The Key Difference Between First Class and Second Class Proteins

December 13, 2025 •

3 min read

Over 99% of all proteins are made from just 20 amino acids, yet they are categorized based on their nutritional quality. The main difference between first class and second class proteins lies in their amino acid composition and biological value, which is crucial for human health.

Quick Summary

First class proteins contain all essential amino acids in sufficient amounts, while second class proteins lack one or more. Sources include animal products for first class and most plant-based foods for second class, but smart food combining can create complete proteins.

Key Points

Amino Acid Profile is Key: The core difference is that first class (complete) proteins contain all essential amino acids, while second class (incomplete) proteins are missing one or more.
Animal vs. Plant Sources: Most first class proteins come from animal sources like meat and eggs, whereas most second class proteins are plant-based, such as legumes and grains.
Biological Value Varies: First class proteins have a high biological value, meaning the body uses them efficiently; second class proteins have a lower value on their own.
Combining for Completeness: You can create a complete protein from second class sources by combining them, a process known as protein complementing (e.g., rice and beans).
Plant-Based Exceptions: Certain plant foods like soy and quinoa are notable exceptions, as they are naturally complete proteins.
Balanced Diet is Crucial: Regardless of the source, a diverse and balanced diet is the best way to ensure all essential amino acids are consumed for proper bodily functions.

Understanding Protein Quality

Proteins are fundamental to life, serving as the building blocks for muscles, bones, skin, and more. The quality of a protein source is primarily determined by its amino acid profile, specifically the presence of all nine essential amino acids that our body cannot produce on its own. It's on this basis that proteins are classified as 'first class' or 'second class'. This classification is less about a protein being good or bad and more about its completeness relative to human nutritional needs.

What are First Class Proteins?

First class proteins, also known as complete proteins, provide all nine essential amino acids in the right proportions. These proteins have a high biological value, meaning they are easily and efficiently used by the body. They are typically sourced from animals, but some plant-based foods can also be considered complete protein sources. For most omnivores, these proteins are readily available in their diet.

What are Second Class Proteins?

Second class proteins, or incomplete proteins, lack one or more of the essential amino acids. While they are still important and beneficial, relying solely on a single source of incomplete protein can lead to a deficiency over time if not properly managed. The missing amino acid is called the limiting amino acid. Most plant-based foods, such as grains, legumes, and nuts, fall into this category. However, this does not mean plant-based diets are inferior; it simply requires more mindful food combinations.

Combining Second Class Proteins for a Complete Amino Acid Profile

For those on vegetarian or vegan diets, or simply seeking to diversify their protein intake, combining different second class protein sources is a smart strategy known as protein complementing. By eating different incomplete proteins throughout the day, the body can assemble a complete set of essential amino acids. For instance, combining legumes (which are often low in methionine) with grains (often low in lysine) creates a complete protein. A classic example is rice and beans.

Some exceptional plant-based sources, like soybeans (including tofu, tempeh, and edamame) and quinoa, are naturally complete proteins and stand as notable exceptions to the general rule. Another is buckwheat. Incorporating these into a diet is an excellent way to ensure complete protein intake without relying on animal products.

The Difference in Detail: First Class vs. Second Class Proteins


Feature	First Class Proteins	Second Class Proteins
Amino Acid Profile	Contain all nine essential amino acids.	Lack one or more essential amino acids (limiting amino acid).
Sources	Primarily animal-based (meat, eggs, dairy, fish), plus some plants (soy, quinoa).	Primarily plant-based (legumes, nuts, seeds, grains, vegetables).
Biological Value	High biological value; easily absorbed and utilized.	Lower biological value; less efficiently utilized on their own.
Completeness	Complete and sufficient on their own.	Must be combined with other sources for completeness.
Dietary Importance	Staple for many diets, high nutritional efficiency.	Important for a balanced diet, requires conscious pairing for vegetarians and vegans.

Examples of First Class Protein Sources

Animal-Based:
- Meat (chicken, beef, lamb)
- Fish and seafood (salmon, tuna)
- Eggs
- Dairy products (milk, cheese, yogurt)
Plant-Based:
- Soybeans and soy products (tofu, tempeh)
- Quinoa
- Buckwheat

Examples of Second Class Protein Sources

Legumes:
- Beans (black, kidney, pinto)
- Lentils
- Peas
Grains:
- Rice
- Wheat
- Corn
Nuts and Seeds:
- Almonds
- Sunflower seeds
- Peanuts

Conclusion

Understanding the difference between first class and second class proteins is fundamental to building a healthy, balanced diet. It’s not about one being inherently better than the other, but about knowing how to combine foods to ensure your body gets the complete set of essential amino acids it needs. For those following a plant-based diet, protein complementing ensures all essential amino acids are consumed. For omnivores, the distinction helps appreciate the high biological value of certain animal products and some plant-based alternatives. Regardless of your dietary choices, a balanced approach combining various sources is key to meeting your nutritional requirements. For more information on dietary proteins and nutritional guidelines, visit the National Institutes of Health website for authoritative resources.

Frequently Asked Questions

Essential amino acids are nine specific amino acids that the human body cannot produce on its own and must be obtained through diet. They are crucial for protein synthesis, tissue repair, and nutrient absorption.

A well-planned vegan diet is not lacking in protein. While many plant-based proteins are 'second class,' combining various sources throughout the day, such as legumes with grains, ensures the body receives all essential amino acids.

No, it is not necessary to eat complementary proteins in the same meal. The body maintains a 'pool' of amino acids from various foods consumed over a day, so eating different protein sources at different times is sufficient.

Biological value is a measure of how efficiently the body absorbs and utilizes a protein. First class proteins typically have a higher biological value because they provide all essential amino acids in one source.

Generally, yes. Most animal protein sources like meat, eggs, and dairy are considered first class because they contain all essential amino acids in sufficient quantities.

Yes, you can build muscle with only second class proteins by ensuring you consume a variety of them to get all essential amino acids. The key is total protein intake and the amino acid profile over the course of a day.

Simple examples include peanut butter on whole wheat bread, hummus with pita, or a classic combination of rice and beans. The combination provides all essential amino acids that might be missing from the individual components.