What is Not Considered a Complete Protein Source?

November 29, 2025 •

3 min read

According to nutrition science, a complete protein is one that contains all nine essential amino acids. What is not considered a complete protein source, therefore, is any food that is deficient in or entirely lacks one or more of these crucial amino acids.

Quick Summary

Incomplete proteins are sources, typically plant-based like legumes, nuts, and grains, that lack one or more of the nine essential amino acids. While not nutritionally inferior, they require combination with other plant foods to form a complete amino acid profile throughout the day.

Key Points

Definition: An incomplete protein is a food that is missing or is very low in one or more of the nine essential amino acids.
Plant-Based Sources: Most plant-based foods, including legumes, grains, nuts, and seeds, are considered incomplete protein sources.
Combining Proteins: Combining different incomplete protein sources, like rice and beans, can create a complete amino acid profile.
Timing is Outdated: The idea that complementary proteins must be eaten at the same meal is an outdated myth; your body can pool amino acids from foods eaten throughout the day.
Nutritional Value: Incomplete proteins are not nutritionally inferior and often provide valuable fiber, vitamins, and minerals that complete proteins may lack.
Complete Plant Sources: Some plant-based foods, such as soy, quinoa, and buckwheat, are naturally complete proteins.
One Animal Exception: Collagen, an animal product, is an exception, as it is an incomplete protein lacking tryptophan.

Understanding the Building Blocks of Protein

Proteins are fundamental to virtually every function in the human body, from building and repairing tissues to producing enzymes and hormones. They are made up of smaller units called amino acids. There are 20 total amino acids, but only nine are considered "essential." These nine must be obtained through your diet because your body cannot synthesize them on its own.

A complete protein source contains all nine essential amino acids in sufficient quantities. In contrast, an incomplete protein source is lacking or low in one or more of these essential amino acids. This is the key distinction for understanding what is not considered a complete protein source.

Incomplete Protein Sources: The Primary Category

Most plant-based proteins fall into the category of incomplete protein sources. These foods provide protein, fiber, vitamins, and minerals but lack a full spectrum of essential amino acids in one serving.

Common examples of incomplete protein sources include:

Legumes (beans, lentils, peas)
Nuts (almonds, walnuts, peanuts)
Seeds (flax, chia, hemp, pumpkin)
Whole Grains (wheat, oats, rice)
Most Vegetables
Collagen (lacks tryptophan)

The Importance of Protein Complementation

For those following vegetarian or vegan diets, consuming a variety of plant foods ensures all essential amino acids are met. Combining different incomplete proteins throughout the day, rather than in a single meal, provides a complete amino acid profile. This is known as protein complementation.

Here are a few classic examples of complementary protein pairings:

Rice and beans
Hummus and pita bread
Peanut butter on whole-grain toast
Lentil soup with whole-grain bread

Comparison of Complete vs. Incomplete Protein Sources


Feature	Complete Protein Sources	Incomplete Protein Sources
Essential Amino Acids	Contains all nine essential amino acids.	Lacks or is low in one or more essential amino acids.
Primary Sources	Animal products, soy, quinoa, buckwheat.	Most plant foods.
Nutritional Composition	May be higher in fat; good source of B vitamins, iron.	High in fiber, carbs, vitamins, minerals; typically lower in fat.
Dietary Flexibility	Provides all essential amino acids in one source.	Requires consuming a variety of foods throughout the day.
Environmental Impact	Generally requires more resources.	More environmentally friendly.

Plant-Based Exceptions: The Complete Protein Club

Some plant-based foods are complete proteins and valuable for those on plant-based diets.

Soy products (edamame, tofu, tempeh)
Quinoa
Buckwheat
Chia and Hemp Seeds (often considered complete)
Spirulina

The Misconception of 'Lesser Quality' Protein

Incomplete proteins are not nutritionally inferior to complete proteins. Plant-based incomplete proteins are rich in fiber, antioxidants, and various vitamins and minerals. A varied diet ensures you obtain all essential amino acids needed for bodily functions.

Conclusion: Strategic Eating for Nutritional Adequacy

Understanding what is not considered a complete protein source, mainly most plant-based foods, helps in dietary planning. While incomplete proteins lack one or more essential amino acids, protein complementation throughout the day from a varied plant-based diet easily provides all necessary amino acids. The key is dietary variety, allowing individuals to enjoy the health benefits of a diverse, plant-rich diet.

Resources

For further information on nutrition and protein sources, consult the resources from the Harvard T.H. Chan School of Public Health: The Nutrition Source.

Frequently Asked Questions

An incomplete protein source is a food that does not contain all nine of the essential amino acids in sufficient quantities. Examples include most plant-based foods like grains, legumes, nuts, and seeds.

No, you do not need to combine different plant proteins at every meal. Your body can store amino acids and draw from a pool throughout the day. As long as you eat a varied diet, your body will get all the essential amino acids it needs.

If you fail to consume all essential amino acids over the course of the day, you could eventually experience health issues like muscle mass loss, fatigue, or a weakened immune system. A varied diet of plant foods, however, easily prevents this.

No, incomplete proteins are not less healthy. Many incomplete protein sources are high in other important nutrients like fiber and antioxidants. The key is to consume a variety of them to ensure all amino acids are obtained.

Classic examples of complementary protein combinations include rice and beans, peanut butter on whole-grain toast, hummus and pita bread, and lentils with a side of wheat bread.

A few plant-based foods are naturally complete proteins, including soy (tofu, tempeh), quinoa, buckwheat, and spirulina.

Yes, you can build muscle with a varied diet of incomplete proteins. While complete proteins may be more efficiently utilized for muscle synthesis, a diverse intake of plant-based foods will provide all the necessary building blocks for muscle repair and growth.