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Battle of the Proteins: Comparing Plant and Animal Sources

By Mark J. Smith, Ph.D., Chief Science Officer
October 24, 2020
Battle of the Proteins: Comparing Plant and Animal Sources image

There has been much discussion recently about plant-based diets. A common question is whether the quality of plant protein is comparable to that of animal protein.

Before addressing this question, it is important to understand what determines protein quality, generally. There are two main factors: the amino acid profile of the protein and the bioavailability of the protein. The two methods most commonly used for determining protein quality are the protein digestibility-corrected amino acid score (PDCAAS); there’s also the more recently developed digestible indispensable amino acid score (DIAAS). [1]

The PDCAAS method doesn’t account for where proteins have been digested; for example, amino acids that pass beyond the terminal ileum are less likely to be absorbed for use in protein synthesis. So, while they could pass out of the body and be measured in the feces, they could also become absorbed by bacteria, which could make it appear, incorrectly, that they have been digested. It also doesn’t account for amino acids, assumed to have been digested, lost due to antinutritional factors present in foods.

Due to these limitations, in 2013, the Food and Agricultural Organization of the United Nations (FAO) proposed changing to DIAAS. [2] This method determines amino acid digestibility, at the end of the small intestine, providing a more accurate measure of the amounts of amino acids absorbed by the body and the protein’s contribution to human amino acid and nitrogen requirements. Essentially, the DIAAS does a much better job of examining the digestibility of the protein.

Of the 20 common amino acids, nine are considered essential, as they are not synthesized by mammals and must be consumed. Three of the essential amino acids—isoleucine, leucine, and valine—are called branched-chain amino acids and are extremely important for muscle health. Healthy muscle tissue dictates metabolic health.

It’s not just athletes that typically benefit from a higher protein intake. Important evidence from the PROT-AGE Study Group has shown that, compared to younger adults, older adults need more dietary protein to support good health, promote recovery from illness, and maintain functionality. [3] This increase is due to age-related changes in protein metabolism, such as high splanchnic extraction and declining anabolic responses to ingested protein.

Additional protein is also needed because of the inflammation and catabolism associated with the diseases typically seen with aging. Because of these findings, the group recommends protein intakes for older people (>65 years) similar to young adults engaged in intense physical activity, with an average daily intake in the range 1.0 to 1.2 grams per kilogram (g/kg) body weight per day. For those with either acute or chronic diseases, it is recommended to increase the protein intake to 1.2 to 1.5 g/kg body weight per day. (Older individuals with severe kidney disease not on dialysis are exempted from this recommendation.) More specifically, it is further recommended that the per-meal anabolic threshold of dietary protein/amino acid intake of 25 to 30 grams protein per meal, containing about 2.5 to 2.8 g leucine.

When it comes to protein quality, whether we’re examining the amino acid profile or the protein digestibility, the scientific data is very clear that animal proteins, pound for pound, are significantly superior to plant proteins.

When differentiating between plant protein and animal protein, the essential amino acid profile is the most pertinent, and particularly the branched-chain amino acids. As an example, the adult recommended daily allowance (RDA) for leucine is 42 mg/kg body weight, which is 2.94 grams for a 70 kg person. [4] That RDA can be accomplished with the consumption of just 100 grams of beef skirt steak—that’s only a 3.5-ounce steak. [5]

One would have to eat twice the amount of firm tofu or almost five times the amount of canned navy beans to obtain the RDA for leucine. That said, one can get the RDA for leucine by eating an additional 25 percent more of pumpkin seeds. However, plant protein is typically low in leucine, lysine, and methionine—all essential amino acids.

The RDA for lysine is 38 mg/kg body weight which is 2.66 grams for a 70 kg person. 100 grams of pumpkin seeds contains 1.24 grams of lysine, whereas 100 grams of beef skirt steak contains 3.31 grams. Thus, one would have to eat over 2.5 times the amount of pumpkin seeds compared to beef skirt steak to obtain the same amount of lysine.

So, while pumpkin seeds aren’t far behind with respect to leucine, they fall way short for lysine—and their methionine content is about two thirds that of beef skirt steak. With respect to amino acid content, this is typical for plant foods in that, while they may compare favorably to animal proteins for one essential amino acid, they lack one or more of the other essential amino acids. Conversely, animal proteins do not.

While mixing plant foods can accomplish a complete RDA for the essential amino acids (so long as you understand how to accomplish this), it does so while forcing an increase in both caloric and carbohydrate consumption, which can be problematic for individuals looking to improve metabolic health.

When looking at the DIAAS figures for animal sources versus plant sources, the lowest animal protein is chicken at 1088, and the highest plant protein is chickpeas at 836. When it comes to protein quality, whether we’re examining the amino acid profile or the protein digestibility, the scientific data is very clear that animal proteins, pound for pound, are significantly superior to plant proteins.

Now, let’s compare the quality of proteins between different animal sources. We can examine both the essential amino acid profile of some different animal proteins along with their DIAAS, where available.

Table 1 shows the essential amino acid quantities (grams per 100 grams) for some select animal proteins, and chickpeas, as well as the RDA for each amino acid in mg/kg/d and in grams for a 70 kg person. Examining the table, one can quickly see that beef is by far the best animal source, in terms of obtaining essential amino acids. It has a very high DIAAS score and eight out of nine RDAs satisfied with just one 3.5-ounce serving. However, further examination shows that all of the animal products can easily satisfy the RDA with three servings in a day, and obviously many 70 kg individuals would consume more than a 3.5 oz. Serving.

In summary, realize that one can easily satisfy the essential amino acid RDA with just three relatively small servings of any unadulterated animal protein, and while you can certainly favor what you enjoy the most, it is usually beneficial to vary your animal protein sources at least now and again.

Battle of the Proteins: Comparing Plant and Animal Sources image

Table 1. DIAAS and essential amino acid quantities* [grams/100 grams (3.5 oz)] for select cooked animal proteins and chickpeas, and the RDA for each amino acid in mg/kg/d and in grams for a 70 kg person. Green shaded boxes indicate RDA is met for a 70 kg person with a 3.5-ounce serving.

*United States Department of Agriculture FoodData Central database (


[1] Marinangeli CPF, House JD. Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health. Nutr Rev 2017;75:658–67.

[2] Dietary protein quality evaluation in human nutrition. Report of an FAQ Expert
Consultation. FAO Food Nutr Pap. 2013;92:1-66. PMID: 26369006.

[3] Bauer J, Biolo G, Cederholm T, Cesari M, Cruz-Jentoft AJ, Morley JE, et al. Evidence-Based Recommendations for Optimal Dietary Protein Intake in Older People: A Position Paper From the PROT-AGE Study Group. J Am Med Dir Assoc 2013;14:542–59.

[4] Institute of Medicine (2002). "Protein and Amino Acids". Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. pp. 589-768.

[5] United States Department of Agriculture FoodData Central database (

[6] Phillips SM. Current Concepts and Unresolved Questions in Dietary Protein Requirements and Supplements in Adults. Frontiers Nutrition 2017;4:13.

[7] Ertl P, Knaus W, Zollitsch W. An approach to including protein quality when assessing the net contribution of livestock to human food supply. Animal 2016;10:1883–9.

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