Busting the Myth of Incomplete Plant-Based Proteins

Like unicorns, myths can live forever unless we find their Achilles heel.
“I would like to go vegetarian but I’m afraid that I won’t get complete proteins.”

The belief that plant-based proteins are lower quality, specifically, that they are ‘incomplete’ because they are ‘missing’ amino acids has been around for decades. Yet, as a biologist, I am highly skeptical of this so-called fact.

My skepticism — and that of my fellow scientists — is rooted in these two facts:

1) Plants use the same twenty amino acids that we do to build their proteins (the genetic code is universal).

2) All animals ultimately get their protein from plants (or plant-like phytoplankton)— either directly or indirectly through the food chain.

Over the years, numerous troublesome scientists have raised their hand and tried to get ‘expert bodies’ to revise their erroneous statements about plant-based proteins. I particularly like this exchange between physician-scientist Dr. John McDougall and the American Heart Association back in 1992. Despite such efforts, the myth of incomplete plant proteins is still going strong, decades later.

In this article, I do something that most plant-protein bashers haven’t done: show real data. I looked at how much of each essential amino acid various foods provide and asked whether or not this is ‘enough’. I compared various plant-based sources to meat and dairy.

I emerged from my research with a call to action. We need to change the way we talk and think about the quality of plant-based proteins.

Calling plant-based proteins incomplete is like calling milk an ‘incomplete’ source of calcium because it takes more than one glass to meet your daily needs.

Note: Meeting your overall protein needs is a separate topic — and is in my queue!

Extra! Extra! Read all about it!

Breaking News: Plant-based Proteins Are NOT Missing Any Amino Acids!!!

You read that right. The statement that all or most plant-based proteins are missing one or more amino acids is false. All plants (and meats and dairy) contain at least moderate amounts of all nine essential amino acids. Note: Fruits were not included due to low total protein content.

Within plant-based protein sources, the worst-case scenario is a diet where all your protein comes from nuts. Even then, you would only fall short for one amino acid, lysine, at 84% of your daily needs. I would hardly call this ‘missing’!

The term ‘incomplete’ is trickier to define and analyze, but we will get to that.

Before we look at the data, let’s get our definitions straight.

Think of a protein as a long string of beads where each bead is an amino acid, with 20 letters to choose from.

What Is a Complete Protein?

Proteins are long chains of amino acids. Picture an alphabet bead necklace where each bead is an amino acid. Each protein is defined by its unique sequence of amino acids (beads).

  • Learn more about proteins here.
  • Your body can make most of the amino acids it needs. An essential amino acid (also called indispensable) is one that your body cannot make, so must come from your diet.
  • There are nine essential amino acids (also called indispensable) in healthy adults. The other eleven key amino acids are considered ‘non-essential’. Some amino acids are ‘conditionally’ essential — needed only in certain stages of life / certain diseases.
  • A food (or combination of foods) is considered to be a complete protein source if it provides enough (or more) of each of the nine essential amino acids. Your definition of ‘enough’ and your choice of serving size for getting ‘enough’ will influence your incomplete vs complete label.

Comparing Plant and Animal Proteins at the Amino Acid Level

Photo by Evan Phillip on Unsplash

How would different diets fare in a world where all our protein comes from powders (ugh!)? This thought experiment helps us get to the heart of the matter.

I compared proteins from difference sources at the amino acid level (meat, dairy, beans, veggies, grains, nuts, or seeds). To make an apples-to-apples comparison, I looked at how much of each amino acid is in a day’s worth of protein (60 grams in my case).

My graph below shows the results for all nine essential amino acids, colour-coded by protein source. The key is to look at the height of each bar relative to the 100% line, which marks your recommended daily intake. A bar height of 200% means the food provides twice your daily needs, whereas 50% would mean half of your daily needs.

See my Appendix for more information on my analysis.

Each bar shows how much of a specific amino acid is found in a group of foods. A bar height of 100% means daily needs are met.

Major findings

Photo by Paweł Czerwiński on Unsplash

Green light:

  • If you eat a mix of plants, you will get roughly twice as much as you need for seven of the nine essential amino acids. This assumes you get enough total protein.
  • Meat and diary protein provide roughly twice as much as you need for all nine essential amino acids.

Yellow light (caution):

  • Lysine: If grains and seeds were your only source of protein, you would get enough lysine, but not by much (about 105%).
  • Methionine: If beans and veggies were your only source of protein, you would get enough methionine, but not by much (about 105%).

Red Flag (watch out!):

  • Lysine: If nuts were your only source of protein, you would likely fall just short on lysine (16% below your target). Not missing, but below 100%.

Cracking the Case

Photo by Jonas Dücker on Unsplash

I think I finally cracked the case as to why the incomplete protein myth is so entrenched. It comes down to a misconception about serving sizes.

Many people think that they need meet their essential amino acid needs in a single meal. In reality, we are talking about a DAILY requirement. We should be looking at meeting our needs throughout entire day.

This brings us full circle to my earlier assertion:

Calling plant-proteins ‘incomplete’ is like calling milk an incomplete source of calcium because it takes more than one glass to meet your daily needs!

A related, underappreciated fact is that when your body makes a new protein, it uses building blocks (amino acids) both from your latest meal AND from your reservoir ‘pool’. This pool comes from the natural process of amino acid recycling in your body.

Learn more about the lifecycle of proteins in your body in my Protein Primer 2.

Is There Truth Behind the Myth?

Detective Davis is on the case!

Many myths have at least a kernel of truth behind them and this one is no exception.

  • Meeting your needs for all essential amino acids is essentially foolproof if you eat moderate amounts of meat or dairy.
  • Meeting your needs for all essential amino acids on a plant-only diet is easy but not foolproof.
  • This is because the ‘weakest link’ for plant-based proteins is weaker than the weakest link for dairy and meat proteins.

