Understanding the Fundamentals of Protein Quality
Protein quality is a measure of how efficiently the body can use dietary protein for functions like growth and repair. This efficiency depends on the food's essential amino acid (EAA) profile and the protein's digestibility. High-quality proteins provide all nine essential amino acids that the body cannot produce itself.
The Role of Amino Acid Profile and Digestibility
A complete protein contains all essential amino acids in adequate amounts. A shortage of even one EAA can limit the body's ability to synthesize new proteins, making that the "limiting amino acid". While animal proteins generally have a balanced EAA profile and are highly digestible, many plant proteins are deficient in one or more EAAs. However, combining different plant protein sources, such as legumes and grains, can provide all essential amino acids. Protein digestibility, or how well the body breaks down and absorbs protein, is also crucial. Processing methods and certain plant compounds can affect digestibility.
The Primary Methods to Calculate Protein Quality
Protein quality is evaluated using scoring systems like the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the more modern Digestible Indispensable Amino Acid Score (DIAAS).
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
The FAO/WHO introduced PDCAAS, which compares a protein's EAA content to a standard and adjusts for digestibility. Key drawbacks include scores capped at 1.0, making differentiation of high-quality proteins difficult. It also uses fecal digestibility, which may not accurately reflect absorbed amino acids.
Digestible Indispensable Amino Acid Score (DIAAS)
DIAAS, a newer method, is considered more accurate. It measures digestibility in the ileum, better indicating absorbed amino acids. Unlike PDCAAS, DIAAS scores are uncapped, allowing better distinction, with scores categorized as "Good," "Excellent," or "High". DIAAS also assesses individual EAA digestibility. Calculating DIAAS requires specialized lab analysis of ileal digestibility.
Comparison of Protein Scoring Methods
| Feature | PDCAAS | DIAAS |
|---|---|---|
| Digestibility Measurement | Fecal digestibility (entire gut) | Ileal digestibility (end of small intestine) |
| Accuracy | Prone to overestimation due to bacterial activity | More accurate, reflects amino acid absorption |
| Scoring Range | Truncated at 1.0 | Uncapped, allowing for differentiation of high-quality proteins |
| Amino Acid Assessment | Based on single limiting amino acid and overall protein digestibility | Assesses digestibility for each individual essential amino acid |
| Industry Standard | Still widely used, especially for nutritional labeling | Recommended by FAO; adoption pending more extensive data |
| Reference Pattern | Originally based on amino acid needs of 2-5 year olds | Updated reference patterns for different age groups |
How to Apply this Knowledge to Your Diet
A varied diet with whole foods, including animal and plant sources, helps ensure adequate high-quality protein.
High-Quality Protein Food Examples (Typically high DIAAS or equivalent):
- Eggs
- Dairy (milk, whey, casein)
- Fish
- Lean Meats
Good-to-Excellent Quality Plant Protein Sources (Can be combined for higher quality):
- Soy (tofu, edamame)
- Quinoa
- Legumes (chickpeas, lentils)
- Grains (rice, wheat)
Practical Tips for Maximizing Protein Quality:
- Mix Plant Proteins: Combine sources like grains and legumes for a complete profile.
- Prioritize Variety: A diverse diet naturally increases essential amino acids.
- Balance Meals: Include mixed protein sources throughout meals.
Conclusion
While methods like PDCAAS and DIAAS detail how to calculate protein quality, practical nutrition principles are simpler. Quality depends on amino acid composition and digestibility. DIAAS is the scientific standard, more accurate than PDCAAS due to ileal digestibility measurement. Eating a varied diet with both animal and plant protein is the best way to get high-quality protein.
