Understanding Protein Quality: DIAAS vs. PDCAAS
Protein quality is not just about the total grams of protein in a food, but also about its amino acid profile and digestibility. The amino acid profile determines if a protein is 'complete'—meaning it contains all nine essential amino acids in adequate amounts. Digestibility measures how well the body can absorb and use those amino acids. For years, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality. However, PDCAAS had limitations, such as using fecal measurements and capping scores at 1.0, which could misrepresent true protein quality.
In 2011, the FAO introduced the Digestible Indispensable Amino Acid Score (DIAAS) as a more accurate replacement. DIAAS measures amino acid absorption at the ileum, providing a better reflection of what the body actually absorbs. Unlike PDCAAS, DIAAS scores can exceed 1.0, giving a clearer picture of superior protein sources. Using this modern metric, we can identify which plant-based proteins truly earn the highest ratings.
The Top Contenders for the Highest Rating
While no single, unblended plant protein consistently rivals the highest-rated animal proteins like whey or egg in isolated form, certain plant-based options achieve top scores. A key takeaway is that combining complementary protein sources can significantly boost the overall score, an effect supported by the additivity of DIAAS.
Soy Protein
Soy is the standout among plant proteins for its naturally high-quality profile. Soy protein isolate has received high scores under both the old PDCAAS system (often capped at 1.0) and the newer DIAAS metric. Products like tofu, tempeh, and edamame derive from whole soybeans, offering a robust protein content and a complete set of essential amino acids. Its rich amino acid composition and good digestibility make it a cornerstone for plant-based diets seeking high-quality protein. However, its amino acid profile, particularly regarding sulfur-containing amino acids, may still place it slightly below animal-derived isolates under DIAAS.
Quinoa and Amaranth
Both quinoa and amaranth are pseudocereals that are naturally complete proteins, containing all nine essential amino acids. While their individual DIAAS scores are not consistently listed at the top, they are excellent whole-food options. A cup of cooked quinoa provides a solid amount of protein along with other essential nutrients. Their versatility and nutrient density make them valuable additions to a balanced, plant-based diet.
Blended Plant Proteins
The ultimate strategy for achieving the highest possible plant-based protein rating lies in blending. By combining plant proteins with different limiting amino acids, the blend can achieve a DIAAS score of 1.0 or higher. A classic example is the combination of pea protein and brown rice protein. Pea protein is high in lysine but low in sulfur-containing amino acids (SAA), while rice protein is the opposite. Combining them creates a complementary profile that scores very high on the DIAAS scale, with some commercial products achieving scores of 1.30 or higher.
How to Create the Best Plant Protein Combinations
It is a common myth that complementary proteins must be consumed in the same meal. The body maintains a pool of amino acids, so eating a variety of plant proteins throughout the day is sufficient. This allows for incredible dietary flexibility while ensuring all amino acid requirements are met.
- Pair grains with legumes: A classic combination like rice and beans or whole-grain toast with peanut butter effectively complements amino acid profiles.
- Add seeds to your meals: Sprinkling hemp seeds (a complete protein) on a salad or chia seeds into your oatmeal can increase the overall amino acid intake.
- Use fermented soy: Tempeh and miso, made from fermented soybeans, offer highly digestible protein. The fermentation process can also break down anti-nutritional factors that interfere with absorption.
Comparison of High-Quality Protein Sources (DIAAS)
| Protein Source | Typical DIAAS Score | Composition | Limiting Amino Acid(s) |
|---|---|---|---|
| Blended Plant Protein (e.g., Pea + Rice) | Up to 1.30+ | Blend of complementary isolates | None (when blended correctly) |
| Soy Protein Isolate | ~0.89 | Soybeans (isolate) | Methionine + Cysteine |
| Potato Protein Isolate | ~0.85 | Potatoes (isolate) | Histidine |
| Pea Protein Concentrate | ~0.65 | Peas (concentrate) | Methionine + Cysteine |
| Quinoa (whole grain) | Not consistently rated | Pseudocereal | Varies, can be low in Lysine |
This table highlights why engineered blends often achieve the highest functional scores for plant-based proteins. For whole-food sources, strategic pairings are key.
Conclusion: Rating Plant Proteins in a Modern Context
The concept of the highest rating for plant sources of protein is nuanced. Based on the gold-standard DIAAS metric, the highest rating is typically found in carefully formulated, complementary plant protein blends, which can even surpass some individual animal proteins in score. For whole foods, unprocessed soy products like tofu and tempeh, along with pseudocereals like quinoa and amaranth, provide all nine essential amino acids. However, the best approach for overall health is a varied plant-based diet, which naturally combines complementary protein sources over the course of the day. This provides all essential amino acids and a spectrum of other beneficial nutrients. For those seeking the absolute highest score, engineered plant protein isolates offer a clear, measurable path to premium nutritional quality. For more information, visit the Food and Agriculture Organization's website on protein quality evaluation.
