Busting the Myth of Incomplete Plant-Based Proteins
Getting ‘complete’ proteins from plants is easier than you think
If you think that you need to eat copious amounts of rice and beans to get “complete” plant-based proteins, think again. Covering your essential amino acids from a plant-based diet is easier than you think.
The widely held belief that plant-based proteins are “incomplete” or “missing” amino acids traces back to a 1971 book, Diet for a Small Planet by Frances Moore Lappé, who trumpeted the lower environmental toll of a plant-based diet. Lappé recommended “protein complementing” — strategically pairing plant proteins at each meal, in order to achieve a certain balance of amino acids. While there was a kernel of truth at the root of her advice, she was way off the mark in terms of what it takes to cover one’s amino acid needs.
In the tenth anniversary of her book (1981), Lappé retracted her emphasis on strategic food pairings and clarified the very limited scope of this concern.
“With three important exceptions, there is little danger of protein deficiency in a plant food diet. The exceptions are diets very heavily dependent on fruit, or on some tubers, such as sweet potatoes or cassava, or on junk food (refined flours, sugars, and fat). Fortunately, relatively few people in the world try to survive on diets in which these foods are virtually the sole source of calories. In all other diets, if people are getting enough calories, they are virtually certain of getting enough protein.”
She hit the nail on the head, but the damage was done. The myth of incomplete plant-based proteins is incredibly sticky, and continues to thrive despite decades later, despite clear evidence to the contrary.
Indeed, it’s clear that something is amiss based purely on two basic tenets of biology:
- Plants use the same 20 amino acids as humans to build their proteins (the genetic code is universal).
- All animals ultimately get their protein from plants (or plant-like phytoplankton)— either directly or indirectly through the food chain.
To remove any lingering doubts, this article provides hard data on the levels of all nine essential amino acids across a range of foods (plant-based, meats, and dairy) and explains how easy it is to get “enough”.
Note: Meeting your overall protein needs is a separate topic — discussed here.
To get started, let’s get our definitions straight.
What is a complete protein?
A food (or combination of foods) is considered to be a complete protein source if it provides “enough” of each of the nine essential amino acids.
This seemingly straightforward definition is actually very fuzzy. Whether or not a food provides “enough” of any given amino acid depends on how you define “enough” — and on how much of it you consume.
As a reminder, 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).
In healthy adults, nine of the 20 amino acids are considered essential (also called indispensable). Your body cannot make them, so they must come from your diet. The other 11 key amino acids are considered “non-essential,” as your body can make them. Some amino acids are “conditionally” essential — needed only in certain stages of life or for certain diseases.
Let’s dig into the data.
Amino Acid Profiles Of Plant And Animal Proteins
Using publicly available amino acid profiles and requirements, I created a chart that shows how much of each amino acid you get from a variety of foods (meat, dairy, beans, veggies, grains, nuts, or seeds).
To make an apples-to-apples comparison, the levels for each food are shown assuming 60 grams of protein (my daily requirement). These amounts are expressed as a fraction of the official daily requirement for that amino acid.
To make sense of the chart, focus on the 100% line. At 100%, the food provides, on a per gram basis, just enough of that amino acid. Values over 100% can help balance out values below 100%. See Appendix for more information.
In meat and dairy, all nine essential amino acid levels are plentiful. In fact, they are so abundant that you’d get enough even if you only ate half your daily protein needs. For plant-based foods, seven of the nine essential amino acids are plentiful and two of them (lysine and methionine) are borderline. This difference is the kernel of truth behind the myth.
This chart also helps explain the origins of food pairing advice — like eating rice and beans together. For rice, and other grains, the least abundant (limiting) amino acid is lysine. Yet, beans are loaded with lysine, serving nearly double the requirement (per gram of protein). The reverse is true for methionine — this is the least abundant (limiting) amino acid in beans and legumes. Conveniently, rice and other grains knock it out of the park.
Should you worry about lysine or methionine?
