almost 10 lbs more lean muscle than those who were long term vegetarians.
Basically, with the literature that we do have on the subject, there seems to be a biochemical ‘sweet spot’ for the lacto-ovo-vegetarian that is under the age of 65. Optimal muscle protein synthesis seems to occur when eating at least 1.2 gram of total protein per kg of body weight. For those who are over 65, there just does not appear to be a volume of plant protein that will make up for the lack of an omnivorous diet.
For those that are vegan, there does not appear to be a ready answer to this problem. When one looks at the sheer volume of food that needs to be eaten just to ingest a proper amount of leucine and other EAA’s, it becomes a ‘less-than-practical’ exercise. For example, quinoa is a wonderful, dense source of plant protein and yet one must eat over a half pound at each meal to provide adequate leucine. One could argue that this is an over-simplification and I might agree. However it could also be called an under-simplification because we are merely looking at one food and one dietary parameter. Thus to even get close to dietary requirements for optimal muscle protein synthesis, one must perform myriad calculations and machinations while planning one’s daily intake. This requires a level of preparation, effort and engagement that few are able or willing to take on.
For those that are vegan, there does not appear to be a ready answer to this problem.
So you may be wondering, should EVERYone be eating animal proteins? Absolutely not. There are people with religious, social and ethical considerations which lead them to varying degrees of vegetarianism or veganism. And as a libertarian, I respect the rights of a person to choose what they’d like to put into their bodies! But, it should be noted that there are often consequences to those decisions. I, for instance, have religious convictions about certain behaviors and activities. This may mean that I am ‘missing out’ on certain possible aspects of life…but I can live very well with my decisions. People who choose to avoid animal proteins may lose out when it comes to muscle protein synthesis and maintain other nutritional deficits, but this is simply a consequence of an intentional action. Some people fastidiously avoid eating vegetables and therefore do not defecate but once a week: action and consequence. For whatever you decide – simply make your decision and avoid adding to the vitriol on either side just because another makes a different choice.
What we can and will continue to argue about is the amount of protein that is optimal for human health, development and maintenance. But this is a subject that I will tackle at another time.
Andrea Crane, MS HNFM, BS is a licensed, clinical nutritionist with a practice in Colorado. She holds a Bachelor of Science in Molecular Biology, a minor in Biochemistry and a Master of Science degree in Human Nutrition and Functional Medicine. Andrea sees patients both in person and remotely for functional nutrition assessments related to chronic and complex disease states. Additionally, she provides nutritional counseling to clients who desire to optimize their nutritional status. She runs a private FaceBook group called, “Get Functional with Andrea” where she provides free information on diet, health and wellness related topics. She can be contacted by email: email@example.com. Articles and posts can be found at www.inallyourways.com.
Aubertin-Leheudre M, Adlercreutz H. Relationship between animal protein intake and muscle mass index in healthy women. British Journal Of Nutrition [serial online]. December 28, 2009;102(12):1803-1810. Available from: CINAHL, Ipswich, MA
Bos C, Metges C, Tomé D, et al. Postprandial kinetics of dietary amino acids are the main determinant of their metabolism after soy or milk protein ingestion in humans. The Journal Of Nutrition [serial online]. May 2003;133(5):1308-1315. Available from: MEDLINE, Ipswich, MA.
Campbell WW, Barton ML, Jr., Cyr-Campbell D, Davey SL, Beard JL, Parise G, Evans WJ. Effects of an omnivorous diet compared with a lactoovovegetarian diet on resistance-training- induced changes in body composition and skeletal muscle in older men. Am J Clin Nutr 1999;70:1032–9.
Haub M, Wells A, Tarnopolsky M, Campbell W. Effect of protein source on resistive-training- induced changes in body composition and muscle size in older men. The American Journal Of Clinical Nutrition [serial online]. September 2002;76(3):511-517. Available from: MEDLINE, Ipswich, MA.
Norton L, Layman D. Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. The Journal Of Nutrition [serial online]. February 2006;136(2): 533S-537S. Available from: MEDLINE, Ipswich, MA.
Proenza A, Palou A, Roca P. Amino acid distribution in human blood. A significant pool of amino acids is adsorbed onto blood cell membranes. Biochemistry And Molecular Biology International [serial online]. November 1994;34(5):971-982. Available from: MEDLINE, Ipswich, MA
van Vliet S, Burd N, van Loon L. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. The Journal Of Nutrition [serial online]. September 2015;145(9):1981-1991. Available from: MEDLINE, Ipswich, MA.
Vianna D, Teodoro G, Torres-Leal F, Tirapegui J. Protein synthesis regulation by leucine. BJPS. 2010; 46 (1):29-36.
Wolfe RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition. 2017;14:30. doi:10.1186/ s12970-017-0184-9.