Updated November 2025: This article was originally published on August 20, 2023, and has been revised with the latest research on protein quality, amino acid bioavailability, and the Digestible Indispensable Amino Acid Score (DIAAS). References and recommendations for both plant and animal proteins have been updated for accuracy.
Some people are considering going “plant-based” for perceived health reasons or for ethical considerations, and while these are important, evaluating plant protein quality is also necessary. Assessing protein quality using the Digestible Indispensable Amino Acid Score (DIAAS) can help identify the best sources of protein for maintaining and building muscle.
A recent study found that essential amino acids from animal protein are more bioavailable than those from plant protein [1]. These findings are especially important for older adults who need to preserve muscle mass, and for active adults wanting to build or repair muscle.
Amino Acids and Bioavailability
Amino acids are the building blocks of protein, including muscle. There are twenty amino acids, divided into two groups: essential amino acids (EAA) and non-essential amino acids.
Bioavailability refers to the degree to which essential amino acids in food can be used by the body to build its own proteins [2], such as muscle tissue.
Essential amino acids, including leucine, must be consumed in the diet because the body cannot make them. Leucine plays a key role in muscle growth and repair by triggering mTOR signaling in muscle cells, which stimulates protein synthesis [3].
Dietary recommendations for older adults emphasize obtaining about 2.3 g of leucine per meal to support muscle growth [4], and around 3 g per meal to rebuild muscle after exercise [5].
Recent research suggests a practical target range of 2.5–3.0 g of leucine per main meal, roughly equivalent to 25–30 g of high-quality protein. This level best supports muscle protein synthesis in older adults and enhances recovery when paired with light resistance exercise — such as a breakfast of eggs or Greek yogurt.
Plant Protein vs. Animal Protein for Building Muscle
Plant proteins generally contain lower levels of leucine than animal proteins [6]. People choosing a more plant-based diet often rely on legumes like beans or lentils for protein. However, most legumes are incomplete proteins, meaning they lack one or more essential amino acids.
For example:
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Lentils contain about 0.7 g of leucine per half cup.
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Chickpeas contain about 0.42 g of leucine per half cup.
An older adult would need to eat more than 3 cups of lentils or 5½ cups of chickpeas at one meal to reach the 2.3 g leucine threshold needed to trigger muscle protein synthesis.
Is Plant Protein Equivalent to Animal Protein?
A randomized, investigator-blinded, crossover study was conducted with both young and older adults [1]. Researchers compared ounce-equivalents (oz-eq) of animal-based protein (lean pork or whole eggs) with plant-based protein (black beans or sliced almonds) in a mixed meal. The goal was to determine how efficiently the body uses amino acids from each source.
The Dietary Guidelines for Americans (DGA) defines ounce-equivalents to identify protein foods with similar nutritional content [7]. However, the researchers noted that the assumption of “equivalence” between these sources is questionable.
For instance, one ounce-equivalent of pork loin provides about 7 g of protein, while one ounce-equivalent of almonds provides only 3 g [1]. Beyond quantity, protein quality — the digestibility and bioavailability of essential amino acids — also differs substantially between these sources.
The Bioavailability of Plant Protein – Digestible Indispensable Amino Acid Score (DIAAS)
In 2013, the Food and Agriculture Organization (FAO) introduced the DIAAS method to replace the older PDCAAS system for assessing protein quality [9].
DIAAS measures how well individual essential amino acids are digested and absorbed at the end of the small intestine, giving a clearer picture of how well the protein supports the body’s needs.
High-quality proteins have a DIAAS ≥100, while scores of 75–99 are considered good, and <75 cannot make a quality protein claim [8].
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Most grains and legumes score below 75.
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Pea and soy protein range from 75–100.
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Animal proteins such as beef typically score well above 100 [10].
Although DIAAS provides a more accurate measure of protein quality, it has not yet been adopted for food labeling in Canada or the U.S. Implementation will require regulatory review and harmonized data across food categories. Until then, dietitians and consumers can use academic data to understand the real quality of different protein sources.
Plant Protein versus Animal Protein – What Studies Show
The recent trial found that meals containing animal-based proteins resulted in higher blood levels of essential amino acids compared with meals made from plant-based proteins — in both young and older adults [1].
Additionally, lean pork produced greater amino acid bioavailability than eggs, and no difference was found between black beans and almonds.
Researchers concluded that it is inaccurate to treat different protein sources as equivalent based on ounce-equivalents alone.
Final Thoughts
If you are an active adult wanting to build and repair muscle, or an older adult aiming to maintain strength, it’s essential to choose high-quality proteins that contain all essential amino acids — especially leucine. While the DGA lists one ounce of meat, one egg, 0.25 cups of beans, or 0.5 ounces of nuts as equivalent, this study shows they are not truly equivalent in quality or bioavailability.
If you are exploring a more plant-based lifestyle, understanding the science of protein bioavailability can help you make informed, health-supporting choices.
For those following a vegetarian or vegan diet for ethical or cultural reasons, combining complementary plant proteins and choosing higher-DIAAS options like soy or pea protein can help you meet your needs.
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References
[1] Connolly G, Hudson JL, Bergia RE, Davis EM, Hartman AS, Zhu W, Carroll CC, Campbell WW. Effects of Consuming Ounce-Equivalent Portions of Animal- vs. Plant-Based Protein Foods, as Defined by the Dietary Guidelines for Americans, on Essential Amino Acid Bioavailability in Young and Older Adults: Two Cross-Over Randomized Controlled Trials. Nutrients. 2023; 15(13): 2870. https://doi.org/10.3390/nu15132870
[2] Gaudichon C, Calvez J. Determinants of Amino Acid Bioavailability from Ingested Protein in Relation to Gut Health. Curr Opin Clin Nutr Metab Care. 2021 Jan; 24(1): 55-61. doi: 10.1097/MCO.0000000000000708. PMID: 33093304; PMCID: PMC7752214.
[3] Norton LE, Layman DK. Leucine Regulates Translation Initiation of Protein Synthesis in Skeletal Muscle after Exercise. J Nutr. 2006 Feb; 136(2 Suppl): 533S–537S. doi: 10.1093/jn/136.2.533S.
[4] Bauer J, Biolo G, Cederholm T, Cesari M, 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 Aug; 14(8): 542–59. doi: 10.1016/j.jamda.2013.05.021.
[5] Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016 Mar; 48(3): 543–568. doi: 10.1249/MSS.0000000000000852. Correction in Med Sci Sports Exerc. 2017 Jan; 49(1): 222.
[6] Berrazaga I, Micard V, Gueugneau M, Walrand S. The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review. Nutrients. 2019 Aug 7; 11(8): 1825. doi: 10.3390/nu11081825. PMCID: PMC6723444.
[7] U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025, 9th ed. Washington, DC: U.S. Government Publishing Office; 2020. Available from: https://www.dietaryguidelines.gov
[8] Herreman L, Nommensen P, Pennings B, Laus MC. Comprehensive Overview of the Quality of Plant- and Animal-Sourced Proteins Based on the Digestible Indispensable Amino Acid Score. Food Sci Nutr. 2020; 8(9): 5379–5391. https://doi.org/10.1002/fsn3.1809
[9] Food and Agriculture Organization of the United Nations. Dietary Protein Quality Evaluation in Human Nutrition (FAO Food and Nutrition Paper 92). Rome, Italy: FAO; 2013. Available from: https://www.fao.org/3/i3124e/i3124e.pdf
[10] 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 Aug 1; 75(8): 658–667. doi: 10.1093/nutrit/nux025. PMCID: PMC5914309.
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