What is classed as high-quality protein? A guide to essential amino acids and digestibility

October 12, 2025 •

5 min read

High-quality proteins are more efficiently digested and utilized by the body, providing all nine essential amino acids. So, what is classed as high-quality protein? It is a measure of a food's ability to supply the body with the right building blocks for optimal health and function.

Quick Summary

High-quality protein is defined by a complete essential amino acid profile and high digestibility, typically measured by the DIAAS method. Sources range from animal products like eggs and dairy to plant-based options such as soy and quinoa.

Key Points

Complete Amino Acid Profile: High-quality protein contains all nine essential amino acids that the body cannot produce on its own.
High Digestibility: The protein must be efficiently broken down and absorbed by the body for optimal use.
DIAAS is the Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most modern and accurate metric for evaluating protein quality.
Diverse Sources: High-quality protein comes from both animal-based sources (eggs, dairy, meat) and select plant-based sources (soy, quinoa).
Combine Incomplete Proteins: Plant-based diets can achieve high-quality protein by combining different sources, such as grains and legumes, throughout the day.
Needs Change with Age: Protein requirements and the importance of high quality increase for certain populations, such as older adults and athletes.
Processing Matters: Food processing methods like heating and fermentation can influence a protein's digestibility and quality.

The Defining Characteristics of High-Quality Protein

Not all protein is created equal. The quality of a protein source is determined by two main factors: its amino acid profile and its digestibility. These two characteristics determine how effectively the body can use the protein for muscle building, hormone production, and other vital functions.

Amino Acid Profile: The Building Blocks

Proteins are long chains of amino acids. Of the 20 amino acids your body needs to function, nine are classified as "essential" because the body cannot produce them on its own. They must be obtained through food. A protein is considered "complete," and therefore high-quality, if it contains all nine of these essential amino acids (EAAs) in sufficient amounts. Animal-based foods, such as meat, eggs, and dairy, are almost always complete proteins. In contrast, most plant-based proteins are incomplete, meaning they are low in or missing at least one EAA. However, some plant foods like soy, quinoa, and hemp seeds are naturally complete.

Protein Digestibility and Bioavailability

For protein to be useful, the body must be able to break it down and absorb its amino acids. Digestibility measures how well this process occurs. A protein with a high digestibility score means that a larger portion of its amino acids are absorbed and available for the body's use, a concept also known as bioavailability. Animal proteins generally have higher digestibility than plant proteins, partly because plant-based foods contain anti-nutritional factors and fiber that can hinder absorption. Cooking and processing can also influence a protein's digestibility.

Scoring Protein Quality: DIAAS vs. PDCAAS

For many years, the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality. However, it has been largely replaced by a more accurate method called the Digestible Indispensable Amino Acid Score (DIAAS).

PDCAAS (Outdated): This method, adopted by the FDA in 1993, used an estimate of total protein digestibility based on fecal samples from rats and capped scores at 1.0, which limited its accuracy. Capping scores meant that superior proteins couldn't be distinguished from good proteins, and fecal sampling could overestimate digestibility.
DIAAS (Current Standard): Proposed by the Food and Agriculture Organization (FAO) in 2013, DIAAS is a more precise measure. It assesses the digestibility of each individual essential amino acid at the end of the small intestine (the ileum), which gives a much more accurate picture of what the body actually absorbs. DIAAS scores are also not capped, allowing for better comparison of protein sources.

Top Sources of High-Quality Protein

To ensure you're getting the best protein, incorporate a variety of the following sources into your diet.

Animal-Based Sources

Eggs: Often considered the "gold standard" for protein, eggs contain all essential amino acids and are highly digestible.
Whey and Casein: These dairy proteins are celebrated for their completeness. Whey is known for its rapid digestion and muscle recovery benefits, while casein digests more slowly.
Lean Meats and Poultry: Foods like skinless chicken breast, turkey, and lean beef provide a dense source of complete protein along with important micronutrients like iron and zinc.
Fish and Seafood: Salmon, tuna, and cod are excellent sources of complete protein and often contain heart-healthy omega-3 fatty acids.
Dairy Products: Milk, Greek yogurt, and cottage cheese are packed with complete proteins and calcium.