While lysine levels aren’t sky-high in grains, seeds, or nuts, it’s definitely not missing. Nuts are lowest, at around 15% lower than ideal, while grains and seeds are right around 100%. Thus, to get enough lysine, you need to make sure you hit your total daily protein (and don’t get all your protein from nuts!)
It’s a similar story with methionine, which is lowest in beans and veggies. If you got all your protein from beans, you’d barely cover your daily methionine needs. Of course, eating only beans is not going to fly for many reasons!
Nerd Note: Methionine+ is the sum of methionine plus cysteine, the only two sulfur-containing amino acids (SAA). These amino acids share a common daily requirement. Similarly, Phenylalanine+ represents the sum of phenylalanine (Phe) and tyrosine (Tyr), the two aromatic amino acids.
How to meet your essential amino acid needs on a plant-based diet
Meeting your essential amino acid needs from plants comes down to two simple rules:
Eat enough protein.
To get enough protein, know your protein needs. I target ~1 gram of protein per kilogram of my weight. This is slightly more than the national guidelines — yet less than most North Americans consume.
Getting enough protein is easy with a balanced plant-based diet but it’s worth doing a bit of homework to be sure, especially if you are aiming higher than the basic guidelines. Learn more in my article about the protein content of different plant-based foods.
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 familiar advice may sound boring, but it’s true — and serves you well for other nutrients too.
Does Timing Matter?
For protein, and amino acids, you should be mindful of your daily needs, but don’t need to worry about each meal.
In fact, your body has a large reservoir “pool” of amino acids that it can draw on even when you are fasting. This pool comes from the natural process of amino acid recycling in your body. I discuss protein turnover here.
The bottom line
As long as you’re eating enough total plant-based protein, and incorporating some diversity in your sources, you should be covering your essential amino acids. You don’t have to eat endless servings of rice and beans — unless you want to!
To fail, you’d need to either not eat enough protein, or eat a severely limited diet, neither of which is advised for many reasons.
At the root of the myth lies the way that we define “complete” and “enough.” To get a “complete” protein, we need to get “enough” of the nine essential amino acids, in one full day. Yet, we judge proteins as complete or not based on a single serving, not based on a day’s worth of protein.
One way to think about “completeness” is to consider another nutrient, calcium. Is milk an incomplete source calcium because it takes more than one serving to meet your daily needs?
Call to action: Choose your words carefully
I’m not the first scientist to raise a red flag regarding the misrepresentation of plant-based proteins (see, for instance, this 1992 exchange between physician-scientist Dr. John McDougall and the American Heart Association), and I won’t be the last.
Deeply rooted beliefs are hard to displace. Let’s work together to tighten up our language, so that this myth will go the way of the dodo bird!
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 20 amino acids — in at least moderate amounts.
2. Consider using the word “limiting” to clarify which amino acid is the weakest link.
3. Be careful and clear when using the word “complete” or “incomplete” to describe a protein. We should be thinking about complete diets — meeting our daily needs — rather than complete proteins. It is not necessary to meet your daily needs at every meal.
- How much protein do I need? Is more better? Can I get enough protein on a plant-based diet?
- Is soy safe?
- Learn more about the lifecycle of proteins in your body in my Protein Biology 1 & Protein Biology 2.
- Learn more and subscribe at Fueled by Science.
The information provided here should not be viewed as medical advice. Consult your physician if you have nutritional concerns.
Appendix: Gory details and FAQs
- 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.
How did you calculate % RDI (Recommended Daily Intake)?
Step 1: Calculate your recommended daily intakes (RDI)
- RDI were based on the World Health Organization and the Institute of Medicine. When the two recommendations differed, I chose the higher number. Actual figures shown further below.
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, Yogurt, 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 RDIs were used?
Here are my recommended DAILY intakes for all nine essential amino acids.
- They range from 5 mg/kilogram per day (tryptophan) to 42 mg/kilogram per day (leucine).
- I calculated my needs by taking the RDI Factor and multiplying by 60 kilograms (132 pounds).
- 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 weigh 60 kilograms) and is slightly above the standard recommendation of 0.8 grams of protein per kilogram. Most people eat more than this!