Understanding Second Class Protein: An Example

October 24, 2025 •

3 min read

Over 70% of the world's population relies on plant-based diets, which are a primary source of second class protein. A prime example is a lentil, which is considered a second class protein because it lacks one or more essential amino acids needed for optimal human health.

Quick Summary

Second class, or incomplete, proteins are typically plant-based foods such as legumes and grains that lack one or more essential amino acids. Combining different plant-based foods can create a complete protein profile.

Key Points

Definition: A second class protein is incomplete because it is deficient in one or more of the nine essential amino acids.
Example: Lentils are a classic example of a second class protein, lacking sufficient amounts of methionine.
Sources: Most second class proteins come from plant-based foods, including grains, legumes, and nuts.
Complementation: Combining two or more second class protein sources (e.g., lentils and rice) creates a complete amino acid profile.
Dietary Importance: Understanding second class proteins is crucial for those on vegetarian or vegan diets to ensure all essential amino acids are consumed.
Biological Value: Second class proteins have a lower biological value compared to complete proteins, meaning the body utilizes them less efficiently on their own.

What is a Second Class Protein?

Proteins are fundamental to life, forming the structural basis of our cells, tissues, and organs, as well as acting as enzymes and hormones. They are composed of smaller units called amino acids. There are 20 different types of amino acids, nine of which are considered 'essential' because the human body cannot produce them and must obtain them from the diet. Proteins are classified based on their content of these essential amino acids.

Second class proteins, also known as incomplete proteins, are those that are deficient in one or more of these nine essential amino acids. This deficiency means that eating a single source of second class protein may not provide all the building blocks the body needs for new protein synthesis. Most second class proteins are derived from plant sources.

Example: Lentils as a Second Class Protein

Lentils are a perfect example of a second class protein. They are a nutritional powerhouse, rich in fiber, vitamins, and minerals, and provide a good amount of protein. However, lentils are notably low in the essential amino acid methionine. Because of this specific deficiency, they cannot be considered a 'complete' protein source on their own.

Combining Second Class Proteins: The Concept of Protein Complementation

Despite being incomplete individually, second class proteins can be combined to form a complete protein meal. This practice, known as protein complementation, is a common and effective strategy in vegetarian and vegan diets. The missing amino acid in one food is supplied by another. For example, while lentils are low in methionine, many grains, such as rice, are a good source of it. Conversely, grains tend to be low in lysine, an amino acid that lentils provide in abundance.

Classic Complementary Pair: The combination of rice and lentils, a dish found in many cultures (like the Indian dish khichdi), provides all the necessary essential amino acids.
Other Examples of Complementation:
- Hummus (chickpeas) with whole-wheat pita bread
- Peanut butter on whole-grain bread
- Beans and corn tortillas

This nutritional synergy highlights that it is not a single meal that determines protein adequacy, but rather the variety of foods consumed over the course of a day.

Why Understanding Protein Class is Important

For individuals following plant-based diets, knowing which amino acids are limiting in their primary protein sources is crucial. It allows for the intentional pairing of foods to ensure all essential amino acids are consumed regularly. This strategic approach prevents nutritional deficiencies and supports muscle repair, immune function, and other vital bodily processes.

First Class vs. Second Class Protein: A Comparison

To fully understand the significance of a second class protein, it is helpful to compare it to a first class protein.


Feature	First Class Protein (Complete)	Second Class Protein (Incomplete)
Essential Amino Acids	Contains all nine essential amino acids in adequate amounts.	Lacks or is deficient in one or more essential amino acids.
Primary Sources	Typically derived from animal sources, such as meat, fish, eggs, and dairy. Some plant sources like soy and quinoa are complete.	Predominantly from plant sources, including legumes, grains, nuts, and seeds.
Biological Value (BV)	Generally has a high biological value, indicating efficient utilization by the body.	Characterized by a lower biological value, as it is less efficiently used by the body on its own.
Complementation	Not required, as the food provides a complete amino acid profile.	Can be combined with other incomplete protein sources to achieve a complete amino acid profile.
Dietary Importance	Provides a straightforward way to consume all essential amino acids.	Requires conscious food pairing for those on a vegetarian or vegan diet to ensure complete intake.

Conclusion

The term second class protein is not an indication of a food's inferiority, but rather a classification based on its amino acid profile. A lentil is a prime example of a second class protein because it is missing sufficient amounts of the essential amino acid methionine. However, by practicing protein complementation—pairing lentils with grains like rice—the nutritional gaps are filled. For individuals on plant-based diets, combining different protein sources is a simple and effective strategy for ensuring a complete intake of all essential amino acids, proving that a diverse diet can easily provide all the protein the body needs.

Frequently Asked Questions

No, not all plant proteins are second class. While the majority are, a few notable exceptions exist, such as soy and quinoa, which are considered complete proteins because they contain all nine essential amino acids.

Protein complementation is the practice of combining two or more incomplete protein sources to create a complete protein meal. For example, pairing legumes (low in methionine) with grains (high in methionine) provides a full amino acid spectrum.

Animal proteins, such as meat, eggs, and dairy, are considered first class because they naturally contain all nine essential amino acids in sufficient proportions. This makes them a reliable and complete source of protein.

No, you don't need to consume complementary proteins in the same meal. For a healthy individual, eating a variety of plant-based foods throughout the day will provide all the essential amino acids needed. The body can store and combine amino acids as needed.

Other examples of second class proteins include grains like wheat and rice (often low in lysine), nuts, and seeds. Combining these with other plant foods is key to a balanced diet.

Second class protein is not unhealthy. While less complete on their own, these foods are often packed with other essential nutrients like fiber and vitamins. A varied diet that includes complementary proteins ensures all nutritional needs are met.

Both first and second class proteins can support muscle building. However, first class proteins offer a complete amino acid profile in one source, which may be more efficient. Combining second class proteins achieves the same result, providing all the necessary building blocks for muscle repair and growth.