The Building Blocks of Protein: Amino Acids
Proteins are large, complex molecules made up of smaller units called amino acids. The human body requires 20 different amino acids to function properly, nine of which are considered essential, meaning the body cannot produce them and they must be obtained from food. A protein's quality is largely determined by its essential amino acid content and how well it can be digested and absorbed.
Essential vs. Non-Essential Amino Acids
- Essential Amino Acids (EAAs): There are nine EAAs: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Foods that contain all nine EAAs in sufficient amounts are known as "complete proteins".
- Conditionally Essential Amino Acids: These are typically non-essential but may become essential under specific circumstances, such as illness or stress. Examples include cysteine and tyrosine.
- Non-Essential Amino Acids: The remaining amino acids can be synthesized by the body and do not need to be acquired directly from the diet.
Metrics for Measuring Protein Quality
For decades, protein quality has been assessed using various scoring systems. While some older methods exist, the industry has evolved towards more precise measurements.
- Protein Digestibility Corrected Amino Acid Score (PDCAAS): This method, once recommended by the FAO/WHO, ranks protein quality by comparing its amino acid profile to human needs and correcting for digestibility based on fecal measurements. A major limitation is that all scores above 1.0 are capped at 1.0, failing to distinguish between exceptionally high-quality proteins.
- Digestible Indispensable Amino Acid Score (DIAAS): Introduced in 2013, DIAAS is the new standard proposed to replace PDCAAS. It offers a more accurate measure by focusing on ileal digestibility—how well amino acids are absorbed at the end of the small intestine—rather than fecal digestibility. DIAAS scores are not capped, allowing for better comparative ranking.
High-Quality Protein Sources
High-quality protein sources provide a complete profile of essential amino acids and are easily digestible. While animal proteins are typically complete, some plant-based sources also qualify, and many can be combined to form a complete profile.
Animal-Based Complete Proteins
- Eggs: Often considered the gold standard with a perfect DIAAS score.
- Dairy (Whey, Casein, Milk): Milk-derived proteins are highly digestible and rich in essential amino acids.
- Meat (Beef, Poultry, Fish): Lean meats are excellent sources of complete protein.
Plant-Based Complete Proteins
- Soy (Tofu, Edamame): Soy is one of the few plant-based proteins that is naturally complete.
- Quinoa: A grain that contains all nine essential amino acids.
- Combining Incomplete Proteins: Eating a variety of plant proteins, such as combining legumes (e.g., beans) and grains (e.g., rice) throughout the day, provides a complete amino acid profile.
Comparison of Protein Quality Metrics: PDCAAS vs. DIAAS
| Feature | PDCAAS (Protein Digestibility Corrected Amino Acid Score) | DIAAS (Digestible Indispensable Amino Acid Score) |
|---|---|---|
| Digestibility Measurement | Corrected for fecal digestibility of protein. | Corrected for ileal digestibility of individual amino acids, providing a more accurate measure of absorption. |
| Score Truncation | Scores capped at 1.0, limiting the ability to compare proteins that exceed this threshold. | Scores are not truncated, allowing for a better quantitative ranking of all dietary proteins. |
| Reference Pattern | Based on amino acid requirements for 2-5 year-old children, considered the most demanding group. | Provides reference values for different age groups, offering a more up-to-date and specific measure. |
| Limitation Identification | Based on the most limiting essential amino acid in a protein. | More detailed, as it accounts for the individual digestibility of each indispensable amino acid. |
How to Select a Quality Protein Supplement
When choosing a protein supplement, assessing its quality is vital. A high-quality protein powder can aid muscle growth, recovery, and overall nutrient intake.
- Check the Protein Source: Opt for complete proteins like whey, casein, egg, or blends of plant-based proteins (e.g., pea and rice). Whey is fast-digesting, while casein is slower, offering different benefits.
- Look for Third-Party Certifications: Since supplements are not strictly regulated, certifications like NSF Certified for Sport or Informed Choice ensure the product has been tested for banned substances and contains the ingredients listed on the label.
- Read the Ingredient List: Prioritize products with minimal additives, fillers, and artificial sweeteners. A shorter ingredient list is often a sign of a cleaner supplement. Some people prefer natural sweeteners like stevia or monk fruit.
- Consider Your Dietary Needs: If you are lactose intolerant, vegan, or vegetarian, plant-based protein blends or isolate options may be more suitable.
- Evaluate Your Fitness Goals: Whey is excellent for post-workout recovery due to its fast absorption, while casein provides a sustained release of amino acids, making it good for overnight recovery or satiety.
Conclusion
Understanding what makes good quality protein is about more than just the amount of protein in a food. It depends on two main factors: the amino acid profile and the protein's digestibility. High-quality proteins, whether from animal or plant sources, provide all nine essential amino acids and are easily absorbed and utilized by the body. Using modern metrics like DIAAS can offer a more precise comparison of protein sources. Ultimately, incorporating a variety of high-quality proteins into a balanced diet is the most effective way to meet nutritional needs and support optimal health.
Protein Quality Key Takeaways
- Amino Acid Profile: High-quality protein contains all nine essential amino acids, which the body cannot produce on its own.
- Complete vs. Incomplete: Complete proteins (e.g., eggs, meat, dairy, soy) have all essential amino acids, while incomplete ones (most plant proteins) can be combined to form a complete profile.
- Digestibility Matters: How well your body can break down and absorb protein is critical for its overall quality and bioavailability.
- DIAAS is the New Standard: The Digestible Indispensable Amino Acid Score is a more accurate metric than the older PDCAAS, as it considers ileal digestibility and provides uncapped scores.
- Third-Party Certified Supplements: When choosing a protein powder, look for certifications to ensure product quality, purity, and that it contains what is listed on the label.
FAQs
Q: Are plant proteins lower quality than animal proteins? A: Not necessarily. While most individual plant proteins are incomplete, a varied diet that includes different plant sources throughout the day, such as legumes and grains, can easily provide all the essential amino acids. Some plant proteins like soy and quinoa are naturally complete.
Q: What is the difference between whey and casein protein? A: Both are high-quality, complete proteins from milk. The main difference is digestion speed; whey is digested and absorbed quickly, making it ideal for post-workout. Casein digests slowly, providing a sustained release of amino acids.
Q: Why is DIAAS considered better than PDCAAS? A: DIAAS provides a more accurate assessment because it measures digestibility at the end of the small intestine (ileal digestibility), where absorption primarily occurs. It also provides an uncapped score, allowing for a clearer comparison between different high-quality proteins.
Q: Can processing affect protein quality? A: Yes, food processing like heating can sometimes affect the availability of amino acids, especially lysine. DIAAS calculations are better equipped to account for these effects than the older PDCAAS system.
Q: Do I need a protein supplement? A: Most people can meet their protein needs through a balanced diet of whole foods. Protein supplements are convenient for athletes or those with high protein goals but are not strictly necessary.
Q: What does a limiting amino acid mean? A: A limiting amino acid is the essential amino acid present in the smallest quantity in a protein relative to the body's needs. It determines the protein's overall quality, as the body can only utilize the other amino acids up to the level of the most limiting one.
Q: How does digestibility affect protein utilization? A: A protein with higher digestibility allows for greater amino acid absorption, meaning more of the consumed protein can be used for essential bodily functions like tissue repair and muscle synthesis. Animal proteins are generally more digestible than plant proteins.
