Why Plant and Animal Sources Are Not Equivalent

NOTE: This article was originally posted on August 20, 2023, and was revised and reposted on April 19, 2026.

Practitioner’s Preface

Protein quality determines how effectively the body can use amino acids to build and repair muscle tissue. Evaluating food sources using current tracking metrics helps identify optimal options for maintaining muscle mass, which is especially vital for aging adults and active individuals.

Some people are considering going “plant-based” for perceived health reasons or 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 important for older adults who need to preserve muscle mass and for active adults who want to build or repair muscle.

How Can Food Labels Underestimate Protein Content?

Food labels list the total grams of protein per serving but do not reflect the actual amount of protein that the body absorbs. Protein bioavailability metrics help determine how easily these nutrients are digested, making it essential to look beyond raw numbers.

In my almost 18 years of private clinical practice, I frequently see people think that because a product label says that it has 17g of protein per serving, all of that protein is available to their body. This provides me with the opportunity to explain that there is a difference between the amount of amino acids in the food and their bioavailability. For people who are vegetarians, it is essential that they understand that some foods have more protein bioavailability than others.

What Are Essential Amino Acids and Protein Bioavailability?

Essential amino acids are crucial nutritional building blocks that the body cannot manufacture and must obtain through food. Bioavailability represents the efficiency with which the digestive tract absorbs these compounds to support muscle growth and trigger internal repair mechanisms.

Essential amino acids are crucial nutritional building blocks that the body cannot manufacture and must obtain through food, and bioavailability is the efficiency with which the digestive tract absorbs these compounds to support muscle growth and tissue repair.

• 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, and 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.

How Do Plant and Animal Proteins Compare for Building Muscle?

Animal proteins generally offer much higher concentrations of the core amino acids required for muscle growth than plant sources. Because common legumes are incomplete proteins, individuals must consume significantly larger serving sizes of beans or lentils to trigger muscle synthesis.

Plant proteins generally contain lower levels of leucine than animal proteins [6]. People choosing a more plant-based diet often rely on legumes, such as beans or lentils, for protein. For example:

• Lentils contain about 0.7 g of leucine per half cup.
• Chickpeas contain about 0.42 g of leucine per half cup.

Since older adults require significant amounts of leucine to retain muscle mass, 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 required to trigger muscle synthesis.

Are Ounce-Equivalents of Plant and Animal Protein Equal?

Standard dietary guidelines often equate different protein foods by weight, but they are not metabolically identical in the body. Clinical studies reveal that animal-based portions deliver far more bioavailable amino acids and better overall digestibility compared to plant options.

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 2020–2025 Dietary Guidelines for Americans (DGA) defined ounce-equivalents to identify protein foods with similar nutritional content [7]. However, 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 protein quantity, protein quality as assessed by the digestibility and bioavailability of essential amino acids also differs substantially between plant and animal sources.

What Is the DIAAS Method for Measuring Protein Quality?

The Digestible Indispensable Amino Acid Score evaluates protein quality by measuring how effectively the small intestine absorbs individual nutrients. This advanced rating system demonstrates that animal-based options consistently achieve high scores, while most plant sources fall short of requirements for quality claims.

In 2013, the Food and Agriculture Organization (FAO) introduced the DIAAS method to replace the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS) adopted by the FAO in 1991 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 scores of <75 cannot make a quality protein claim [8].

• Most grains and legumes score below 75.
• Pea and soy protein range from 75–100.
• Animal proteins such as beef typically score well above 100 [10]

How to Improve Plant Protein Bioavailability in Vegetarian Meals?

Combining lower-scoring plant options with small amounts of high-quality animal proteins dramatically increases total nutrient absorption. Adding small portions of dairy or eggs provides the specific missing amino acids required to complete the nutritional profile and satisfy physical demands.

While isolated plant proteins, such as wheat or pea protein, have lower DIAAS scores, consuming them in a mixed meal that includes small amounts of high-DIAAS animal protein, such as dairy or eggs, provides significantly improved protein and amino acid bioavailability[11].

• This has significant implications for ovolacto-vegetarians, as the addition of 10-15g of egg or dairy, such as cheese, to a plant-based meal (i.e., 30-40% of total protein) provides the limiting amino acids, including lysine or methionine, that all plant-based meal sources lack.

• This also has significant implications for older adults, as adding only 10–15g of high-quality animal protein (30-40% of total protein) to a plant-based meal provides the protein and leucine required for older adults.

How to Slow Muscle Loss in Older Vegetarian Adults?

Ensuring optimal nutrient absorption is critical for preventing sarcopenia (muscle wasting) and maintaining bone mass during aging. Tailoring plant-based dietary strategies to prioritize highly bioavailable foods ensures that older individuals can easily meet the protein requirements to protect physical strength and functional independence.

Some people choose a vegetarian lifestyle for religious or ethical reasons, and as a Dietitian, part of my role is to ensure that they know how to choose protein-based foods with the highest bioavailability. This is especially important for older adults who have high protein requirements to ensure they retain their muscle and bone mass to avoid sarcopenia.

What Is the Best Way to Meet Daily Protein Requirements?