Let me elaborate:

  • Dairy and meat have a ‘weakest link’ amino acid level that is twice your daily needs (in a day’s worth of protein). This means you can meet all your daily essential amino acid needs in only half a day’s worth of total protein — or a very large, protein heavy meal.
  • Most individual plants (eg. rice alone or black beans alone) have a ‘weakest link’ amino acid level just below or just 100%. Thus, if a single plant were your sole source of protein, you would need a full day’s worth of protein (and a full day’s worth of food) to meet your needs.

There is a lot to digest in this article so I’ll re-iterate my takeaways. First, a constructive view:

Two Simple Rules For Meeting Your Essential Amino Acid Needs on a Plant-Based Diet

“Colorful fruit and vegetables on a market stall” by ja ma on Unsplash
  1. Eat enough protein.
  • Know your needs and your sources. I eat ~1 gram of protein per kilogram of my weight. This is slightly more than the national guidelines, yet less than most North Americans. My results are based on this level of intake.

2. Eat a variety of whole foods.

  • Similar foods tend to have a common ‘weakest link’ so eating a mix of veggies, beans, grains, nuts, and seeds helps them all cover for each other.

This advice may sound boring because you’ve heard it again and again, but it’s true — and easy to follow.

I will present the flipside view in a second, related article (in progress):How to Fail to Meet Your Essential Amino Acid Needs on a Plant-Based Diet

Call to Action: Choose Your Words Carefully

Photo by rawpixel on Unsplash

I think much of the confusion about this myth comes from using language loosely. If we work together to tighten things up, a clearer picture will emerge for all.

Here are my recommendations:

1. Stop saying that plants are ‘missing’ amino acids. This is completely false. The word ‘missing’ implies zero or very low, whereas all plants use all twenty amino acids — in at least moderate amounts.

2. Be careful and clear when using the word ‘complete’ or ‘incomplete’ to describe a protein. We should be thinking about a day’s intake rather than a single serving. It is not necessary to meet your daily needs at every meal.

3. Consider using the word ‘limiting’ to clarify which amino acid is the weakest link.

Check out my Appendix to better understand my analysis.

Future Questions

This analysis raises several questions:

  • How can I get enough protein on a plant-based diet?
  • How can I be sure to get enough methionine and lysine? Does it have to be beans and rice?
  • Is soy safe (for men? for women? for kids?)
  • Is more better or is ‘just enough’ better? Does it depend on the amino acid?

The first question is the subject of a previous article and the rest of the topics are all in my queue. Subscribers get to shape my choice of future topic!

Learn more and subscribe at https://fueledbyscience.com


  • The information provided here should not be viewed as medical advice. Consult your physician if you have nutritional concerns.
  • I have not incorporated amino acid digestibility. This tends to be lower for plant-based foods relative to animal foods but the difference is not enough to change the basic message.
  • RDI results shown are averages (medians) by food group. I chose four to seven foods per category. Some foods will be lower than the average by food group.
  • RDI results will vary depending on which foods are chosen to represent each category. I tried to choose popular items.
  • Amino acid needs can vary between individuals by age and health status. I have used standard needs for healthy adults.

Appendix: Gory Details and FAQs

How did you calculate % RDI (Recommended Daily Intake)?

Step 1: Calculate your recommended daily intakes (RDI)

Step 2: Look up the essential amino acid content of foods of interest

  • Source: USDA Food Composition Database — Try it!

Step 3: Calculate how well each food meets my RDI for each of the nine amino acids

  • % RDI = amino acid content of food / RDI for amino acid
Which foods were included? How were they chosen?

For meats and dairy, I chose popular foods that span range of fat content. For plants, I chose popular foods and biased towards foods I eat.

  • Meats: Steak, Ground beef, Chicken, Salmon, Bacon
  • Dairy: Eggs, 1% Milk, Yoghurt, Cheddar Cheese, Cream Cheese
  • Veggies: Spinach , Kale, Broccoli, Cauliflower, Celery, Potato, Carrot
  • Beans: Tofu, Edamame, Black Beans, Chickpeas, Lentils, Peas
  • Seeds: Hemp Seeds, Sunflower Seeds, Sesame Seeds, Pumpkin Seeds
  • Grains: Quinoa Oats, Whole Wheat, Pasta, Brown Rice
  • Nuts: Peanut Butter, Almonds, Walnuts, Cashews
What Recommended Daily Intakes (RDIs) were used?

Here are my recommended DAILY intakes for all nine essential amino acids.

  • They range from 5 mg/kg per day (tryptophan) to 42 mg/kg per day (leucine).
  • I calculated my needs by taking the RDI Factor and multiplying by 60 kg (132 lb).
  • Note that the daily RDI factors shown here only apply to healthy adults.

RDA (mg per kilogram)

  • Methionine+: 19
  • Lysine: 38
  • Phenylalanine+: 33
  • Tryptophan: 5
  • Threonine: 20
  • Isoleucine: 20
  • Leucine: 42
  • Valine: 26
  • Histidine: 14

Note: *RDIs for some amino acids are given as combinations of two amino acids (essential plus non-essential). Methionine+ represents Methionine + Cysteine (the two sulfur containing AAs).; Phenylalanine+ represents Phenylalanine + Tyrosine (both aromatic AAs).

Why did you choose 60 grams of protein?

I wanted to be both realistic and conservative. I based my analysis on the amount of protein I typically eat as part of my plant-based diet — 60 grams. This is about 1 gram per kilogram of my weight (I weight 60 kg) and is slightly above the standard recommendation of 0.8 grams of protein per kilogram. Most people eat more than this!