What Exactly Defines Low-Quality Protein?
Protein is a crucial macronutrient made up of smaller building blocks called amino acids. The human body can produce 11 of these, but nine are considered “essential” and must be obtained through our diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The quality of a protein is determined by its ability to provide these essential amino acids (EAAs) and how well the body can digest and absorb them.
Proteins that contain all nine EAAs in sufficient quantities are called “complete” or high-quality proteins. Conversely, low-quality proteins—also known as incomplete proteins—are those that are low in or missing one or more of these essential amino acids. This doesn't mean they are worthless, but rather that they need to be consumed alongside other complementary protein sources to provide a complete amino acid profile for the body.
The Digestible Indispensable Amino Acid Score (DIAAS) is a modern metric used to measure protein quality, taking into account the digestibility of each individual essential amino acid. This provides a more accurate picture than older methods and helps explain why some proteins, even if they contain all EAAs, might be considered lower quality if their absorption is poor.
Examples of Low-Quality Protein Sources
While high-quality proteins are predominantly found in animal products like meat, eggs, and dairy, most plant-based protein sources are naturally incomplete. However, this is easily addressed with a varied and balanced diet. Some common examples include:
- Legumes: This category includes foods such as beans (pinto, black, and kidney), peas, and lentils. Legumes are typically low in the essential amino acid methionine.
- Grains: Many whole grains, like wheat, rice, and oats, are low in the essential amino acid lysine. This is why the classic combination of rice and beans is so effective for creating a complete protein.
- Nuts and Seeds: Most nuts and seeds are incomplete proteins. For example, nuts often lack or are low in lysine, and seeds can be low in lysine or methionine.
- Vegetables: While vegetables provide many nutrients, they offer smaller amounts of protein, and the protein they do contain is often incomplete. For instance, spinach is missing several EAAs.
- Gelatin and Collagen: This is a notable example of an incomplete protein derived from animals. Gelatin and its parent protein, collagen, are deficient in the essential amino acid tryptophan. Therefore, relying on collagen supplements as a primary protein source is not recommended for muscle building.
Complementary Proteins: Making 'Incomplete' Complete
For those following a plant-based diet, the concept of complementary proteins is key. By combining different incomplete protein sources, you can ensure your body receives all the essential amino acids it needs. The best part is that you don't have to combine them in the same meal, as your body can pool amino acids throughout the day.
Here are some classic complementary protein pairings:
- Rice and Beans: This classic combination provides all nine essential amino acids. The methionine missing in beans is supplied by the rice, and the lysine that rice lacks is found in the beans.
- Hummus and Pita Bread: Hummus (from chickpeas) and pita bread (from wheat) is another excellent example. The chickpeas provide the missing amino acids in the wheat, and vice versa.
- Peanut Butter on Whole Wheat Toast: Similar to hummus and pita, the legumes (peanuts) and grains (wheat) complement each other perfectly.
- Corn and Legumes: In many cultures, corn-based dishes are served with beans or other legumes. The tryptophan and lysine found in legumes complement the amino acids in corn.
Comparison of High-Quality vs. Low-Quality Protein
| Feature | High-Quality Protein | Low-Quality Protein |
|---|---|---|
| Essential Amino Acids | Contains all nine EAAs in sufficient amounts. | Lacks or is low in one or more EAAs. |
| Primary Sources | Animal products (meat, eggs, dairy), soy, quinoa. | Most plant-based foods (grains, legumes, nuts, seeds), gelatin. |
| Bioavailability | Generally high, meaning the body easily digests and absorbs the amino acids. | Can be lower depending on the source; plant foods often contain anti-nutrients that can affect digestibility. |
| Completeness Strategy | Inherently complete, no special combinations needed. | Requires combining different sources (protein complementation). |
| Example | Chicken, milk, eggs, soybeans. | Rice, lentils, wheat, gelatin. |
| Added Benefits | Often high in specific micronutrients like B12, iron, and zinc (animal sources). | Rich in fiber, vitamins, minerals, and antioxidants. |
Conclusion
The concept of low-quality protein is based on a food's amino acid profile, and while most plant-based foods fall into this category, they are far from being nutritionally inferior. By understanding the principle of protein complementation, individuals can easily create complete protein meals by combining different plant-based sources throughout the day. A varied diet, whether omnivorous, vegetarian, or vegan, is the most important factor in ensuring you receive a balanced intake of all essential amino acids and the many other benefits these foods provide. For those concerned about meeting specific nutritional needs, especially with a strictly plant-based diet or in situations like aging, focusing on high-quality protein sources or strategically combining incomplete ones becomes even more important. A balanced plate that includes a variety of protein sources is the surest way to meet your body's demands and maintain optimal health.