Meeting daily protein requirements demands focusing heavily on both the total quantity and the underlying quality of food choices. Active adults and aging individuals can maximize muscle repair by choosing high-scoring protein sources or pairing complementary plant items.

For active adults wanting to build and repair muscle, or older adults aiming to maintain strength, it’s essential to choose high-quality proteins that contain all essential amino acids, especially leucine. While one ounce of meat, one egg, 0.25 cups of beans, or 0.5 ounces of nuts may be shown as being equivalent portion sizes, they are not equal in terms of protein quality or bioavailability. For those following a vegetarian diet for ethical or cultural reasons, combining complementary plant proteins, choosing higher-DIAAS options such as soy or pea protein, or adding 10-15 g of cheese or egg can help you better meet your protein requirements.

More Info

Customized professional nutritional planning helps individuals navigate their unique needs to ensure adequate muscle protein synthesis. Working with professional guidance simplifies the process of identifying optimal food sources and structuring daily meals to achieve long-term health goals.

If you would like support in ensuring that you or someone you love eats sufficient high-quality protein to trigger muscle synthesis, please view the Comprehensive Dietary Package that I offer. You can learn about me here.

To your good health,

Joy

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Clinical Summary

Clinical data confirms that plant and animal proteins are not metabolically equal due to distinct differences in digestibility and nutrient scores. Maximizing muscle preservation requires a clear understanding of daily protein quality and the strategic selection of highly bioavailable dietary options.

Q: Is plant protein equivalent to animal protein for building muscle?

A: Clinical research shows that ‘ounce-equivalents’ of plant vs. animal protein are not metabolically equal. Animal proteins typically have higher DIAAS scores (≥100) and contain significantly more leucine, which is the primary amino acid required to trigger muscle protein synthesis via mTOR signaling.

Q: How can plant protein bioavailability be improved in a vegetarian diet?

A: Adding a small amount (10–15g) of high-quality animal protein, such as dairy or egg, to a plant-based meal significantly improves the bioavailability of essential amino acids. This synergy helps ‘complete’ the protein profile and provides the necessary leucine levels for muscle maintenance.

Q: What is DIAAS and why is it important for protein quality?

A: The Digestible Indispensable Amino Acid Score (DIAAS) is the modern gold standard for assessing protein quality. It measures how well individual essential amino acids are digested and absorbed in the small intestine, providing a clearer picture of how effectively a protein source supports the body’s needs compared to older metrics like PDCAAS.

References

1. Connolly, G., Hudson, J. L., Bergia, R. E., et al. (2023). 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, 15(13), 2870. [https://doi.org/10.3390/nu15132870]
2. Gaudichon, C., & Calvez, J. (2021). Determinants of Amino Acid Bioavailability from Ingested Protein in Relation to Gut Health. Curr Opin Clin Nutr Metab Care, 24(1), 55-61. [https://pubmed.ncbi.nlm.nih.gov/33093304/]
3. Norton, L. E., & Layman, D. K. (2006). Leucine Regulates Translation Initiation of Protein Synthesis in Skeletal Muscle after Exercise. J Nutr, 136(2 Suppl), 533S–537S. [https://www.sciencedirect.com/science/article/pii/S0022316622080956]
4. Bauer, J., Biolo, G., Cederholm, T., et al. (2013). 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, 14(8), 542–59. [https://pubmed.ncbi.nlm.nih.gov/23867520/]
5. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc, 48(3), 543–568. [https://pubmed.ncbi.nlm.nih.gov/26891166/]
6. Berrazaga, I., Micard, V., Gueugneau, M., & Walrand, S. (2019). The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review. Nutrients, 11(8), 1825. [https://pubmed.ncbi.nlm.nih.gov/31394788/]
7. U.S. Department of Agriculture and U.S. Department of Health and Human Services. (2020). Dietary Guidelines for Americans, 2020–2025, 9th ed. [https://www.dietaryguidelines.gov]
8. Herreman, L., Nommensen, P., Pennings, B., & Laus, M. C. (2020). Comprehensive Overview of the Quality of Plant- and Animal-Sourced Proteins Based on the Digestible Indispensable Amino Acid Score. Food Sci Nutr, 8(9), 5379–5391. [https://doi.org/10.1002/fsn3.1809]
9. Food and Agriculture Organization of the United Nations. (2013). Dietary Protein Quality Evaluation in Human Nutrition. [https://www.fao.org/3/i3124e/i3124e.pdf]
10. Marinangeli, C. P. F., & House, J. D. (2017). Potential Impact of the Digestible Indispensable Amino Acid Score as a Measure of Protein Quality on Dietary Regulations and Health. Nutr Rev, 75(8), 658–667. [https://pubmed.ncbi.nlm.nih.gov/28969364/]
11. Rolands MR, Mainardi F, Bochud M, Lê KA. Identifying the optimal ratio from protein foods for protein and nutrient quality in plant-based meals using a non-linear optimization approach. Front Nutr. 2025 Oct 1;12:1624633. doi: 10.3389/fnut.2025.1624633. PMID: 41103323; PMCID: PMC12520914.

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