Understanding Protein Quality Metrics
To effectively learn how to calculate a good protein source, you must first move beyond a simple protein-per-gram mindset. The quality of a protein is primarily determined by its amino acid profile and its digestibility. The human body requires nine essential amino acids (EAAs) that it cannot synthesize on its own. A complete protein contains adequate levels of all nine EAAs, while an incomplete protein is deficient in one or more. However, modern science has developed more precise scoring systems to rank protein quality.
The Digestible Indispensable Amino Acid Score (DIAAS)
The DIAAS is the current gold standard for measuring protein quality, recommended by the FAO in 2013 to replace the older PDCAAS method. This method addresses the shortcomings of its predecessor by measuring the digestibility of individual amino acids at the end of the small intestine (the ileum), which provides a more accurate reflection of what the body actually absorbs. A DIAAS score can also exceed 100%, allowing for a more accurate ranking of high-quality proteins. For example, skim milk powder can have a DIAAS score well over 100%, indicating a superior protein source for children and adults.
The Protein Digestibility Corrected Amino Acid Score (PDCAAS)
Before DIAAS, the PDCAAS was the most widely used metric for over two decades. This system measures protein quality by comparing a food's amino acid profile to a reference standard and correcting it for overall protein digestibility based on fecal analysis. Its primary limitations are that the score is capped at 1.0, and it uses a less accurate measure of digestibility. Many top-tier proteins, like whey and egg, all receive a score of 1.0, making it impossible to differentiate between them.
Other Historical Metrics: BV and NPU
- Biological Value (BV): This older method measures the proportion of absorbed protein from a food that is retained and utilized by the body. It primarily focuses on nitrogen retention but is less relevant for human nutrition than modern, amino acid-focused scoring systems.
- Net Protein Utilization (NPU): Similar to BV, NPU assesses the percentage of dietary protein that is retained in the body but also accounts for digestibility. It is calculated from nitrogen ingested rather than absorbed.
Practical Steps to Evaluate Protein Sources
Here is a step-by-step approach to apply these concepts when evaluating a protein source for your diet.
- Check for Completeness: For single-source plant proteins, check if it is a complete protein like soy, quinoa, or chia seeds. Otherwise, plan to combine it with other plant proteins over the day, such as combining legumes and grains. Animal products are almost always complete proteins.
- Use the 'Protein Per Calorie' Trick: For a quick assessment, multiply the grams of protein per serving by 10. If the result is equal to or higher than the calories per serving, it's a very good protein source relative to its energy density.
- Consider DIAAS if Possible: For a more precise measure, look for foods with a high DIAAS score. Since DIAAS data isn't always on product labels, you can refer to published charts or scientific studies. Foods like whey protein concentrate, whole milk powder, and eggs have excellent DIAAS scores, while many plant proteins score lower as a single source.
- Prioritize Digestibility: Even if a protein has a great amino acid profile, poor digestibility reduces its value. Animal proteins are generally highly digestible, while some plant-based sources have lower digestibility, which modern DIAAS accounts for. This is why processing methods, like heating or fermenting, can improve the quality of plant proteins.
- Look Beyond the Numbers: Don't forget that a complete, nutrient-dense diet is about more than just protein quality scores. The overall mix of vitamins, minerals, fiber, and phytonutrients in whole foods is crucial for health. A varied diet of both plant and animal foods is an effective way to ensure a complete intake of all essential amino acids and micronutrients.
Comparison of Protein Quality Metrics
| Metric | Primary Measurement | Key Feature | Advantage | Limitation | Example Scores |
|---|---|---|---|---|---|
| DIAAS | Ileal Amino Acid Digestibility | Accounts for digestibility of individual amino acids at the end of the small intestine. | More accurate and can score over 100% to rank high-quality proteins. | Data is less common on product labels and requires specific testing. | Whey (>100%), Milk (>100%), Pea (≈82%) |
| PDCAAS | Fecal Protein Digestibility | Compares amino acid profile to a reference standard. | Widely used and understood historically. | Score is capped at 1.0, and fecal data is less accurate. | Casein (1.0), Soy (0.98), Chickpeas (0.74) |
| Biological Value (BV) | Nitrogen Retention | Measures nitrogen absorption and retention. | Gives an indication of how much protein is used by the body. | Does not account for digestion efficiency and is not a modern standard. | Egg (100+), Beef (80), Soy (74) |
Conclusion: Making Smarter Protein Choices
Calculating a good protein source is a nuanced task that requires looking beyond simple protein counts to consider the complete amino acid profile and digestibility. While modern metrics like DIAAS offer the most accurate assessment, practical strategies such as combining incomplete proteins and prioritizing nutrient-dense whole foods are effective for everyone. Whether you're an omnivore or plant-based, a varied diet and an understanding of protein quality metrics will ensure you get the most out of every gram of protein you consume.
