Understanding the Building Blocks: The Amino Acid Profile
Proteins are composed of amino acids, with nine being essential and obtainable only through diet. A protein is deemed 'complete' if it contains all nine essential amino acids in sufficient amounts, typically found in animal sources like meat and dairy. 'Incomplete' proteins, common in most plant-based foods such as grains and legumes, lack or are low in one or more essential amino acids. Strategic combining of plant foods is important for those on plant-based diets to ensure a complete amino acid intake over the day.
The Crucial Role of Digestibility
Protein quality also hinges on digestibility—the body's ability to absorb and utilize amino acids. Highly digestible proteins, common in animal sources, are easily broken down. Plant protein digestibility can be lower due to factors like cell walls and anti-nutritional compounds, but methods such as cooking can enhance it.
Measuring Protein Quality: DIAAS vs. PDCAAS
The Food and Agriculture Organization (FAO) recommends the Digestible Indispensable Amino Acid Score (DIAAS) over the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS) for a more accurate protein quality assessment.
DIAAS is considered superior because:
- It measures digestibility at the ileum (end of the small intestine), a more accurate indicator of absorption than PDCAAS's fecal measurements.
- Unlike PDCAAS, DIAAS is not capped at 1.0, allowing for better differentiation of high-quality proteins, which can score over 100. For example, whey and soy protein isolates have DIAAS scores of 1.09 and 0.90, respectively, demonstrating a clear difference not captured by PDCAAS.
- It uses age-specific amino acid requirement patterns.
Combining Incomplete Proteins for a Complete Profile
It's a misconception that plant-based protein pairings must occur in a single meal. The body can draw from an amino acid pool throughout the day. Consuming a variety of plant protein sources daily ensures the intake of all essential amino acids.
Examples of Complementary Plant Protein Pairings:
- Grains and Legumes: Rice and beans.
- Nuts/Seeds and Legumes: Hummus with whole-grain pita.
A Comparison of Protein Sources and Quality
| Feature | Animal-Based Protein | Plant-Based Protein |
|---|---|---|
| Amino Acid Profile | Usually complete. | Often incomplete, with limiting amino acids. |
| Digestibility | Generally very high. | More variable; can be lower. |
| DIAAS Score | Typically high (often >100). | More variable; lower for some, high for soy and potato. |
| Environmental Impact | Significantly higher. | Generally lower. |
| Dietary Context | Requires moderation for certain types. | Can be combined strategically. |
Examples of high-quality protein sources
- Eggs: High digestibility and complete profile.
- Dairy: Milk, yogurt, and cheese offer excellent complete protein.
- Meat and Poultry: Provide complete and highly digestible protein.
- Soy: Tofu, tempeh, and edamame are complete plant proteins.
- Quinoa: A complete plant-based protein.
- Hemp and Chia Seeds: Also complete proteins.
Examples of lower quality or processed proteins
- Processed Red Meats: Linked to health risks.
- Sugary Protein Bars and Yogurts: Can contain added sugars and inferior proteins.
- Gelatin: Very incomplete amino acid profile.
Conclusion: A Holistic View of Protein Quality
Good or bad protein quality depends on amino acid composition, digestibility, and dietary context. While animal proteins often provide a complete, highly digestible source, varied plant-based diets can also meet essential amino acid needs over the day. Focusing on the DIAAS metric and prioritizing diverse whole food sources—both plant and animal—is the best approach for optimal health.
Learn more from an authoritative source at The Nutrition Source by Harvard T.H. Chan School of Public Health.
