What Is Protein Quality and Why It Matters
Protein quality refers to a protein's capacity to provide essential amino acids to the body. Essential amino acids are the building blocks your muscles need for repair and growth, and your body cannot produce them on its own; they must come from food. For athletes, choosing high-quality protein is key because it directly influences muscle protein synthesis—the process that rebuilds and strengthens muscle tissue after exercise.
When you eat protein, its quality is determined by how well it supplies the essential amino acids in the right amounts and how easily your body can digest and absorb them. A high-quality protein provides all essential amino acids in sufficient quantities and is highly digestible, ensuring your muscles get the materials they need to recover and adapt to training.
Sources: Journal of the International Society of Sports Nutrition
Key Metrics: PDCAAS, DIAAS, and Amino Acid Profile
To compare protein quality, scientists use scoring systems that combine amino acid composition with digestibility. The two most common are the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) and the more recent Digestible Indispensable Amino Acid Score (DIAAS).
PDCAAS is currently the method used by the US FDA for food labeling, but it has limitations. It measures protein quality based on the amino acid profile and overall digestibility, but it tends to overestimate digestibility because it does not account for amino acids that are absorbed in the colon through bacterial fermentation. Additionally, PDCAAS scores are capped at 1.00, even if a protein provides more than enough essential amino acids.
DIAAS is considered a more accurate assessment because it measures the digestibility of individual essential amino acids at the end of the small intestine (the ileum). Unlike PDCAAS, DIAAS scores can exceed 1.00, allowing for a better differentiation of high-quality proteins.
Beyond these scores, the amino acid profile itself matters, particularly the content of leucine, an essential amino acid that plays a key role in triggering muscle protein synthesis. Animal-based proteins generally contain more leucine per serving than plant-based proteins, which is an important consideration for athletes.
Sources: American Dairy Products Institute, Cardiac Longevity Clinic
Animal vs. Plant Protein Quality
Protein sources from animal and dairy products contain higher percentages of essential amino acids and generally have higher DIAAS scores compared to most plant proteins. This translates to a greater ability to stimulate muscle protein synthesis and, over time, can lead to greater hypertrophy and strength gains when combined with resistance training. For athletes, this means that animal-based proteins like whey, eggs, meat, and dairy are often more efficient per gram at promoting muscle recovery and adaptation.
However, plant proteins can still play a valuable role in an athlete's diet. Soy protein stands out as a complete plant protein with a quality score comparable to many animal sources. While most plant proteins are lower in one or more essential amino acids, they can be combined to provide a complete amino acid profile. This is where the concept of protein complementation comes in.
Sources: Journal of the International Society of Sports Nutrition, Cardiac Longevity Clinic
Top High-Quality Protein Foods
When it comes to high-quality protein, dairy proteins are among the best available, with whey protein scoring exceptionally well on the DIAAS scale. Here are some top sources for athletes, based on their protein quality and amino acid profile.
- Whey protein: DIAAS above 1.0, rich in leucine and quickly digested
- Milk and dairy products: high in both whey and casein, slow and fast digesting proteins
- Eggs: excellent amino acid profile and high digestibility
- Chicken and meat: high-quality animal proteins with all essential amino acids
- Fish: similar to other animal proteins, with a complete amino acid profile
- Soy protein: a complete plant protein, suitable for vegetarians and vegans
| Protein Source Category | Typical DIAAS Score |
|---|---|
| Whey protein isolate | 1.0 or higher |
| Milk protein concentrate | 1.0 or higher |
| Egg white | 1.0 |
| Chicken breast | ~1.0 |
| Beef | ~0.9 |
| Soy protein isolate | ~0.9 |
| Pea protein concentrate | ~0.8 |
Sources: American Dairy Products Institute, Cardiac Longevity Clinic
Combining Plant Proteins for Completeness
Plant proteins are often called 'incomplete' because they lack sufficient amounts of one or more essential amino acids. The classic example is grains, which are low in lysine, and legumes, which are low in methionine. By eating a combination of different plant proteins throughout the day, you can ensure your body receives all essential amino acids in adequate amounts.
The practical approach is to include a variety of plant protein sources in your diet, rather than worrying about combining them at every single meal. Your body maintains a pool of amino acids, so what matters is the overall balance over the day. For example, having rice and beans together or peanut butter on whole-grain bread can easily create a complete amino acid profile.
If you are following a vegetarian or vegan diet and looking to maximize muscle growth, focusing on higher-leucine plant sources like soy and pea protein, or adding a vegan protein blend that includes complementary proteins, can help you meet your needs.
