The difference between protein quantity and quality
Understanding the distinction between the amount of protein and its quality is fundamental to nutrition science. Protein quality refers to how well a food's protein can provide the body with the amino acids needed for various metabolic processes, including tissue repair and muscle growth. In contrast, the amount of protein simply refers to the total weight or mass of protein contained within a food item.
A food might be high in total protein but low in certain essential amino acids, making it an 'incomplete' protein source. For example, many plant proteins contain significant amounts of protein but are low in one or more essential amino acids, known as limiting amino acids. Animal proteins, such as those found in meat, eggs, and dairy, typically contain all essential amino acids in sufficient proportions, making them 'complete' protein sources. To compensate for the low quality of plant proteins, combining different plant sources with complementary amino acid profiles can create a complete protein meal.
Key measures of protein quality
Several scientific methods are used to determine protein quality, focusing on amino acid composition and digestibility. The shift in scientific consensus has led to the adoption of more accurate measures over time.
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the industry standard for many years, adopted by the FDA in 1993. It evaluates protein quality by correcting a protein's amino acid score for its fecal digestibility. However, this method has a significant limitation: it caps the maximum possible score at 1.00. This means that proteins of extremely high quality, such as whey, are not differentiated from proteins that barely meet the requirements. Furthermore, using fecal digestibility overestimates the true absorption of amino acids, especially in plant-based proteins.
Digestible Indispensable Amino Acid Score (DIAAS)
Recognizing the limitations of PDCAAS, the FAO proposed the Digestible Indispensable Amino Acid Score (DIAAS) in 2013. DIAAS is a superior method because it addresses several of PDCAAS's shortcomings:
- It uses true ileal digestibility, which measures amino acid absorption at the end of the small intestine, providing a more accurate reflection of what the body actually absorbs.
- DIAAS does not cap scores at 1.00, allowing for a more accurate ranking and differentiation of higher-quality protein sources.
- It measures the digestibility of individual amino acids, not just overall protein, which offers a more precise assessment of a protein's true value.
Biological Value (BV)
Biological Value (BV) is an older measure that assesses the proportion of absorbed protein that is incorporated into the body's proteins. It primarily focuses on how efficiently the body utilizes the protein that has been absorbed. BV is determined by measuring nitrogen intake versus nitrogen excretion and is expressed as a percentage. A food with a high BV is a good source of essential amino acids. However, BV doesn't account for how readily the protein is digested and absorbed in the first place, which is a major drawback.
Net Protein Utilization (NPU)
Net Protein Utilization (NPU) is another traditional method that measures the ratio of amino acids converted to protein in the body relative to the total amino acids supplied in the diet. NPU is considered a better measure than BV because it includes a factor for digestibility, as NPU = Digestibility × BV. However, like BV, NPU is often determined in animal studies and may not perfectly translate to human needs.
Protein Efficiency Ratio (PER)
The Protein Efficiency Ratio (PER) is an older method that determines protein quality by measuring weight gain in rats in relation to their protein intake. It is a simple method but has been heavily criticized because it assumes that all protein is used for growth and does not reflect the needs of adult humans for maintenance and repair. The FDA in the US now uses PDCAAS for food labeling, but PER is still used in some regulations.
Comparison of protein quality measures
| Feature | PDCAAS | DIAAS | Biological Value (BV) | Net Protein Utilization (NPU) | Protein Efficiency Ratio (PER) |
|---|---|---|---|---|---|
| Mechanism | Amino acid score corrected for fecal digestibility | Amino acid score corrected for ileal digestibility | Nitrogen retention relative to nitrogen absorbed | Nitrogen retention relative to nitrogen ingested | Weight gain per gram of protein |
| Accuracy | Good, but has a capped score and less precise digestibility correction. | Superior; uses ileal digestibility and no capped score. | Measures absorbed protein utilization, but ignores digestibility. | Includes digestibility, making it more comprehensive than BV. | Measures growth in rats, less relevant for adult human needs. |
| Digestibility | Uses fecal digestibility. | Uses true ileal digestibility. | Excludes digestibility. | Measures overall dietary protein retention. | Reflects digestibility implicitly through weight gain. |
| Capped Score | Yes, capped at 1.00. | No capped score. | No, can exceed 100% relative scale. | Not capped. | Not capped. |
| Current Use | Widely used, but being superseded by DIAAS. | The new gold standard for regulatory purposes. | Largely for historical context and specific research. | Largely for historical context and specific research. | Still used in some niche applications, notably in Canada. |
Why the amount is not a measure of quality
The total amount of protein in a food is simply a quantitative figure, telling you nothing about its nutritional completeness or bioavailability. For instance, you could have a supplement with 50 grams of protein, but if that protein is poorly digested or lacks essential amino acids, your body won't be able to effectively use it for synthesis. The true value lies in the protein's amino acid profile, particularly the balance of essential amino acids relative to human requirements, and its digestibility. This is why scoring systems like DIAAS and PDCAAS were developed—to provide a qualitative measure that goes beyond a simple number on a nutrition label.
For most people consuming a varied diet, combining different food sources with complementary protein profiles (e.g., beans and rice) can provide a complete range of essential amino acids. For those relying on fewer sources or single-protein supplements, understanding the quality of the protein becomes more crucial. For example, whey protein consistently scores high in quality tests due to its rich amino acid profile and high digestibility, while some plant-based sources may need to be consumed in larger quantities to meet the same nutritional needs.
Conclusion
In summary, the total amount of protein in a food is simply a measure of quantity, not quality. True protein quality is a more complex nutritional concept defined by a protein's amino acid composition, its digestibility, and how efficiently the body can use it. The scientific methods for measuring this, from the outdated PER to the modern and more accurate DIAAS, reflect this complexity. Consumers aiming for optimal nutrition should consider the quality of their protein sources, not just the quantity, to ensure their body receives the full range of essential amino acids it needs to function properly. For those interested in deeper nutritional data, examining the DIAAS score can provide a more accurate assessment of a food's protein value.
For more detailed information on dietary protein requirements and evaluation, the FAO's publication provides a comprehensive overview(https://www.fao.org/ag/humannutrition/36254-04f7c223a54b679a61421f6e1f0e4b525.pdf).
Note: The concept of 'protein quality' is most relevant for those with specific dietary requirements or limited food choices, as a varied diet typically provides a balance of all necessary amino acids. However, understanding these measures helps inform a more nutritionally robust diet for everyone.
