Which form of protein does not contain all 9 essential amino acids?

January 5, 2026 •

3 min read

Over 40% of the world's population relies on a vegetarian or vegan diet, which often includes protein sources that are considered incomplete. A food that is an incomplete protein does not contain all 9 essential amino acids, which are vital for human health. This guide delves into what makes a protein incomplete, which foods fall into this category, and how to balance your diet effectively.

Quick Summary

An incomplete protein is a dietary protein source that lacks or has insufficient quantities of one or more of the nine essential amino acids required for optimal bodily function. The majority of incomplete proteins are plant-based, but they can be combined effectively.

Key Points

Incomplete Protein Defined: An incomplete protein is a food source that lacks one or more of the nine essential amino acids needed by the human body.
Plant-Based Sources: Most plant-based foods, such as grains, legumes, nuts, and seeds, are considered incomplete protein sources.
Complementary Proteins: You can obtain all nine essential amino acids by consuming a variety of plant-based incomplete proteins over the course of a day.
No Meal-by-Meal Pairing Necessary: The old belief that incomplete proteins must be combined in a single meal is outdated; your body can draw from an amino acid pool.
Notable Exceptions: While most animal products are complete proteins, some, like collagen, are incomplete. Some plant sources like soy and quinoa are complete.

What are Incomplete Proteins?

Proteins are the building blocks of life, responsible for cell and tissue repair, muscle growth, and hormone production. They are made up of smaller compounds called amino acids, which can be either essential or non-essential. Our bodies can produce 11 of the 20 necessary amino acids, but the other nine, known as essential amino acids, must be obtained from food. A complete protein source provides all nine essential amino acids in adequate amounts.

In contrast, an incomplete protein source is one that is deficient in one or more of these nine essential amino acids. The term 'incomplete' is not a judgment on the food's nutritional value, but rather a classification based on its amino acid profile. Most incomplete proteins come from plant-based sources.

Limiting Amino Acids

Incomplete proteins are often characterized by having a 'limiting amino acid.' This is the essential amino acid that the food contains in the lowest quantity. For example, grains are often low in lysine, while legumes can be low in methionine. This is why combining different plant-based foods can be a successful strategy for ensuring complete amino acid intake.

Sources of Incomplete Protein

For those on a plant-based diet, understanding sources of incomplete proteins is crucial. Here are some of the most common examples:

Grains: Rice, wheat, and corn are excellent sources of carbohydrates and other nutrients but are typically low in the amino acid lysine.
Legumes: Beans, lentils, and peas are packed with fiber and protein, yet often have insufficient levels of methionine.
Nuts and Seeds: Almonds, cashews, and sunflower seeds offer healthy fats and protein but can be deficient in one or more essential amino acids.
Vegetables: While nutrient-dense, many vegetables do not provide all nine essential amino acids in the necessary proportions.
Collagen: Interestingly, one animal-derived protein, collagen, is considered incomplete because it lacks the essential amino acid tryptophan.

Complementary Proteins: The Key to a Balanced Diet

For a long time, it was believed that vegans and vegetarians needed to combine complementary proteins in the same meal to ensure they received all essential amino acids. However, modern nutrition research shows this isn't necessary. The body maintains a pool of amino acids throughout the day, so consuming a variety of plant-based protein sources over a 24-hour period is sufficient.

Classic examples of complementary pairings include:

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

Complete vs. Incomplete Protein Comparison Table


Feature	Complete Proteins	Incomplete Proteins
Essential Amino Acids	Contain all nine essential amino acids in adequate amounts.	Deficient in one or more essential amino acids.
Primary Sources	Animal products (meat, fish, eggs, dairy) and select plant sources (soy, quinoa).	Most plant-based foods, including legumes, grains, nuts, and seeds.
Dietary Requirement	Can be a one-stop source for essential amino acids.	Need to be combined with other protein sources throughout the day for a complete profile.
Examples (Plant-Based)	Soy, quinoa, buckwheat, hemp seeds.	Rice, beans, lentils, nuts, most vegetables.
Typical Limiting AA	None, by definition.	Varies by food type (e.g., lysine in grains, methionine in legumes).

Is It Healthy to Eat Only Incomplete Proteins?

Yes, it is perfectly healthy to get your protein from a variety of incomplete sources, provided you consume enough calories and variety. Many plant-based diets are incredibly nutrient-dense, offering additional fiber, vitamins, and antioxidants. The key is dietary diversity, not focusing on single 'complete' foods. A varied intake of legumes, whole grains, nuts, seeds, and vegetables will provide all the essential amino acids your body needs.

Conclusion

To answer the question, the form of protein that does not contain all 9 essential amino acids is an incomplete protein. These are predominantly found in plant-based foods, such as grains, legumes, and nuts. While the term 'incomplete' may sound negative, it simply refers to an amino acid profile rather than a lack of nutritional value. By consuming a diverse range of plant-based protein sources throughout the day, individuals following a vegetarian or vegan diet can easily acquire all the essential amino acids needed to support their health. There is no need for meticulous meal-by-meal combining, as the body is adept at drawing from its amino acid reserves. Emphasizing variety and balance in your diet is the most effective approach to meeting your protein needs.

For further reading on protein and amino acids from an authoritative source, visit the Harvard T.H. Chan School of Public Health's nutrition resource page.

Frequently Asked Questions

The primary form is an 'incomplete protein,' which is a dietary source lacking or containing insufficient amounts of one or more of the nine essential amino acids that the body cannot produce on its own.

No. While most animal proteins are complete (e.g., meat, dairy, eggs), some, like collagen, are not. Additionally, some plant-based proteins like soy, quinoa, and buckwheat are considered complete.

A 'limiting amino acid' is the essential amino acid that is present in the smallest amount within an incomplete protein source. This is the amino acid that limits the body's ability to create complete proteins from that source alone.

No, it is not necessary to combine proteins in every meal. The body has a pool of amino acids it can use to form complete proteins, so consuming a variety of plant-based foods throughout the day is sufficient.

Common examples of incomplete protein sources include most grains (like rice and wheat), legumes (like beans and lentils), nuts, seeds, and most vegetables.

You can create a complete protein by combining different incomplete protein sources that complement each other's amino acid profiles. For example, combining grains (low in lysine) with legumes (low in methionine) results in a complete profile.

Yes, incomplete proteins are healthy and nutritious. Many plant-based foods that are incomplete proteins also provide fiber, vitamins, and antioxidants. A diverse diet ensures all nutritional needs are met.