Understanding the PDCAAS
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method developed by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) to evaluate the quality of a protein. The scoring system measures how well a protein provides the essential amino acids required by the human body after digestion. A score of 1.0 is the highest possible value and is considered the gold standard for protein quality. This score signifies that the protein, after digestion, provides 100% or more of the essential amino acids needed per unit of protein.
Foods That Achieve the Maximum 1.0 PDCAAS Score
Many foods are so rich in high-quality protein that they reach the maximum PDCAAS score of 1.0. These proteins are often referred to as 'complete' proteins because they contain all nine essential amino acids in sufficient quantities. The top-scoring foods include:
- Eggs: Often referred to as the 'perfect protein,' eggs have a very high amino acid profile and excellent digestibility, earning a perfect score. It's worth noting that heating eggs helps to deactivate certain protease inhibitors, increasing the bioavailability of their protein.
- Milk Proteins (Whey and Casein): Cow's milk is composed of two primary proteins: whey and casein. Both have excellent amino acid profiles and achieve a PDCAAS of 1.0. Whey is known for its fast absorption, while casein provides a slower, more sustained release of amino acids.
- Soy Protein Isolate: This highly refined protein source is one of the few plant-based proteins that is considered complete and achieves a PDCAAS of 1.0. It is a popular option for those seeking a high-quality, non-animal source of protein.
- Beef: Some cuts of beef are also cited as reaching a perfect PDCAAS of 1.0, though some older sources show scores closer to 0.92. This variation can depend on the specific cut and preparation method.
PDCAAS vs. DIAAS: A More Nuanced Look
While the PDCAAS has been widely used, it is not without limitations. A major criticism is that the scale caps at 1.0, preventing differentiation between proteins that may offer superior nutritional value. For example, studies show that milk proteins and eggs actually have 'untruncated' PDCAAS values higher than 1.0, but the official score is capped. Another issue is that PDCAAS relies on fecal digestibility, which may overestimate the amount of protein actually absorbed.
In response to these shortcomings, the Digestible Indispensable Amino Acid Score (DIAAS) was developed. DIAAS measures digestibility in the small intestine (ileal digestibility), providing a more accurate assessment of amino acid absorption and does not truncate scores at 1.0. For this reason, some nutrition experts consider DIAAS a more precise tool for comparing protein quality.
Comparison of PDCAAS and DIAAS for Selected Proteins
| Protein Source | PDCAAS | DIAAS (for children 6 months–3 years) |
|---|---|---|
| Milk Protein Concentrate | 1.00 | 1.18 |
| Whey Protein Isolate | 1.00 | 1.09 |
| Egg (hard-boiled) | 1.00 | 1.13 |
| Beef | 0.92–1.00 | 1.116 |
| Soy Protein Isolate | 0.98–1.00 | 0.898–0.90 |
| Pea Protein Concentrate | 0.89 | 0.82 |
| Chickpeas | 0.78 | 0.83 |
| Wheat | 0.42 | 0.40–0.48 |
Lower-Scoring Proteins and Complementary Combinations
While the foods above represent the pinnacle of protein quality, many other common foods provide beneficial protein but with lower PDCAAS scores. This is often because they lack sufficient amounts of one or more essential amino acids, which are then considered 'limiting'.
- Legumes: Foods like lentils, beans, and chickpeas generally have lower PDCAAS scores than animal proteins, primarily due to insufficient methionine.
- Cereals: Grains such as wheat and rice have lower scores, with lysine typically being the limiting amino acid.
However, a lower score doesn't mean a protein is worthless. By combining different protein sources, you can create a complete amino acid profile. A classic example is combining grains and legumes, such as rice and beans, to ensure all essential amino acids are consumed. This is known as protein complementation.
Conclusion: Focus on Variety
To answer the original question, multiple foods, including eggs, whey, casein, and soy protein isolate, share the distinction of having the highest PDCAAS, each achieving a perfect score of 1.0. While these foods provide the most complete and digestible protein per unit, newer scoring systems like DIAAS offer a more granular differentiation. For the average person, focusing on a varied and balanced diet that includes a mix of these high-quality sources, rather than fixating on a single winner, is the most practical approach to ensuring adequate protein intake. For more detailed nutritional information and data, consult reputable sources like the National Institutes of Health.
List of high PDCAAS protein sources:
- Eggs
- Milk (Whey, Casein)
- Soy Protein Isolate
- Beef
List of lower PDCAAS protein sources:
- Lentils
- Peanuts
- Wheat Gluten
- Rice
Further Reading
For a deeper dive into protein quality and the differences between measurement methods, resources from organizations like the FAO and NIH are invaluable. Understanding the science behind these scores can help make more informed dietary choices.
