Why is Plant-Based Protein Not Complete?

November 13, 2025 •

4 min read

According to nutritional science, proteins are composed of 20 different amino acids, nine of which are considered essential because the human body cannot produce them on its own. This is the fundamental reason why is plant-based protein not complete, as most individual plant sources are deficient in one or more of these crucial building blocks.

Quick Summary

This guide explains why most plant proteins are considered incomplete due to missing essential amino acids. It covers the science behind complete protein, debunks common myths, and provides practical ways to combine complementary plant foods to get all essential amino acids.

Key Points

Essential Amino Acids: A complete protein contains all nine essential amino acids, which the body cannot produce on its own.
Limiting Amino Acids: Most individual plant proteins are considered incomplete because they are low in or lack at least one of these essential amino acids.
The Power of Variety: Eating a variety of plant foods throughout the day, such as combining grains and legumes, ensures you get a full amino acid profile.
Not Required at Every Meal: The body stores amino acids, so complementary proteins do not need to be consumed at the same meal to be effective.
Naturally Complete Plant Proteins: Some plant sources, including soy, quinoa, and hemp seeds, are complete proteins on their own.
Bioavailability and Digestibility: While some plant proteins are less digestible than animal proteins, a varied diet and sufficient calorie intake generally prevent deficiencies.

What Defines a Complete Protein?

Proteins are large, complex molecules that play a vital role in virtually every biological process, from building tissues and muscles to synthesizing hormones and enzymes. These large molecules are made up of smaller units called amino acids. Of the 20 amino acids the body uses, nine are classified as "essential" because they must be obtained through the diet. A protein source is labeled as "complete" if it contains sufficient quantities of all nine of these essential amino acids. Conversely, a protein source is considered "incomplete" if it is low in or missing one or more of the essential amino acids.

The Limiting Factor: Why Most Plant Proteins Fall Short

While animal-based proteins like meat, eggs, and dairy are all complete protein sources, the amino acid profiles of individual plant foods differ significantly. The concept of a "limiting amino acid" is key to understanding why most plant-based protein is not complete. A limiting amino acid is the essential amino acid present in the smallest proportion relative to the body's needs.

For example, grains like rice and wheat are typically low in lysine. Legumes, such as beans and lentils, often lack sufficient methionine and tryptophan. This is why combining different types of plant proteins is so important for those following a plant-based diet. By pairing a grain with a legume, the strengths of one food compensate for the deficiencies of the other, effectively creating a complete protein profile.

Combining Complementary Proteins

The outdated advice that complementary proteins must be eaten at the same meal has been debunked by modern nutritional science. The body maintains a pool of amino acids throughout the day, allowing it to combine them from various food sources as needed. This flexibility makes it relatively easy for those on a plant-based diet to meet their protein needs by simply eating a varied diet.

Examples of Complementary Protein Pairings

Grains + Legumes: This is one of the most classic combinations. A few well-known examples include beans and rice, peanut butter on whole-wheat bread, and hummus with pita bread.
Legumes + Nuts/Seeds: Combining these two food groups can also create a complete protein. Try a lentil soup topped with sunflower seeds or a salad with chickpeas and crushed nuts.
Grains + Dairy/Eggs (Lacto-Ovo Vegetarian): For non-vegans, combining grains with dairy or eggs easily forms a complete protein. Think oatmeal with a side of yogurt or scrambled eggs with whole-grain toast.

Plant-Based Foods That Are Complete Proteins on Their Own

While the concept of combining proteins is fundamental, several plant-based foods are considered complete proteins on their own, containing all nine essential amino acids in sufficient amounts.

List of Complete Plant Proteins

Soy Products: Tofu, tempeh, and edamame are derived from soybeans and contain all essential amino acids.
Quinoa: This ancient grain is a standout for being a complete protein and is also naturally gluten-free.
Buckwheat: Another nutrient-dense pseudocereal, buckwheat provides all nine essential amino acids.
Hemp Seeds: These seeds are a versatile, complete protein source rich in healthy fats.
Chia Seeds: These tiny seeds offer a complete amino acid profile and can be used in a variety of dishes.
Spirulina: A blue-green algae, spirulina is a potent source of complete protein.
Ezekiel Bread: Made from a mixture of sprouted grains and legumes, this bread is a naturally complete protein.

Incomplete vs. Complete Protein: A Nutritional Comparison

To highlight the differences between individual plant-based and animal-based protein sources, here is a comparison based on their amino acid profiles. This table illustrates why relying on a single plant source is less effective than a varied diet or a naturally complete source.


Protein Source	Type	Amino Acid Profile	Considerations for Completeness
Chicken Breast	Animal-based	Complete	High-quality, complete protein source.
Black Beans	Plant-based (Legume)	Incomplete (low in methionine)	Needs to be combined with grains to form a complete protein.
Brown Rice	Plant-based (Grain)	Incomplete (low in lysine)	Needs to be combined with legumes to form a complete protein.
Tofu (Soy-based)	Plant-based	Complete	A stand-alone complete protein source.
Quinoa	Plant-based (Pseudocereal)	Complete	A stand-alone complete protein source.
Whey Protein	Animal-based (Dairy)	Complete	A high-quality, complete protein, often used in supplements.

Conclusion: Variety Is the Key to Plant-Based Protein

The notion that plant-based protein is inherently inferior is a widespread misconception rooted in an outdated understanding of nutrition. While most single plant sources are indeed incomplete, a varied and balanced plant-based diet easily provides all the necessary essential amino acids over the course of a day. The modern understanding of nutrition has moved away from the need for perfect protein combinations at every single meal, emphasizing a varied diet for overall health. By incorporating a mix of legumes, grains, nuts, and seeds, or by choosing naturally complete plant proteins like soy and quinoa, individuals on a plant-based diet can easily and adequately meet their protein needs. The key is to embrace variety and focus on a well-rounded diet rather than fixating on the completeness of a single food item.

For more in-depth nutritional information, visit the American Institute for Cancer Research's resource on how to get enough protein on a plant-based diet: https://www.aicr.org/resources/blog/how-to-get-enough-protein-on-a-plant-based-diet/.

Frequently Asked Questions

A complete protein contains all nine essential amino acids in sufficient amounts, which are the amino acids the human body cannot produce. An incomplete protein is low in or missing one or more of these essential amino acids.

No, you do not. This is an outdated concept. The human body maintains a pool of amino acids, so as long as you eat a varied diet throughout the day, you will get all the essential amino acids you need.

Common examples include combinations of grains and legumes, such as rice and beans, peanut butter on whole-wheat bread, and hummus with pita bread.

Yes. Several plant foods are complete proteins, including soy products (tofu, tempeh, edamame), quinoa, buckwheat, hemp seeds, and chia seeds.

While some plant proteins are slightly less bioavailable than animal proteins, a well-planned, varied plant-based diet can provide sufficient protein for muscle growth and maintenance, even for athletes.

Grains and cereals, such as rice and wheat, are typically low in the essential amino acid lysine.

Legumes like beans, peas, and lentils are often low in methionine.