The Building Blocks: What are Amino Acids?
Proteins are fundamental to life, playing a crucial role in building and repairing tissues, creating enzymes and hormones, and supporting overall physiological function. They are composed of smaller units called amino acids. While the body needs 20 different amino acids, it can only produce 11 of them. The other nine are deemed “essential amino acids” (EAAs), as they must be obtained from dietary sources.
The Two Pillars of Protein Quality
When discussing protein quality, nutritionists consider two primary characteristics of a protein source: its amino acid profile and its digestibility. A protein source that supplies all nine essential amino acids in sufficient amounts is considered a 'complete protein'. Digestibility measures how well the body can break down and absorb the amino acids from a food. A protein with a favorable amino acid profile is only useful if the body can effectively absorb and utilize those amino acids. For this reason, assessing protein quality requires evaluating both factors together.
Complete vs. Incomplete Proteins
- Complete Proteins: These sources provide all nine essential amino acids in adequate proportions. Animal-based foods are the most common examples, including meat, fish, poultry, eggs, and dairy. However, several plant-based foods also qualify, such as soy, quinoa, and buckwheat.
- Incomplete Proteins: These proteins are lacking or have low levels of one or more essential amino acids. The majority of plant-based proteins, including nuts, seeds, most grains, and legumes, fall into this category.
The Importance of Combining Proteins
For those on a plant-based diet, it is vital to combine different incomplete proteins throughout the day to ensure all essential amino acid needs are met. For example, legumes are low in the sulfur-containing amino acid methionine, while grains are low in lysine. Combining the two, as in the classic dish of rice and beans, provides a complete amino acid profile. The outdated notion that these pairings must happen in a single meal is incorrect; the body can pull from its amino acid pool over a 24-hour period.
Modern Protein Scoring: DIAAS vs. PDCAAS
To determine a protein's quality, scientific methods have evolved. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was the long-standing gold standard but has been largely replaced by a more accurate method, the Digestible Indispensable Amino Acid Score (DIAAS).
- PDCAAS (Older Method): The PDCAAS compares a food's amino acid composition to a standard reference pattern and adjusts it for fecal digestibility. A key limitation is that it truncates all scores at 100%, meaning two different proteins might both score 100% despite one being nutritionally superior.
- DIAAS (Current Method): The DIAAS method is preferred for several reasons. It measures ileal digestibility, providing a more accurate measure of amino acids absorbed by the small intestine. The score is not capped, allowing for a clearer differentiation between protein sources of the highest quality. It also uses updated, age-specific reference patterns for humans.
High-Quality Protein Sources: Plant vs. Animal
Generally, animal proteins are considered higher quality due to their more complete essential amino acid profile and higher digestibility. However, advancements in food processing have improved the quality of some plant-based protein isolates. While animal sources provide a more efficient supply of EAAs, the nutritional benefits of a diverse, whole-food plant-based diet are well-documented.
A Comparison of Protein Quality Scores
| Protein Source | Typical DIAAS Score | Notes |
|---|---|---|
| Animal Sources | ||
| Bovine Milk | >100% | High quality, excellent amino acid profile. |
| Whey Protein Isolate | ~109% | Scores very high due to rapid digestion and high leucine content. |
| Casein | ~145% | Scores very high, but digests more slowly than whey. |
| Beef | >100% | Complete protein source, typically with good digestibility. |
| Plant Sources | ||
| Soy Protein Isolate | ~90-100% | High quality for a plant-based option, complete EAA profile. |
| Pea Protein | ~70-90% | Improved with processing, often limited by methionine/cysteine. |
| Quinoa | ~83% | A rare complete plant protein, also rich in fiber. |
| Rice Protein | ~50-58% | Lower quality due to limitations, especially lysine. |
Excellent sources of high-quality protein
- Animal-Based: Lean meats (chicken, beef), fish (salmon, tuna), eggs, and dairy products (milk, yogurt, cheese) are all excellent sources of complete, high-quality protein.
- Plant-Based: Soy products (tofu, tempeh), quinoa, and buckwheat are complete plant protein options. Combining foods like rice and beans, or hummus and whole-wheat pita, effectively creates a complete protein meal.
The Health Impact of High-Quality Protein
Beyond simply meeting a daily protein quota, focusing on the quality of your protein intake offers numerous health advantages. It is particularly crucial for muscle growth and repair, especially after exercise. Consuming high-quality proteins can also increase satiety, which helps with weight management, and supports better blood sugar control. For older adults, who often experience age-related muscle loss (sarcopenia), adequate intake of high-quality protein is especially important to mitigate muscle wasting and preserve function. The availability of EAAs, especially leucine, is a key driver for stimulating muscle protein synthesis, and animal-based proteins often contain higher levels of leucine.
Conclusion
Understanding what is meant by good quality protein goes beyond simply counting grams. A protein’s quality is determined by its complete essential amino acid profile and its digestibility. While animal proteins are typically higher in quality based on current metrics like DIAAS, a well-planned, varied plant-based diet can provide all the necessary amino acids. For optimal health, it is wise to incorporate a variety of quality protein sources, whether animal or plant-based, to support muscle growth, repair, and overall well-being. Ultimately, the best dietary approach prioritizes nutrient density and a balanced intake of all essential nutrients from a variety of whole foods.
