Meal Planning

Are Complementary Proteins Necessary?

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Have you ever heard that plant protein is “incomplete” and wondered exactly what this means? Maybe you’ve heard that you should mix certain complementary plant proteins, such as rice and beans, to create a complete protein. However, are complementary proteins necessary? Whether you are a vegetarian or a meat-eater who frequently consumes plant-based, vegetarian meals, these are questions many of us may ask in an attempt to increase our intake of plant-based meals.

My husband and I aren’t vegetarian, but we love (ok me…I love) to eat and cook vegetarian meals. My favorite plant-based source of protein is definitely beans. I find that vegetarian meals can be really easy on our food budget. They often save us a ton of money while being quick and simple to cook.

If your family is like ours you may find yourselves enjoying vegetarian meals as well. Or perhaps, you are a vegetarian and have wondered if combining protein sources to complement each other is truly necessary. Read on to discover the truth about plant-based protein.

bags of grains and other incomplete proteins

The Difference Between Essential and Nonessential Amino Acids

First, let’s cover the basics of amino acids. The most basic definition of amino acids is they are the building blocks of protein. There are a total of 20 amino acids that the body needs and uses.

The protein foods we eat actually break down into amino acids once they are digested. However, amino acids aren’t only found in the food we eat. The body is actually capable of making some, but not all amino acids. This is where the difference between essential and nonessential amino acids comes in.

Essential Amino Acids

The human body needs and uses 20 different amino acids. However, only 9 of these 20 are essential amino acids. These nine are considered essential because the human body is not able to produce them at a sufficient rate without outside input (i.e. food). The body can synthesize the other 11 amino acids. However, since the body cannot synthesize these essential amino acids, it’s imperative that we consume them in our diet. This is what makes them “essential.”

What are the 9 essential amino acids?

The 9 essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Non-Essential Amino Acids

Whereas the human body cannot synthesize essential amino acids, nonessential amino acids can be made in the body using other components. While non-essential amino acids still have important functions in the body, it is not imperative to get them through the diet since they can be synthesized.

The function of nonessential amino acids, similar to essential amino acids is to create new proteins. The only difference lies in whether the body can synthesize them or not.

What are the nonessential amino acids?

Most sources claim the 11 non-essential amino acids are alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine. However, if we look further, 5 of these are often classified as conditionally essential amino acids or semi-essential.

What are conditionally essential amino acids?

Different sources claim up to 6 amino acids as conditionally essential. Basically, in healthy normal circumstances, the body has no trouble keeping up with production, but in states of catabolic distress, the body may be unable to provide enough. These 6 amino acids that can be classified as semi-essential are arginine, cysteine, glutamine, glycine, proline, and tyrosine.

Complete and Incomplete Proteins: What It Means

Let’s get down to what it means exactly for a protein to be complete or incomplete.

The distinction between complete and incomplete protein is where things start to get interesting. In its simplest definition, if a protein is complete then it contains all 9 of the essential amino acids. In general, animal protein sources are complete proteins. Some examples of complete proteins include eggs, meat, fish, and dairy products. According to traditional thought, plant proteins lack one or more essential amino acids. Thus, we often refer to them as incomplete proteins.

However, it’s actually not that simple. In fact, the more technical classification of a complete protein seems to change things:

To be a complete protein, a food must supply 100% of the essential amino acids the body needs when consumed at the level that would provide the body with adequate protein. 

You may need to read that again, I did. So basically, if you ate nothing but black beans in an entire day (to provide protein), then you would be getting an inadequate intake of the essential amino acids. Therefore, black beans are not a complete protein.

But who eats one food item all day long? Not me. Therefore, the whole idea of complementary proteins is suggested to be somewhat irrelevant today.


Eggs - a complete protein

Complementary Proteins

What are complementary proteins and how are they formed? Since plant proteins have traditionally been considered “incomplete” sources of protein, it used to be recommended to combine plant proteins in order to make a complete protein when meal planning. In this way, the two complementary foods would between them provide all 9 of the essential amino acids. For example, the most common complementary protein is beans and rice. These complete each other as legumes are low in methionine and the grains (rice) in lysine. Therefore, combining the two which is also known as protein complementation would provide all 9 essential amino acids in adequate ratios.

