Why Are Animal Proteins Considered Higher Quality Than Plant Proteins?

December 9, 2025 •

4 min read

According to nutritional experts, animal proteins are generally classified as higher quality than most plant proteins. This is because they contain all nine essential amino acids in sufficient quantities and are more easily absorbed by the human body. Understanding why animal proteins are considered higher quality can help you make informed dietary choices.

Quick Summary

Animal proteins are typically deemed higher quality due to their superior amino acid profile and better digestibility compared to plant proteins. The differences impact muscle synthesis and bioavailability of nutrients, making animal sources efficient for fulfilling essential amino acid needs. However, a varied diet can overcome plant protein limitations.

Key Points

Complete Amino Acid Profile: Animal proteins are classified as 'complete' because they contain all nine essential amino acids needed by the human body.
Incomplete Plant Proteins: Most plant proteins lack sufficient quantities of one or more essential amino acids, making them 'incomplete'.
Higher Digestibility: The bioavailability of animal proteins is generally higher, meaning they are more easily and efficiently digested and absorbed.
Impact of Antinutrients: Plant-based sources contain antinutritional factors and fiber that can inhibit protein digestion and nutrient absorption.
Protein Complementation Strategy: To get all essential amino acids, individuals on plant-based diets must combine various plant foods, like legumes and grains, throughout the day.
Protein Quality Scores: Metrics like the DIAAS and PDCAAS consistently rank animal proteins higher due to their superior amino acid profile and digestibility.
Context Matters: The concept of 'quality' is context-dependent, and while animal proteins are technically more efficient, a well-planned plant-based diet can be nutritionally sufficient.

The Core Difference: Amino Acid Profiles

Protein quality is primarily determined by its amino acid composition, especially the presence and ratio of the nine essential amino acids (EAAs). Our bodies cannot produce these nine EAAs, so they must be obtained through our diet.

Complete vs. Incomplete Protein Sources

Complete Proteins: Animal proteins, such as those found in meat, poultry, eggs, and dairy, are considered "complete" because they provide all nine EAAs in adequate amounts.
Incomplete Proteins: Most plant proteins are "incomplete," meaning they are deficient in one or more of the nine EAAs. For example, legumes are often low in methionine, while grains can be low in lysine.

Digestibility and Bioavailability

Beyond the amino acid profile, protein quality is also influenced by how well the protein is digested and absorbed by the body. This is known as bioavailability.

Factors Affecting Digestibility

Structural Differences: Animal proteins are structured differently than plant proteins, making them more susceptible to breakdown by human digestive enzymes.
Antinutritional Factors: Plant proteins often contain compounds like phytic acid and tannins, which can inhibit protein digestion and mineral absorption. While processing methods like cooking can reduce these effects, they still contribute to lower overall digestibility compared to animal sources.
Cell Wall Content: The fibrous cell wall structure of plants can also impede the digestion of their proteins, another factor that animal proteins do not have.

Comparing Absorption Rates

Studies show that animal proteins generally have higher true digestibility values. For instance, milk and egg protein have digestibility scores approaching 97%, while the range for plant and legume proteins typically falls between 75% and 85%.

The Role of Limiting Amino Acids

For an incomplete plant protein, the essential amino acid present in the lowest amount relative to the body's needs is called the "limiting amino acid." For protein synthesis to occur efficiently, all EAAs must be available simultaneously. If even one EAA is missing or insufficient, protein production can be hampered. This is why a varied diet with complementary plant proteins is necessary to ensure all EAAs are consumed.

Protein Quality Scoring Systems

Several methods are used to measure and compare protein quality. Two of the most common are:

Protein Digestibility-Corrected Amino Acid Score (PDCAAS): This scoring method evaluates protein quality based on both its amino acid profile and its digestibility. Animal proteins often score very high on the PDCAAS scale.
Digestible Indispensable Amino Acid Score (DIAAS): This newer method is considered more accurate than PDCAAS because it measures amino acid digestibility at the end of the small intestine, providing a more precise assessment of bioavailability. Many animal sources receive excellent DIAAS scores, while many plant sources have lower values due to poor ileal digestibility.

How to Build Complete Proteins from Plant Sources

While animal proteins offer a more direct route to acquiring a complete set of EAAs, a well-planned plant-based diet can also easily achieve this through protein complementation. By combining different plant foods, you can ensure all essential amino acids are present in your diet over the course of a day. This includes mixing different plant-based foods, such as:

Legumes and grains (e.g., beans and rice)
Nuts or seeds with whole grains (e.g., peanut butter on whole wheat toast)
Seeds and legumes (e.g., salad with sunflower seeds and chickpeas)

Comparison Table: Animal vs. Plant Protein


Feature	Animal Protein	Plant Protein
Complete Amino Acid Profile	Yes, contains all nine essential amino acids.	Typically incomplete, with limiting amino acids.
Digestibility	High (e.g., ~97% for eggs/milk).	Lower (e.g., 75-85%) due to inhibitors and fiber.
Bioavailability	High, with easy absorption by the body.	Lower due to antinutritional factors and structure.
Protein Quality Score (e.g., DIAAS)	Consistently high scores across most sources.	Variable scores, often lower due to limiting amino acids.
Nutrient Density	Often rich in Vitamin B12, Vitamin D, and iron (heme).	Often rich in fiber, antioxidants, and various phytonutrients.
Health Considerations	Red meat linked to higher risk of some chronic diseases.	Linked to lower risk of chronic diseases and better overall health.
Environmental Impact	Generally higher carbon footprint, water, and land use.	Generally lower environmental footprint.

Conclusion: Making Informed Choices

The perception that animal proteins are considered higher quality than plant proteins is rooted in scientific facts concerning amino acid completeness and bioavailability. However, this does not mean that animal proteins are inherently superior for all diets or that a plant-based diet is nutritionally inferior. A varied plant-based diet, rich in a range of legumes, grains, and nuts, can provide a complete amino acid profile to meet the body's needs. Ultimately, the best protein sources for an individual depend on their dietary goals, personal values, and health considerations. For many, a balanced approach combining both high-quality animal and complementary plant proteins can be the most effective strategy for optimal nutrition.

Frequently Asked Questions

Essential amino acids are nine specific amino acids that the human body cannot produce on its own. They must be obtained through dietary sources to support muscle growth, tissue repair, and other vital bodily functions.

While most plant proteins are considered incomplete, a few exceptions, like soy, quinoa, and buckwheat, contain all nine essential amino acids in sufficient amounts. However, the overall digestibility of even these sources can still be lower than animal proteins.

Not necessarily. A well-planned plant-based diet that incorporates a variety of complementary plant proteins throughout the day can provide all the essential amino acids needed. Deficiencies are more likely to occur with poor planning rather than the protein source itself.

Bioavailability refers to the proportion of a nutrient that is absorbed from the diet and used for normal body functions. In the context of protein, it measures how effectively your body can digest and utilize the amino acids.

The health impact is nuanced. Animal proteins offer a higher quality, more readily available source of essential amino acids. However, diets rich in plant proteins are often linked to a lower risk of chronic diseases due to their high fiber and nutrient content. The healthiest approach often involves a balanced intake of both, focusing on lean animal options.

Processing techniques can significantly improve the digestibility of plant proteins by reducing antinutritional factors and altering their structure. Protein isolates, for example, have much higher digestibility than unprocessed whole plant foods.

You can combine a legume with a grain to create a complete protein. Examples include pairing beans with rice, hummus with pita bread, or lentils with oats. You don't need to eat them in the same meal, but ensuring variety throughout the day is key.