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What is an Incomplete Protein?

By M.R. Anglin
Updated: Feb 07, 2024

Everyday the human body repairs or replaces damaged and dead cells. In order to do this, the body needs to manufacture different proteins out of amino acids. Some amino acids can be generated by the body, while others have to be imported via an outside food source. A complete protein is a food source that provides the body with all the amino acids it needs to function. In contrast, an incomplete protein is a food source that does not have all the essential amino acids.

There are about twenty amino acids that the body needs to create the various proteins it requires. Of the twenty, an adult human can manufacture all but nine. These nine essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Without these substances, the body may be hard pressed to repair itself. Since they are not able to be manufactured by the body, these nine have to come from an outside food source.

Since they are so essential to human function, the proper amount of essential amino acids must be consumed every day. If a food source has all nine of these amino acids, it is called a complete protein. Should it lack one or more of the nine essential amino acids, it is an incomplete protein. Commonly, plant proteins are classified as an incomplete protein. Different types of plant proteins can be combined, however, to make up a complete protein.

Complementary proteins are two or more incomplete proteins from different food sources that combine to make up a complete protein. While one food may be lacking in a certain amino acid, another may have it in abundance. For example, many beans are rich in protein but are still considered an incomplete protein. Combining it with rice, however, creates a complete protein because what was missing in the beans is supplied by the rice. This is another reason why it is generally wise to eat a variety of foods daily.

While plants often contain incomplete proteins and animals complete proteins, this isn't always the case. There are some plants that have complete proteins and some animal sources that do not. For example, soy beans and the products made from them are usually complete proteins. Gelatin, on the other hand, comes from an animal but is an incomplete protein. A balanced diet typically will provide all the essential amino acids required.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon1003555 — On Jul 23, 2020

There are only 6 scientifically-proven facts you need to know (all of which blow this blog/article out of the water). Are you ready?......

1- All essential amino acids are made by plants (they cannot be made by animals/humans at all).

2- Every plant contains a complete essential amino acid profile. No plant protein is incomplete or lacking any essential amino acid. (This has been known -and proven- for decades).

3- Preformed animal proteins increase IFG-1 hormone production (which is a known cause of cancer) - plant-based proteins do not elevtae IFG-1 production.

4- On average, everyone (meat-eaters, vegetarians and vegans included) consume 70% more protein than they need, every day. "Too much protein" is the problem.

5- Over 95% of people are deficient in fiber. Because most people get their "protein" from fish, meat, eggs and dairy, they do not get enough (if any) dietary fiber from animal proteins. This is the number 1 leading cause of major disease.

6- The only diet ever to have been scientifically and consistently proven to prevent and reverse major diseases (such as heart-disease, diabetes, stroke and cancer) is a whole-food, plant-based diet.

By bagley79 — On Oct 12, 2012

I wonder if most vegetarians get enough incomplete and complete protein? I know some who are very particular about eating certain foods together, and this must be one of the reasons.

I have used both a whey and soy protein supplement in the past. Even though these are plant based, the supplement contains all of the essential amino acids.

I try to take a couple scoops of this every day so I know I am getting the recommended amount of protein I need. Protein is also good for your muscle mass and helps your body stay lean and strong.

By golf07 — On Oct 12, 2012

Tryptophan is an incomplete protein and is in one of my favorites foods which is turkey. This amino acid is also one that is known to help with sleep. Many times I have read that eating turkey may make you feel sleepy, and I think this is because of the tryptophan in it.

Many people enjoy turkey on Thanksgiving, so are eating it with a lot of other foods that would probably give them a complete protein when combined with the turkey. Some may argue that eating a big meal like that makes anyone feel sleepy, and there is probably a lot of truth to that as well.

By John57 — On Oct 11, 2012

I am not a vegetarian, but don't eat a lot of meat just because I don't care for the taste or texture of it. I try to get all the protein I need from the food that I eat.

In my diet I consume a lot of fruit and vegetables, whole grains and just about any kind of nut. Some of these foods have more protein than others, and I make sure and eat a variety of them every day.

Beans and lentils are also a great source of protein and are low in fat and calories. This article mentions beans and rice together as being a complete protein, and this is one of my favorite meals. It leaves me feeling full and satisfied, and is full of protein and good nutrition.

By julies — On Oct 10, 2012

@anon112646-- I cannot specifically say what would happen if someone didn't get enough incomplete protein, but I am familiar with what happens if your body doesn't get enough protein in general.

If I don't consume enough protein I am much more fatigued, have low energy and am hungry all the time. Protein is what gives me the feeling of being full and the energy I need to make it through the day without crashing.

By anon112646 — On Sep 21, 2010

What would happen if I had too much or too little of incomplete protein? How common is this and can i ever treat it? Please post soon, thanks.

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