Plant-Based Sources

Soy Products: Tofu, tempeh, and edamame are some of the few plant-based foods that are complete proteins, making them a cornerstone of many vegetarian and vegan diets.
Quinoa: This ancient grain is a complete protein and a great source of fiber and minerals.
Combining Incomplete Proteins: By eating a variety of plant-based foods, you can ensure you get all nine essential amino acids throughout the day. For example:
- Legumes with Grains: Such as rice and beans or whole wheat pita and hummus.
- Seeds with Legumes: A salad with chickpeas and sunflower seeds.
- Nuts with Grains: Peanut butter on whole wheat toast.

Comparing Protein Quality: Animal vs. Plant


Feature	Animal-Based Proteins	Plant-Based Proteins
Essential Amino Acids	Most sources are complete, containing all nine EAAs.	Most are incomplete, low in one or more EAAs, though some are complete (e.g., soy, quinoa).
Digestibility	Generally higher due to lack of fiber and anti-nutrients.	Can be lower due to fiber, phytates, and other compounds, but can be improved with processing.
Protein Density	Often higher protein content per serving, especially for lean meats and dairy.	Varies widely; sources like soy and lentils are dense, while others contain less protein per calorie.
Macronutrient Profile	May be higher in saturated fat, requiring selection of leaner cuts.	Naturally contains healthy fats and fiber; can be low in certain nutrients like vitamin B12.
Bioavailability	High bioavailability means more amino acids are absorbed by the body.	Lower overall bioavailability, but can be improved through combining sources.

How to Maximize Your High-Quality Protein Intake

Strategize Plant-Based Meals

For those on a plant-based diet, don't rely on just one source. Combine different types of plant proteins throughout the day to ensure you are consuming a complete range of essential amino acids. A classic example is combining grains and legumes, such as rice and beans, which together form a complete protein source.

Time Your Protein Intake

For athletes and those with an active lifestyle, the timing of protein intake can be important. Consuming high-quality protein, especially one with a fast absorption rate like whey or eggs, soon after exercise can support muscle repair and growth.

Adapt to Life Stages

Protein requirements and the importance of quality change throughout life. Older adults, for instance, often need more protein to counteract age-related muscle loss. Similarly, pregnant or breastfeeding women have higher needs. Focusing on highly digestible, complete proteins can help meet these increased demands effectively.

Conclusion

Ultimately, what is classed as high-quality protein? It's a protein source with a complete profile of all nine essential amino acids and high digestibility. While animal products like eggs, meat, and dairy are classic examples, plant-based options like soy and quinoa also meet this standard. For those relying on other plant proteins, strategic combinations can provide all the necessary building blocks. Understanding protein quality through metrics like DIAAS allows for more informed dietary choices. A well-rounded diet that includes a variety of high-quality protein sources is the best way to ensure your body has what it needs to build and repair tissues, support a healthy metabolism, and thrive.

Frequently Asked Questions

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. They are vital for protein synthesis and cannot be produced by the body.

Most animal proteins are considered high-quality because they are naturally complete and highly digestible. However, some sources, like collagen, are incomplete. It is also important to choose lean options to avoid excessive unhealthy saturated fat.

Vegans and vegetarians can get high-quality protein by consuming complete plant proteins like soy and quinoa or by combining incomplete protein sources. Pairing foods like grains and legumes (e.g., rice and beans) throughout the day ensures all essential amino acids are consumed.

DIAAS (Digestible Indispensable Amino Acid Score) is the current and more accurate method recommended by the FAO. It measures ileal digestibility and provides uncapped scores. PDCAAS (Protein Digestibility-Corrected Amino Acid Score) is an older method that used fecal digestibility and capped all scores at 1.0, making it less precise.

Many protein powders, especially those derived from whey, casein, or soy, can provide high-quality, easily digestible protein. However, consumers should check labels to ensure the product doesn't contain excessive added sugars or fillers.

Digestibility is crucial because it determines how many of the amino acids in a food are actually absorbed and used by the body. A protein with high digestibility provides more nutritional benefit than one with lower digestibility, even if their amino acid profiles are similar.

Yes, cooking and other forms of processing can affect protein quality. While some methods can improve digestibility by breaking down complex structures, excessive heat can cause denaturation, aggregation, or other chemical changes that reduce a protein's nutritional value.