Understanding Protein Quality: What is poor quality protein?

October 14, 2025 •

3 min read

Protein quality is often overlooked in dietary discussions, despite its critical importance for health. Around 25% of older adults in the US do not consume the recommended dietary allowance of protein, highlighting a potential issue with both protein quantity and quality. So, what is poor quality protein and why does it matter for your diet?

Quick Summary

Poor quality protein is a protein source that lacks adequate levels of one or more essential amino acids necessary for human health. Its quality is determined by both its amino acid profile and digestibility. Properly combining diverse protein sources can overcome the limitations of low-quality protein.

Key Points

Incomplete Protein: Poor quality protein lacks one or more of the nine essential amino acids the body cannot produce.
Limiting Amino Acid: The scarcest essential amino acid in a protein source that limits the body's ability to synthesize new proteins.
Complementary Proteins: Combining different incomplete protein sources throughout the day (like rice and beans) to create a complete amino acid profile.
Digestibility Matters: Protein quality is also affected by how easily the body can digest and absorb its amino acids.
Balanced Diet is Key: For most people, a varied diet with multiple protein sources prevents issues associated with poor protein quality.
Health Impacts: Chronic reliance on unbalanced poor quality protein can lead to muscle wasting, weakened immunity, and fatigue.

Defining Poor Quality Protein

Protein is a crucial macronutrient involved in various bodily functions, including tissue repair and immune support. It is made up of amino acids, nine of which are essential and must come from our diet.

A protein's quality is primarily assessed by its essential amino acid (EAA) content and how well it is digested. Poor quality protein, or 'incomplete' protein, lacks sufficient levels of one or more EAAs. This missing amino acid, known as the limiting amino acid, restricts the body's ability to utilize the other amino acids effectively for protein synthesis.

The Role of Limiting Amino Acids

The limiting amino acid is the essential amino acid that is least abundant relative to the body's needs. The specific limiting amino acid varies among different plant protein sources.

Grains (wheat, rice): Often low in lysine.
Legumes (beans, lentils, peas): Typically low in methionine and tryptophan.
Nuts and Seeds: Can be limited in lysine and isoleucine.

A diet relying heavily on these individual sources without variety may not provide a complete set of essential amino acids.

Health Implications of Relying on Poor Quality Protein

While a varied diet usually prevents problems, a diet consistently lacking essential amino acids from poor quality proteins can lead to health issues:

Muscle Wasting: Difficulty in repairing and building muscle tissue.
Impaired Growth: Potential hindrance of growth, especially in children.
Weakened Immune System: Reduced production of immune components.
Fatigue: Lower energy levels due to impact on oxygen transport.
Edema: Fluid imbalance leading to swelling.

These issues are generally a result of overall dietary imbalance, not from consuming individual plant-based foods as part of a varied diet.

How to Overcome Poor Quality Protein: Complementary Proteins

Combining different incomplete protein sources throughout the day, known as consuming complementary proteins, is an effective way to obtain all essential amino acids. It is not necessary to combine these foods in a single meal.

Examples of complementary protein combinations:

Grains and Legumes: Rice and beans provide a complete protein profile.
Legumes and Nuts/Seeds: Hummus or nut butter on whole-grain toast are good examples.
Grains and Dairy: Oatmeal with milk or cheese on whole-grain bread.

Including animal protein sources with plant proteins can also improve the overall amino acid intake.

Comparison Table: High Quality vs. Poor Quality Protein


Feature	High Quality Protein (Complete)	Poor Quality Protein (Incomplete)
Essential Amino Acids	Contains all nine essential amino acids in adequate amounts.	Deficient in one or more essential amino acids.
Biological Value (BV)	Higher BV, indicating more efficient use by the body for protein synthesis.	Lower BV, meaning a smaller proportion of absorbed protein is retained.
Source Type	Primarily animal-based (meat, poultry, fish, eggs, dairy) and a few plant-based sources (soy, quinoa).	Primarily plant-based (nuts, seeds, legumes, grains) with some exceptions.
Example Sources	Egg, whey protein, chicken, beef, fish, milk, soy.	Beans, lentils, peanuts, wheat, corn, gelatin.
Dietary Strategy	Easier to meet all amino acid needs from a single source.	Requires combining different sources throughout the day to achieve a complete amino acid profile.
Digestibility	Generally highly digestible.	Digestibility can vary; some anti-nutritional factors in certain plant foods can inhibit digestion.

Modern Protein Quality Measurement

The Digestible Indispensable Amino Acid Score (DIAAS) is now considered a more accurate method for evaluating protein quality than the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS). DIAAS assesses the digestibility of individual amino acids, offering a more precise measure of a protein's nutritional value. Both systems highlight the importance of essential amino acid balance and digestibility for protein quality.

Conclusion

Understanding what is poor quality protein is key to a balanced diet. These are protein sources lacking sufficient amounts of one or more essential amino acids. While animal proteins and some plant proteins like soy are complete, many plant foods are incomplete. The solution isn't to avoid them, but to consume a variety of protein sources daily to ensure all essential amino acids are obtained by combining complementary proteins. A well-planned diet, including vegetarian and vegan approaches, can easily provide all necessary amino acids. Focusing on dietary diversity is more important than relying on single protein sources.

The Importance of Dietary Protein Quality in Mid- to High-Income Countries

Frequently Asked Questions

High-quality protein, also known as complete protein, contains all nine essential amino acids in adequate amounts. Poor quality protein, or incomplete protein, is deficient in one or more essential amino acids.

Yes, you can get all essential amino acids on a plant-based diet by eating a variety of different protein sources throughout the day. Combining complementary proteins, such as legumes with grains, ensures all amino acid needs are met.

Common examples of poor quality protein sources include most legumes (beans, lentils), nuts, seeds, grains (wheat, rice), and vegetables. Gelatin is an animal-based exception that is also incomplete.

Simply consuming poor quality protein isn't the issue, but if your diet is consistently unbalanced and lacks the full spectrum of essential amino acids, you may experience health problems like muscle loss, fatigue, or a weakened immune system.

No, it is not necessary to combine complementary proteins in the same meal. The body maintains a pool of amino acids, so consuming a variety of different protein sources throughout the day is sufficient to meet your needs.

Protein quality is measured using scoring systems like the Digestible Indispensable Amino Acid Score (DIAAS) or the older Protein Digestibility Corrected Amino Acid Score (PDCAAS). These methods evaluate a protein's amino acid composition and digestibility.

Not necessarily. Many sources of poor quality protein, like beans and whole grains, are rich in other important nutrients like fiber, vitamins, and minerals. They are only 'poor' in the context of their specific essential amino acid profile.