Protein is a fundamental macronutrient, but its effectiveness in the body depends on its quality, not just its quantity. The two primary factors determining protein quality are its amino acid composition and its digestibility. By understanding these factors and how to interpret food labels and scoring systems, you can make more informed dietary choices.
The Role of Amino Acids: Complete vs. Incomplete Proteins
All proteins are made up of building blocks called amino acids. Of the 20 amino acids the body uses, nine are considered "essential" because the body cannot produce them and they must be obtained from food. A protein source's amino acid profile is the most basic measure of its quality. This distinction gives rise to the concepts of complete and incomplete proteins.
- Complete Proteins: These are food sources that contain all nine essential amino acids in adequate amounts. Animal-based products like meat, poultry, fish, eggs, and dairy are classic examples. A few plant-based foods, such as soy, quinoa, and hemp seeds, also qualify as complete proteins.
- Incomplete Proteins: Most plant-based proteins, including legumes, nuts, seeds, and grains, are considered incomplete because they lack or are low in one or more of the essential amino acids. However, by combining different incomplete proteins throughout the day (e.g., rice and beans), you can still acquire all the essential amino acids needed. The old notion that this must be done in the same meal has been debunked.
The Importance of Protein Digestibility and Bioavailability
Even if a food contains a full amino acid profile, its value is diminished if the protein cannot be easily digested and absorbed by the body. This is where digestibility and bioavailability come in.
- Digestibility: This refers to how well the body can break down the protein into amino acids that can be absorbed. Factors like food processing and anti-nutritional compounds in some plants can impact digestibility.
- Bioavailability: This term describes how efficiently the absorbed protein can be utilized by the body. Proteins with high bioavailability, like whey and egg protein, are absorbed quickly and efficiently, making them valuable for muscle repair and recovery.
Protein Quality Scoring Systems: PDCAAS and DIAAS
Nutrition scientists use standardized scoring systems to quantify protein quality, moving beyond the simple complete/incomplete classification.
- Protein Digestibility-Corrected Amino Acid Score (PDCAAS): This older method, once the industry standard, compares a food's amino acid profile to a reference pattern and corrects for fecal digestibility. Scores range from 0 to 1.0, with 1.0 being the highest possible score. However, the 1.0 cap makes it impossible to differentiate between very high-quality proteins.
- Digestible Indispensable Amino Acid Score (DIAAS): Introduced by the FAO in 2013, DIAAS is the newer, more accurate standard. It measures amino acid digestibility at the end of the small intestine (ileal digestibility), providing a more precise bioavailability assessment. Unlike PDCAAS, DIAAS values are not capped, allowing for better ranking of superior protein sources. While DIAAS is gaining traction, PDCAAS is still referenced in many regulatory guidelines.
A Guide to Good Protein Sources
To choose sources of good protein, consider both the amino acid profile and the digestibility. Here is a comparison of common protein sources:
| Food Source | Protein Profile | Digestibility / Score | Key Considerations |
|---|---|---|---|
| Whey Protein | Complete | Excellent (High DIAAS/PDCAAS) | Fast-absorbing, ideal for post-workout. High concentration of BCAAs. |
| Eggs | Complete | Excellent (High DIAAS/PDCAAS) | A highly bioavailable source. Best to consume the whole egg for nutrients. |
| Beef / Red Meat | Complete | High (High DIAAS/PDCAAS) | Excellent source of iron. Digestibility can be slightly lower than whey/egg. |
| Soy Protein Isolate | Complete | High (High DIAAS/PDCAAS) | A high-quality plant option. Isolate form removes some anti-nutritional factors. |
| Quinoa | Complete | Good | One of the few complete plant proteins. Great source of fiber and minerals. |
| Lentils | Incomplete | Moderate | Low in sulfur-containing amino acids. Best combined with grains like rice. |
| Brown Rice | Incomplete | Moderate | Low in lysine. Complements other plant proteins like pea protein. |
| Pea Protein | Incomplete | Moderate | Low in methionine. Pairs well with brown rice protein. |
How to Check for Good Protein on Food Labels
For consumers, navigating the technical details of DIAAS or PDCAAS can be challenging, as these scores aren't always prominently displayed. However, you can still evaluate protein quality by following these steps:
- Check for "Complete Protein" Status: The label may state if the product contains a complete protein, which simplifies your assessment. Common complete plant proteins include soy, quinoa, and hemp.
- Look for Blended Plant Proteins: If the product is plant-based and doesn't explicitly state "complete protein," check the ingredients list for complementary protein sources. A blend of pea and rice protein, for example, is common because it creates a more complete amino acid profile.
- Use the % Daily Value (%DV): The %DV for protein, where included, is a good indicator of protein quality. The FDA requires the %DV to be based on the PDCAAS-corrected protein amount, not just the raw grams. A food with 20% DV provides 10 grams of usable high-quality protein (based on a 50g daily reference), so compare this to the total grams to gauge quality. If a food lists 20g of protein but only has 20% DV, its quality is lower than a food with 10g of protein and 20% DV.
- Perform the "Protein to Calorie" Trick: For a quick, though imperfect, quality check, a simple trick is to take the grams of protein per serving and add a zero (multiply by 10). If this number is greater than or equal to the total calories per serving, the food has a high protein density. This helps differentiate between a protein-rich food and a calorie-dense food that just happens to contain some protein, like certain processed snack bars.
Conclusion
Distinguishing good protein from lesser-quality protein is vital for maximizing your dietary benefits. It's not enough to simply look at the total grams of protein listed on a label. You must also consider its completeness (containing all essential amino acids) and its digestibility (how well the body can absorb it). Animal products like meat, eggs, and dairy are reliably high-quality and complete. Many plant-based foods are incomplete but can form a complete protein when combined effectively over the course of a day. Utilizing label clues like the %DV and comparing protein grams to calories can help, but understanding the underlying science of amino acids, digestibility, and scoring systems like DIAAS offers the most complete picture. A varied diet that includes a mix of high-quality animal proteins and thoughtfully combined plant proteins remains the most effective strategy for meeting your body's needs. For more scientific information on protein quality assessment, consider resources from reputable organizations like the FAO.
