Defining High-Quality Protein
High-quality protein, often referred to as a 'complete' protein, is one that provides all nine essential amino acids (EAAs) that the human body cannot produce on its own. Proteins are constructed from chains of amino acids, and these EAAs are the vital building blocks for various physiological functions, including repairing tissues, creating enzymes, and building muscle. When a protein source contains all nine EAAs in adequate amounts, it is considered complete. Animal-based proteins are almost always complete. However, some plant-based proteins, such as soy and quinoa, are also naturally complete sources.
In contrast, 'incomplete' proteins are low in or missing one or more of these nine EAAs. Most plant-based proteins, like those found in beans, nuts, and grains, are incomplete on their own. However, this is not a concern for those consuming a varied diet, as different plant sources can be combined to form a complete amino acid profile over the course of a day.
The Role of Digestibility and Bioavailability
Beyond amino acid composition, another critical factor is digestibility—the body's ability to break down and absorb the protein. A protein's nutritional value is determined by how much of its amino acids actually get utilized by the body, a concept known as bioavailability. Animal proteins generally have a higher digestibility, around 90–100%, while many plant proteins have lower digestibility, closer to 70%. Processing and cooking methods can also influence a protein's digestibility. For example, soaking and cooking legumes can help inactivate anti-nutritional factors that hinder protein breakdown.
Modern Protein Scoring Methods
For many years, the standard for measuring protein quality was the Protein Digestibility Corrected Amino Acid Score (PDCAAS). However, a newer, more accurate method has since been developed. The Food and Agriculture Organization (FAO) now recommends the Digestible Indispensable Amino Acid Score (DIAAS).
Why DIAAS is a superior measure
- Ileal vs. Fecal Digestibility: PDCAAS measures digestibility by analyzing fecal matter, which can be inaccurate due to the activity of intestinal bacteria. DIAAS, by contrast, measures amino acid digestibility at the end of the small intestine (the ileum), providing a more precise value of what the body truly absorbs.
- Uncapped Scores: PDCAAS scores are capped at a maximum of 1.0, meaning the highest quality proteins, like whey or egg, receive the same score as some slightly lower quality proteins. DIAAS provides uncapped scores, allowing for a clearer differentiation between the highest quality sources.
- Age-Specific Needs: DIAAS offers reference patterns based on different age groups, acknowledging that nutritional requirements change throughout life.
Comparative Protein Quality (PDCAAS vs. DIAAS)
| Food Source | PDCAAS Score | DIAAS Score (0.5-3 yr) | Limiting Amino Acid (in DIAAS) |
|---|---|---|---|
| Milk Protein Concentrate | 1.00 | 1.18 | Met + Cys |
| Whey Protein Isolate | 1.00 | 1.09 | Val |
| Soy Protein Isolate | 0.98 | 0.898 | Met + Cys |
| Egg (hard boiled) | 1.00 | 1.13 | His |
| Beef | 1.00 | 1.116 | - |
| Peas | 0.782 | 0.647 | Met + Cys |
| Rice | 0.616 | 0.595 | Lys |
Top High-Quality Protein Sources
For most people, a diet that includes a variety of protein sources is the best way to ensure all amino acid needs are met. Here are some of the best choices:
Animal-Based Sources
- Eggs: Considered a gold standard of protein quality due to their excellent EAA profile and high digestibility.
- Milk, Greek Yogurt, and Cottage Cheese: Dairy products are rich in both whey and casein, offering a rapid and slow-digesting protein blend.
- Lean Meats: Chicken breast, turkey, and lean beef are excellent sources of complete, high-quality protein packed with other nutrients like iron and zinc.
- Fish and Seafood: Salmon and tuna are not only rich in high-quality protein but also provide heart-healthy omega-3 fatty acids.
Plant-Based Sources
- Soy Products (Tofu, Edamame, Tempeh): Soy is one of the few plant-based sources that offers a complete protein profile, making it especially valuable for vegetarians and vegans.
- Quinoa: This grain-like seed is another complete protein source, offering a good balance of amino acids.
- Legumes (Lentils, Chickpeas, Beans): While incomplete on their own, legumes are a foundational plant-based protein. They can be effectively combined with grains to provide all EAAs.
- Nuts and Seeds: Almonds, peanuts, and chia seeds offer protein alongside healthy fats and fiber. Combining them with other plant sources enhances protein quality.
Putting Protein Complementation into Practice
For those following a primarily plant-based diet, protein complementation is a simple and effective strategy. It means combining different plant foods that have complementary amino acid profiles to ensure you get all nine EAAs. A common and easy practice involves pairing legumes with grains, as the amino acid profile of one compensates for the limitations of the other.
Examples of Complementary Protein Pairings:
- Rice and Beans: A classic pairing where the lysine in beans complements the methionine in rice.
- Hummus and Pita Bread: The chickpeas provide lysine, and the wheat in the pita adds methionine.
- Peanut Butter on Whole Wheat Bread: This common sandwich is a good example of complementary proteins.
- Lentil Soup with Whole-Grain Bread: The amino acid profile of the lentils is balanced by the whole grains.
While these examples demonstrate how to create complete proteins within a single meal, it is not necessary to do so every time. A varied plant-based diet throughout the day will naturally provide all the essential amino acids your body needs.
Conclusion
Ultimately, what is considered good quality protein is a combination of its essential amino acid content and its digestibility. High-quality or complete proteins, like those from animal sources, soy, and quinoa, contain all nine essential amino acids and are highly digestible. While most plant proteins are incomplete on their own, a varied diet that includes complementary plant-based sources can easily provide the full range of essential amino acids. Modern scoring methods, particularly DIAAS, offer a more precise way to measure protein quality, reflecting a better understanding of human digestion. Whether you consume animal or plant-based proteins, focusing on variety and considering digestibility will help you achieve optimal nutritional intake for growth, repair, and overall health. For further reading, consult the National Institutes of Health.