Outbound Link
For a deeper dive into the science of protein, read this resource from Harvard T.H. Chan School of Public Health: The Nutrition Source: Protein.
Keypoints
Defining Low-Quality Protein: Low-quality proteins, or incomplete proteins, lack one or more of the nine essential amino acids the body cannot produce itself. Primary Sources: Most plant-based foods like grains, legumes, nuts, and seeds are examples of incomplete proteins. Animal-Based Exception: Gelatin and collagen are animal-derived incomplete proteins because they are missing the essential amino acid tryptophan. Complementation is Key: A balanced intake of all essential amino acids can be achieved by combining different incomplete proteins, such as rice and beans. Timing is Flexible: You don't need to consume complementary proteins in the same meal, as your body pools amino acids throughout the day. Beyond Protein Content: Incomplete protein sources, particularly plant-based ones, are also rich in other beneficial nutrients like fiber, vitamins, and minerals.
FAQs
Question: Are plant-based proteins bad because they are often low quality? Answer: No, plant-based proteins are not bad. They offer numerous nutritional benefits, including fiber and antioxidants. Their lower quality simply means they lack a complete amino acid profile, which can be easily managed by eating a variety of plant foods throughout the day.
Question: Is quinoa a low-quality protein? Answer: No, quinoa is one of the exceptions to the rule and is considered a complete, high-quality plant-based protein because it contains all nine essential amino acids.
Question: Do vegetarians or vegans need to worry about protein intake? Answer: With careful planning and a varied diet, vegetarians and vegans can meet their protein needs by consuming a wide range of complementary proteins over the course of a day. Combining legumes with grains is a simple strategy to achieve this.
Question: Can I get enough protein from a low-quality source? Answer: You can get enough total protein from low-quality sources, but relying on a single one will result in a deficiency of one or more essential amino acids. Combining different sources is necessary to ensure a complete amino acid intake for optimal health.
Question: What makes gelatin a poor source of protein for muscle growth? Answer: Gelatin is derived from collagen and is missing the essential amino acid tryptophan, which is crucial for muscle protein synthesis. While it contributes to overall protein intake, it should not be your primary source for muscle-building goals.
Question: What are some easy examples of protein complementation meals? Answer: Easy meal examples include beans and rice, peanut butter on whole wheat bread, hummus with pita bread, or a salad with chickpeas and sunflower seeds.
Question: Is soy protein high or low quality? Answer: Unlike most other plant-based proteins, soy (including tofu, tempeh, and edamame) is a complete protein, meaning it contains all nine essential amino acids in sufficient amounts.
Citations
- : Understanding Dietary Protein Quality: Digestible Indispensable Amino Acid Score and Beyond by Gilani, G. S. and Chibbar, R. ScienceDirect.com. July 15, 2025.
- : The Difference Between Incomplete and Complete Proteins, Explained by McDaniel, S. Onepeloton.com. April 18, 2024.
- : Complete versus Incomplete Protein by National Nutrition Council. Nnc.gov.ph. 2025.
- : Breaking Down Dietary Protein: What is it? by National Kidney Foundation. Kidney.org. September 22, 2025.
- : Complete vs. Incomplete Proteins and Examples by Cleveland Clinic. Health.clevelandclinic.org. December 06, 2022.
- : Low biological and high biological value protein by National Nutrition Council. Nnc.gov.ph. 2025.
- : Gelatin. Wikipedia. En.wikipedia.org. 2025.
- : Does Collagen Count as Protein Intake? by Naked Nutrition. Nakednutrition.com. March 05, 2024.
- : The Complete Protein Foods List And Facts by Piedmont Healthcare. Piedmont.org. 2025.
- : Vegan Complete Proteins: 13 Plant-Based Options by Healthline. Healthline.com. May 28, 2024.
- : Consideration of the role of protein quality in determining adequacy of protein intake in the United States by Wolfe, R. R. Frontiers in Nutrition. November 13, 2024.
- : What's the Difference Between Complete & Incomplete Proteins by Kroger. Kroger.com. February 13, 2024.
