What is Protein Quality and How Is It Measured?
To understand what is the protein score of an egg, it is important to first define how protein quality is assessed. Protein quality is not simply about the quantity of protein in a food, but about its nutritional value to the human body. This value is determined by two main factors: the protein's amino acid composition and its digestibility. For a protein to be considered 'complete,' it must contain all nine essential amino acids (EAAs), which the body cannot produce on its own. Eggs contain all nine of these EAAs in optimal proportions. The ease with which the body can absorb and utilize these amino acids is also a crucial component of the score. Two prominent methods for scoring protein quality have been developed over time to standardize this measurement: the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).
The PDCAAS Score of an Egg
The PDCAAS method was formerly the most widely recognized standard for evaluating protein quality, adopted by health organizations like the Food and Agriculture Organization (FAO) and the World Health Organization (WHO). It measures the quality of a protein by comparing its amino acid profile to a standard reference pattern and corrects for how well the protein is digested. On this scale, a perfect score is 1.0. Eggs have consistently earned the highest possible PDCAAS score, originally calculated to be higher than 1.0 (around 1.18 or 1.21), but it was capped at the maximum of 1.0 for reporting purposes. This perfect score signifies that egg protein, once digested, provides 100% or more of the essential amino acids required per unit of protein.
The Modern DIAAS Score for Egg Protein
Recognizing the limitations of the PDCAAS—particularly its capping of scores at 1.0 and its use of less precise digestibility measurements—the FAO proposed the DIAAS method in 2013. The DIAAS is a more accurate and advanced system because it measures digestibility based on the absorption of individual amino acids in the ileum (the final part of the small intestine), rather than based on overall protein digestion in the feces. Critically, DIAAS values are not capped, allowing for a more nuanced comparison between high-quality proteins. For example, a hard-boiled egg has a DIAAS score of 1.13 for toddlers and 1.01 for older individuals, demonstrating its 'excellent' protein quality. This uncapped score clearly positions eggs among the highest-quality protein sources available.
The Role of Bioavailability: Cooked vs. Raw Eggs
One of the most significant factors affecting the practical protein score of an egg is its preparation method. The bioavailability of a protein—or the proportion of a nutrient that is absorbed from the diet and used for normal body functions—is dramatically impacted by cooking. While many believe raw is better, studies show the opposite is true for eggs. The protein in raw eggs is only about 50% digestible, while cooking an egg increases digestibility to approximately 90%. This is because the heat denatures the complex protein structures, unfolding them and making the amino acids more accessible for the body's digestive enzymes to break down and absorb. Consuming raw eggs also poses a risk of bacterial infection, such as Salmonella, and contains avidin, a protein that binds to the essential B-vitamin biotin and inhibits its absorption. For optimal absorption and safety, it is always best to consume cooked eggs.
Comparison of Protein Quality Scores
To put the egg's high score into context, here is a comparison of PDCAAS values for several common protein sources:
| Food Source | PDCAAS Score | Notes |
|---|---|---|
| Egg | 1.00* | Highest score, originally calculated higher than 1.0. |
| Milk (Casein) | 1.00* | Highest score, originally calculated higher than 1.0. |
| Beef | 0.92 | Also a high-quality protein, but slightly lower. |
| Soy Protein Isolate | 1.00* | Often processed to achieve the highest score. |
| Pea Protein | 0.82 | Good source, but contains limiting amino acids. |
| Rice | 0.47 | Lower score, as it lacks a full amino acid profile. |
| Wheat | 0.42 | Significantly lower protein quality score. |
*Note: These PDCAAS scores are capped at 1.0. Uncapped DIAAS scores provide a clearer distinction between these top-tier protein sources.
The Complete Amino Acid Profile of Eggs
The high protein score of an egg is a direct result of its excellent amino acid profile. As a complete protein source, it contains all nine essential amino acids needed for muscle repair, growth, and overall bodily functions. The optimal balance and ratio of these amino acids are what make it such an efficient and valuable protein for human nutrition. The essential amino acids in eggs include:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
This complete profile is why eggs are so frequently recommended by dietitians and nutritionists as a fundamental component of a healthy diet. For athletes, this balance is especially beneficial for supporting muscle protein synthesis and recovery. Beyond essential amino acids, eggs also provide a rich source of other nutrients, vitamins, and minerals, further contributing to their status as a nutritional powerhouse.
Conclusion: The Gold Standard for Protein Quality
In conclusion, the protein score of an egg is among the highest in the world, validated by both the classic PDCAAS method (earning a capped score of 1.0) and the more modern, uncapped DIAAS method (earning an 'excellent' score). This superior quality is attributed to its complete amino acid profile, high bioavailability, and excellent digestibility, particularly when cooked. Whether you are an athlete, a health-conscious individual, or someone simply looking for an affordable, high-quality protein source, the humble egg stands out as a clear leader. For more in-depth information, you can consult research on the benefits of egg protein PMC9316657.