- Pair grains with legumes: rice and beans, hummus with pita, or peanut butter on whole wheat
- Include a variety of plant proteins: soy, pea, hemp, and quinoa are complete or nearly complete
- Consider a vegan protein powder blend that combines pea and rice protein for a full amino acid profile
Sources: American Dairy Products Institute, Cardiac Longevity Clinic, My Sport Science
Practical Tips for Meeting Quality Needs
Most athletes need between 1.6 and 2.2 grams of protein per kilogram of body weight per day. To meet this goal while ensuring high protein quality, follow these guidelines.
First, include a source of high-quality protein at each meal. This could be animal-based options like eggs at breakfast, chicken or fish at lunch, and dairy or whey shake after a workout, or plant-based alternatives like tofu, tempeh, and soy milk.
Second, pay attention to leucine content. Include leucine-rich protein sources at meals to support muscle protein synthesis. Animal proteins are naturally rich in leucine, but if you rely on plant proteins, choose options like soy or a plant protein blend that provides complementary amino acids.
Third, distribute your protein intake evenly across the day. Consuming protein every 3-4 hours, especially after exercise, can help maximize muscle repair and growth. Finally, remember that whole foods should be your primary source of protein, while supplements like whey or plant protein powders are convenient additions, not necessities.
Sources: Journal of the International Society of Sports Nutrition
Choosing Between Animal and Plant Proteins
When deciding between animal and plant protein sources, consider factors like amino acid profile, digestibility, and how they fit into your overall diet. Animal proteins tend to have higher DIAAS scores and more leucine per serving, which may translate to a greater stimulus for muscle protein synthesis per gram. However, plant proteins can still meet your needs when consumed in adequate amounts and varied over the day.
The table below summarizes general considerations for animal and plant protein categories. Note that exact DIAAS values vary by specific food and processing, but the general trends are consistent.
| Category | Typical DIAAS Range | Leucine Content | Completeness |
|---|---|---|---|
| Animal proteins (dairy, meat, egg) | 0.9 to above 1.0 | Higher | Complete |
| Plant proteins (soy, pea, etc.) | 0.8 to 0.9 | Lower to moderate | Some complete; others complementary |
Sources: American Dairy Products Institute, Cardiac Longevity Clinic
Frequently asked questions
Is whey protein higher quality than chicken?
Whey protein is often considered higher in quality due to its very high digestibility and excellent amino acid profile. It scores above 1.0 on the DIAAS scale, while chicken scores around 1.0. Both are complete proteins, but whey is particularly rich in leucine and is absorbed quickly, making it an excellent choice around workouts. Chicken, however, is also a top-quality protein that provides important micronutrients and can be more satiating. For most athletes, the difference in practical terms is small, and both can effectively support muscle growth when consumed as part of a protein-adequate diet.
Sources: Cardiac Longevity ClinicDo I need to combine proteins at every meal?
No, you don't need to combine proteins at every single meal. Your body maintains an amino acid pool, and what matters most is the overall balance of essential amino acids over the course of the day. As long as you eat a variety of protein sources—such as different plant proteins or a mix of plant and animal proteins—throughout the day, you can meet your needs without precise meal-by-meal combinations. However, if you are relying solely on plant proteins and aiming to maximize muscle protein synthesis, it is wise to ensure each main meal contains a higher-leucine source, such as soy, or includes complimentary proteins like grains and legumes.
Sources: American Dairy Products Institute, Cardiac Longevity ClinicCan plant proteins be as effective as animal proteins for building muscle?
Yes, plant proteins can be as effective as animal proteins for building muscle, provided you consume enough total protein and pay attention to amino acid profile. Animal proteins generally stimulate muscle protein synthesis more per gram, especially due to their higher leucine content, but when overall protein intake is adequate and varied, plant-based diets can support similar muscle growth. Soy stands out as a complete plant protein that is nearly as effective as many animal sources. For vegan or vegetarian athletes, focusing on leucine-rich plant foods and possibly using a mixed protein supplement can help you reach the same anabolic response.
Sources: Cardiac Longevity Clinic, My Sport ScienceSources
- International Society of Sports Nutrition Position Stand: protein and exercise — Journal of the International Society of Sports Nutrition
- Protein Quality Guide — American Dairy Products Institute
- Optimal Protein Intake for Athletic Performance and Muscle Growth — Cardiac Longevity Clinic
- Protein sources for athletes: animal, plant and alternative proteins — My Sport Science
- Contemporary Issues in Protein Requirements and Consumption for Resistance Trained Athletes — Journal of the International Society of Sports Nutrition