Protein is a fundamental macronutrient, but not all protein is created equal. The difference between a high-quality protein and a low-quality one can significantly impact your body's ability to repair tissues, build muscle, and produce hormones and enzymes. Two primary factors determine a protein's quality: its essential amino acid (EAA) profile and its digestibility. These metrics help us understand how effectively a protein source can be absorbed and utilized by the body to meet its metabolic needs.
The Role of Essential Amino Acids
Amino acids are the building blocks of protein. While the body can synthesize 11 of the 20 amino acids it needs, there are nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) that must be obtained through your diet.
Complete vs. Incomplete Proteins
A protein is considered 'complete' when it contains all nine essential amino acids in sufficient amounts to meet the body's needs. Animal-based proteins, such as meat, eggs, and dairy, are typically complete proteins. In contrast, most plant-based proteins, like those found in grains, nuts, and legumes, are 'incomplete,' meaning they are low in or missing one or more EAAs.
- Animal Proteins: Excellent sources of all EAAs. Examples include eggs, whey protein, casein, beef, chicken, and fish.
- Plant Proteins: Many are incomplete, but some are complete. Notable complete plant proteins include soy, quinoa, buckwheat, and hempseed. Vegan diets can achieve a complete EAA profile by combining different incomplete protein sources throughout the day, such as pairing rice with beans.
The Importance of Digestibility and Bioavailability
Beyond just the amino acid profile, a protein's quality depends on how well your body can break it down and absorb the amino acids. This is known as digestibility and bioavailability.
- Processing Matters: Cooking and processing methods can affect a protein's digestibility. For example, excessive heat can damage protein structures and reduce the bioavailability of certain amino acids, like lysine through the Maillard reaction. Conversely, methods like fermentation can improve digestibility, especially for plant-based proteins.
- Antinutrients: Plant-based sources may contain antinutrients like phytates and tannins that can inhibit protein digestion. Proper processing, such as soaking, dehulling, and heating, helps reduce these compounds. Animal proteins typically have higher digestibility (90-95%) compared to plant proteins (70-85%).
Measuring Protein Quality: DIAAS vs. PDCAAS
Historically, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality. However, it had several flaws, including its scoring being capped at 1.0 and using fecal digestibility, which overestimates absorption. The new gold standard, recommended by the Food and Agriculture Organization (FAO) in 2013, is the Digestible Indispensable Amino Acid Score (DIAAS).
Why DIAAS is Superior
- Accurate Digestibility: DIAAS measures amino acid digestibility at the end of the small intestine (ileum), providing a more accurate reflection of what is absorbed by the body.
- Better Scoring: DIAAS is not capped at 100%, allowing for a more precise ranking of high-quality proteins. For example, whey protein isolate scores higher on the DIAAS scale than soy protein isolate, better reflecting its superior amino acid profile and digestibility kinetics.
- Individual Amino Acid Digestion: DIAAS accounts for the specific digestibility of each essential amino acid, unlike PDCAAS which uses a single crude protein digestibility value.
Comparison Table: Protein Quality Scores
| Protein Source | Typical DIAAS Score | Notes |
|---|---|---|
| Whey Protein | >109% | A very high-quality dairy protein, rapidly digested and rich in BCAAs. |
| Casein Protein | >100% | A high-quality dairy protein that digests slowly, providing a steady supply of amino acids. |
| Eggs | >100% | Often considered the benchmark for protein quality due to its optimal EAA profile and high bioavailability. |
| Beef | ~91-99% | Excellent source of complete protein, high in iron and zinc. |
| Soy Protein Isolate | ~90% | A complete plant-based protein, though with slightly lower digestibility and leucine content than animal proteins. |
| Pea Protein | ~71-78% | A plant-based source with some limiting amino acids, often paired with rice to complete the profile. |
| Rice Protein | ~50-58% | Plant-based, limited in lysine, and typically combined with other sources for a complete profile. |
| Wheat Gluten | ~46-51% | Lower quality due to limited lysine and moderate digestibility. |
Conclusion
What makes something a high quality protein is a combination of a complete essential amino acid profile and high digestibility. While animal sources generally offer higher quality due to their balanced amino acid content and superior digestibility, many plant-based sources can be combined to achieve a high-quality protein intake. The shift towards the DIAAS scoring method provides a more accurate tool for evaluating protein sources, helping consumers and health professionals make informed nutritional choices for optimal health. Focusing on a variety of protein sources, both animal and plant-based, remains the most practical way to meet all your amino acid needs and ensure a high-quality protein intake.
Visit the Food and Agriculture Organization website to learn more about the DIAAS scoring system
Optimize Your Protein Intake
- Prioritize Variety: Consume a wide array of protein sources throughout the day, including both animal and plant-based foods, to ensure you get all essential amino acids.
- Combine Plant Proteins: If following a plant-based diet, pair complementary proteins, such as beans with rice or lentils with quinoa, to create a complete amino acid profile.
- Consider Timing: For muscle building and repair, consume high-quality, fast-digesting proteins like whey immediately after exercise, and opt for slow-digesting casein before bed.
- Choose Wisely: Be aware that processing can impact protein quality. Opt for minimally processed foods where possible, and ensure plant-based products have been treated to reduce antinutrients.
- Factor in Goals: Higher protein intake may be beneficial for older adults or those with intense physical activity levels, as their amino acid requirements can be higher than the average healthy adult.
