Home / G. Health and nutrition / Part IND
"Plant protein is incomplete."
it's true that many plant proteins, on their own, are low in some essential amino acid, such as lysine in cereals. But no one eats just one food. Plant foods contain all nine essential amino acids, in different proportions, and eating a varied diet over the day provides enough of each. The idea that foods must be combined in the same meal dates from the 1970s and has been abandoned. And traditional pairings such as rice and beans already combine a cereal and a legume well.
Short answer
Plant foods have all the essential amino acids, just in different proportions. Cereals have less lysine, legumes have more; together, over the day, they complete each other. The idea that everything must be combined in the same meal is an old myth, abandoned even by the person who popularized it. Rice and beans, an everyday dish in much of the world, is already an excellent combination. And soy has an amino acid profile close to that of animal proteins.
For children
Protein is like a necklace made of beads of different colors. Every food has all the colors, but some have more of one color and less of another. When you eat a variety of foods during the day, like rice, beans, lentils and peanuts, your body puts all the beads together and makes the complete necklace.
"If rice and beans already combine the amino acids, what's missing?"
Como pode aparecer
"It lacks amino acids."
"Plant protein has low biological value."
"You have to combine everything in the same meal."
"Plant protein doesn't build muscle."
"The amino acid score of plant proteins is lower."
"We need animal protein." (which is G1)
The strongest version
"It lacks amino acids. Plant protein has low biological value. You have to combine everything in the same meal."
Strongest version (steelman):
- P1Proteins are made of amino acids, and nine of them are essential: they must come from the diet.
- P2Many plant proteins have less of some essential amino acid, such as lysine in cereals.
- P3Animal proteins have all the essential amino acids in proportions close to what we need.
- CSo only animal proteins are complete, and plant diets will have deficiencies.
P1, P2 and P3 are true. The problem lies in the conclusion: a diet isn't made of a single food, and variety makes up for the differences between sources.
The argument isn't valid: nothing about a varied diet follows from premises about isolated foods.
Full answer
- a
Reconstruction and diagnosis
The argument assesses isolated foods, but people eat diets:
Source Amino acid in smaller amounts What makes up for it Cereals (rice, wheat, corn) Lysine Legumes Legumes (beans, lentils, chickpeas) Methionine Cereals, nuts and seeds Soy Balanced profile Close to animal proteins - b
Where the argument breaks
1. Plant foods have all the essential amino acids.
What changes is the proportions. There's no common plant food completely lacking an essential amino acid.
2. Combining in the same meal is a myth.
The idea of mandatory "protein combining" was popularized in the 1970s. Today we know the body continuously recycles amino acids and keeps a circulating pool of them, and that variety over the day is enough. The author who popularized the rule, Frances Moore Lappé, revised it herself in the 1981 edition of Diet for a Small Planet.
3. Science confirms adequacy.
A 2019 review concluded that concern about amino acids in vegetarian diets had been overstated, and that legumes, nuts and seeds are enough for protein adequacy in adults.
4. Rice and beans combine well.
The traditional dish joins a cereal, low in lysine, and a legume, low in methionine, and each largely makes up for what the other lacks.
5. "Biological value" measures an isolated food.
The measure is useful in the lab, but it doesn't describe a real diet, which includes many foods.
- c
Data and sources
Source What it says Note Mariotti and Gardner, 2019, *Nutrients* Concern about amino acid deficiency in vegetarian diets has been overstated; minimally varied diets provide enough Scientific review Academy of Nutrition and Dietetics, 2016 Proteins from a variety of plant sources, eaten over the day, provide enough amino acids Official position - d
The strongest counter-reply: you have to eat more
The most serious version grants that plant diets can provide all the amino acids, but notes that because of lower digestibility and the proportions, it may be necessary to eat a little more total protein.
- e
Why the counter-reply doesn't reach the conclusion
Eating a little more is easy.
Just include legumes, soy and nuts regularly.
That's planning, not impossibility.
Every diet requires some planning.
The difference is small for adults with a varied diet.
Nutrition bodies don't consider it an obstacle.
- f
Back to the moral question
Even if one had to eat a few more beans, that wouldn't justify raising and killing animals. The question remains: if plants provide everything we need, what justifies making animals suffer to get the same protein?
Reasoning errors
Logical
Is the form valid?
Fallacy of composition
Epistemological
Does the evidence support it?
Outdated myth
Moral
Is the criterion coherent?
Invented necessity
Biases
Rationalization through the 4Ns
"Necessary" is one of the most used justifications
Confirmation bias
Old technical information is accepted without checking because it confirms the habit
General literature on confirmation bias
Illusory truth effect
The idea of "incomplete" protein was repeated for decades after being revised
Extrapolations
The principle being tested is: "if an isolated food is incomplete, the diet will be incomplete".
If it applied to…MeatTap to see
Meat has no vitamin C, so meat eaters would get scurvy
No one eats only meat
If it applied to…RiceTap to see
Rice eaters would lack lysine
Beans make up for it
If it applied to…Any dietTap to see
Every diet would be incomplete
Variety is what matters
Contradictions
And doesn't think that dish lacks protein.
The concern only comes up with vegans.
Cattle and chickens eat soy and corn.
What the argument gets right
What is true
It's true that plant proteins have amino acid profiles different from animal ones, and that a diet based on a single plant food would be inadequate. Nutrition deserves attention, and the idea of variety is important.
But
But variety is exactly what solves the problem. With legumes, cereals, soy and nuts over the day, plant protein is complete, and the need for animals disappears.
Connections
Usually comes before
Usually comes after