Common Examples of Complementary Protein Combinations:

  • Legumes/Beans paired with Nuts, Seeds, Grains, or Dairy – Some examples may be soups with legumes and seeds/grains; rice and beans; beans on a tortilla; hummus on whole grain toast; or a salad with beans and nuts/seeds/dairy.
  • Dairy with Nuts/Seeds, Grains, or Legumes – This could be a grilled cheese sandwich; pizza; quinoa salad with feta; or yogurt with nuts/seeds.

However although protein complementation and complementary proteins remain a hot topic, we now know that all proteins actually contain all essential amino acids. Just not in the ratio that would designate them to be “complete” as described above.

Vegetarian Salad with Complementary Proteins

Are Complementary Proteins Necessary or Important?

So, the big question: Are complementary proteins necessary to achieve a balanced vegetarian meal? Should we be meticulous in pairing proteins for plant-based meals?

But, what about if you were eating a whole day as a vegetarian or even if you follow a regular vegetarian diet day in and out?

The answer, in short, is no, you don’t need to ensure each meal contains complementary proteins in order to meet essential protein requirements and maintain muscle. Our diets are varied enough throughout the day to make this concept irrelevant and combining proteins at a meal is no longer actively recommended.

In fact, the idea of complete and incomplete proteins is misleading. Studies have shown that when you consume adequate calories throughout the day (from a variety of healthful sources), even vegetarians eating a varied diet will consume all of the essential amino acids throughout the day.

Protein Pairing is Tradition

People often pair proteins in a “complementary” way as a part of culture and tradition. For example, people in Asian communities frequently pair rice and soybeans, while Latin American cultures pair pinto beans and corn, just to name a few. Even succotash (combining of corn, lima beans, and other vegetables) which became popular during the Great Depression, is an example of traditional protein pairing which is still carried on today by culture and tradition.

So, next time you sit down to a vegetarian meal, rest easy knowing that most advice on protein pairing is simply tradition and based on 60+ year old research. When we break down the facts, it’s really not necessary to ensure that each meal is paired perfectly, since over the course of a day we are likely eating more than one food. And, next time someone tries to point out the poor quality of vegetable protein, remember that this is likely based on misconceptions and standards that are outdated and impractical. Enjoy a variety of plant-based proteins such as nuts, seeds, beans, lentils, and tempeh throughout the day and your body will be adequately supplied with the essential amino acids it needs.

beans and legumes - incomplete proteins to make complementary proteins

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American Society for Nutrition: Protein Complementation

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The Dietitian’s Guide to Vegetarian Diets

Position of the Academy of Nutrition and Dietetics: Vegetarian

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  1. I have been needing this for info for awhile now. Thanks for sharing!

  2. I also appreciate your easy to understand explanation of complete/incomplete proteins. Nutrition information changes – often corrected – through the years. Decades ago combining the proteins, complete/incomplete, HAD to be at the same meal. Looking at food intake for the day makes much better sense & welcoming to accomplish!

  3. I disagree with the part about complementary proteins being unnecessary. I am currently studying nutrition so I have been reading a it about this lately. Like you said, most plant proteins do contain all nine essential amino acids. However there is this thing called a “limiting amino acid” which is the amino acid that is either supplied in the lowest amount of all of the amino acids in a food, or is completely absent in a food. Also, with protein production in the body, there is this thing called the “all-or-nothing rule,” which means that in order for your cells to synthesize protein, you must have all of the essential amino acids; if you do not have all of the essential amino acids– if even only one is missing– you body will not make protein and will use the amino acids elsewhere. How do these two things connect? This means that your body can only synthesize as much protein as the amount of the limiting amino acid supplied allows… so lets say the proteins you are eating DO contain all of the essential amino acids, but are low in, say, methionine, and you have high amounts of all of the other amino acids. Once your body runs out of that methionine during protein synthesis, you will not be able to make any more protein… even if you have lots of all of the other 8 essential amino acids, no methionine means no protein at all. If you are not eating complementary proteins, yes you may be getting all of the essential amino acids, but the point of complementary proteins is to ensure that you have enough of all of the amino acids to produce an adequate amount of protein in your body without running out of any of the amino acids.

    1. Hi! By complementary proteins, the article is referring to traditional protein pairing of two foods eaten in combination within a single meal which is the belief that was taught years ago (back when I was in nutrition school, actually). Now, however, we know that while you are correct… a variety of foods IS important…. it is not necessary to consume all of these amino acids within one meal or even a day. It can obviously get quite complex and is boiled down in this post to answer the simple question of my readers as to whether we should be pairing proteins together within a single meatless meal. Thanks for weighing in 🙂

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