What is the difference between a complete vs incomplete protein?

January 2, 2026 •

2 min read

Over 70% of people on a vegetarian or vegan diet consume more protein than they need every day, according to a 2013 study in the Journal of the Academy of Nutrition and Dietetics. This dispels the common myth that plant-based eaters struggle to get enough protein, which is key to understanding the difference between a complete vs incomplete protein.

Quick Summary

The distinction between complete and incomplete proteins hinges on their essential amino acid content. Complete proteins provide all nine essential amino acids, while incomplete proteins lack one or more. However, a varied diet with complementary protein sources can ensure adequate intake of all essential amino acids.

Key Points

Essential Amino Acids: Nine amino acids must be consumed through your diet, as the body cannot produce them.
Complete Proteins: Contain all nine essential amino acids in adequate amounts, commonly found in animal products like meat, eggs, and dairy, as well as soy and quinoa.
Incomplete Proteins: Lack one or more essential amino acids and include most plant-based foods such as nuts, grains, and legumes.
Dietary Variety is Key: The body can combine amino acids from different foods throughout the day, making strict meal-by-meal protein pairing unnecessary.
Plant-Based Success: Vegetarians and vegans can easily get all essential amino acids by eating a variety of plant foods.
Higher Bioavailability: Animal proteins are generally more digestible and have higher bioavailability, but plant-based diets are healthy with proper planning.
Focus on Balance: A diverse diet, rather than focusing on just complete proteins, provides a wider range of beneficial nutrients.

The Building Blocks of Protein: Amino Acids

Proteins are composed of amino acids, and the body needs 20 different types. Eleven of these are non-essential, meaning the body can produce them, while nine are essential amino acids (EAAs) that must come from the diet.

The Nine Essential Amino Acids

The essential amino acids are:

Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine

What Defines a Complete Protein?

A complete protein contains all nine essential amino acids in sufficient quantities. These are primarily found in animal products, which have amino acid profiles well-suited for human needs.

Examples of Complete Proteins

Meat, poultry, fish, and seafood
Eggs and dairy
Soy (tofu, tempeh, edamame)
Quinoa and buckwheat

Some plant-based foods, like soy and quinoa, are notable exceptions as they are complete proteins.

Understanding Incomplete Proteins

Incomplete proteins lack one or more essential amino acids. While not complete on their own, they are often rich in other nutrients like fiber, vitamins, and minerals.

Examples of Incomplete Proteins

Grains (rice, wheat, oats)
Legumes (beans, lentils, peas)
Nuts and seeds
Most vegetables

Protein Complementation: The Outdated Myth

The idea that plant-based eaters must combine incomplete proteins in each meal (protein complementation) is now considered outdated. The body maintains an amino acid pool and can combine amino acids from various foods eaten throughout the day to form complete proteins, provided the diet is varied.

Complete vs. Incomplete Protein: A Comparative Table


Feature	Complete Proteins	Incomplete Proteins
Essential Amino Acids	Contains all nine.	Lacks one or more.
Typical Sources	Primarily animal-based.	Primarily plant-based.
Protein Quality	Generally higher due to EAA completeness and digestibility.	Can be lower quality alone but nutritious.
Bioavailability	Typically higher.	Can be lower.
Dietary Importance	Easy EAA source, good for muscle synthesis.	Requires dietary variety for all EAAs daily.

Why Protein Variety is Important for Everyone

Regardless of diet, consuming a variety of protein sources is beneficial. Animal-based proteins are efficient sources of EAAs, but plant-based foods offer additional nutrients like fiber and antioxidants. A diverse diet ensures comprehensive nutrient intake.

Conclusion

The key difference between a complete vs incomplete protein is the presence of all nine essential amino acids. While complete proteins have them all, incomplete proteins are missing at least one. However, a varied diet provides all necessary amino acids over the day, making strict protein combining unnecessary. Focusing on overall dietary diversity and nutrient intake is more important than the complete/incomplete distinction. Consult a healthcare provider for personalized advice.

For more in-depth nutritional information, the Harvard T.H. Chan School of Public Health is a reliable resource on dietary protein: Protein - The Nutrition Source.

Frequently Asked Questions

Yes, absolutely. By consuming a wide variety of plant-based protein sources like legumes, grains, nuts, and seeds throughout the day, a person can get all the essential amino acids needed to form a complete protein profile.

No, you do not need to combine proteins at every meal. The human body stores amino acids and can combine them as needed from various foods consumed over the course of a day.

A limiting amino acid is the essential amino acid present in the smallest quantity in a food, which limits the body's ability to synthesize new proteins from that source alone.

No, incomplete proteins are not less healthy. They are often packed with fiber, vitamins, and minerals that are crucial for overall health. The distinction is about their amino acid profile, not their overall nutritional value.

Soy is one of the few plant-based foods that is considered a complete protein, as it contains all nine essential amino acids in adequate proportions.

Classic examples of complementary pairings include rice and beans, peanut butter on whole-wheat bread, and hummus with pita bread.

Cooking can affect protein structure and, subsequently, its digestibility. However, for most proteins, cooking helps to denature the protein, making it easier for enzymes to break down and improve digestibility.