Is a Proteinogenic Amino Acid Absent in Plants? Dispelling the Nutritional Myth

November 27, 2025 •

4 min read

Contrary to a common and persistent nutritional myth, no single proteinogenic amino acid is absent in plants. This misconception, which has historically influenced dietary advice, is based on a misunderstanding of what makes a protein 'complete'. In reality, plants are the foundational producers of all 20 standard amino acids for the entire food chain.

Quick Summary

Plants can synthesize all 20 proteinogenic amino acids needed for life. The widespread belief that plants are missing amino acids comes from their varied proportions of essential amino acids for humans, not from a complete absence.

Key Points

No Proteinogenic Amino Acid Is Absent: Plants can synthesize all 20 standard amino acids required for protein synthesis, a capability animals lack.
The "Incomplete Protein" Myth: The belief stems from the fact that individual plant foods may contain lower levels of certain essential amino acids compared to human dietary needs, not a total absence.
Universal Genetic Code: All life, including plants, uses the same set of 20 proteinogenic amino acids, underscoring the universality of the genetic code.
Complementary Proteins Are Not Required Meal-by-Meal: The human body maintains a pool of amino acids, so eating a variety of plant foods throughout the day is sufficient to meet protein needs.
Beyond Protein Building Blocks: Amino acids in plants also serve as precursors for many other critical compounds, including hormones, pigments, and secondary metabolites.
Non-Proteinogenic Amino Acids: Non-protein building blocks like taurine are not found in plants, but this does not affect the completeness of plant protein.

The Biological Imperative: Why Plants Make All Amino Acids

At the most fundamental level of biology, all life forms, from bacteria to plants and animals, use a universal genetic code to build proteins from the same 20 standard amino acids. For a plant to exist and grow, it must have the ability to synthesize every one of these amino acids. Unlike humans and other animals that must obtain 'essential' amino acids from their diet, plants possess the complex biosynthetic pathways to create them all from simple components like atmospheric carbon dioxide and nitrogen from the soil. These metabolic processes primarily occur in the chloroplasts and cytosol of plant cells.

The Source of the 'Incomplete Protein' Myth

The confusion around which amino acid is absent in plants stems from the concept of "incomplete protein," a nutritional idea that gained prominence in the 1970s. This theory suggested that because some individual plant foods, like grains or legumes, are low in one or more of the nine essential amino acids required by humans, they must be combined to form a "complete" protein. However, this is a misinterpretation of a plant's metabolic capacity. A limiting amino acid simply means that it is present in a smaller proportion relative to the human dietary requirement, not that it is entirely missing.

Plant Amino Acid Biosynthesis vs. Animal Amino Acid Acquisition


Feature	Plant Amino Acid Synthesis	Animal Amino Acid Acquisition
Capacity	Can synthesize all 20 standard proteinogenic amino acids.	Cannot synthesize all 20; must acquire 'essential' amino acids from diet.
Pathway	Complex biosynthetic pathways starting from basic inorganic sources like nitrates and sulfates.	Involves digestion of food proteins and breakdown into constituent amino acids.
Primary Source	Internally produced from carbon and nitrogen assimilation.	Obtained externally by consuming other organisms (plants or animals).
Energy Cost	High energy investment in synthesizing all amino acids de novo.	Lower energy cost for acquiring pre-made amino acids via food.

The Role of Limiting Amino Acids and Complementary Proteins

Modern nutritional science has largely debunked the necessity of strict meal-by-meal protein combining. The human body maintains a pool of free amino acids that can be used throughout the day to construct necessary proteins. As long as a varied plant-based diet is consumed, all essential amino acids will be obtained over the course of a day. For instance, grains often have a lower concentration of lysine, while legumes (like beans) are lower in methionine. By consuming a combination of grains and legumes over the day, a vegan or vegetarian easily meets their amino acid needs, though they don't need to be eaten at the same time.

Amino Acid Pathways and Functions in Plants

Plants rely on several major biosynthetic pathways to create the diverse range of amino acids. These pathways are integral to everything from basic growth to specific defense mechanisms.

The Aspartate Family: The synthesis of lysine, methionine, threonine, and isoleucine begins with the precursor aspartate. This pathway is a frequent target for biofortification to increase essential amino acid content in crops.
The Glutamate Family: This pathway produces glutamate, glutamine, arginine, and proline. These are critical for nitrogen transport and storage in the plant.
The Aromatic Family: Tryptophan, phenylalanine, and tyrosine are synthesized via the shikimate pathway. These are precursors for a wide array of secondary metabolites, such as hormones and pigments.
The Pyruvate Family: The branched-chain amino acids valine, leucine, and isoleucine are produced from pyruvate.
The Serine Family: Glycine, cysteine, and serine are created from 3-phosphoglycerate and are involved in various metabolic processes, including stress responses.
The Histidine Pathway: A unique and complex pathway, histidine biosynthesis is critical for plant development and is a conserved pathway across plants and microorganisms.

Special Case: Non-proteinogenic Amino Acids

It is true that some amino acid-like compounds are not found in plants. The most notable example is taurine. Taurine is not a building block for protein but plays other physiological roles, particularly in animals. Since it is not a proteinogenic amino acid, its absence in plants does not make plant protein incomplete. This important distinction separates a specific nutritional lack from a broader metabolic myth.

Conclusion: Plants as the Ultimate Amino Acid Source

In summary, the notion that a proteinogenic amino acid is absent in plants is a fallacy. Plants are masterful biochemists, capable of producing all the necessary building blocks for their own proteins, as well as providing the foundational source for all protein in animal diets. The so-called "incomplete" nature of some plant proteins simply refers to a lower relative concentration of certain essential amino acids for human nutrition, which is easily remedied with a varied diet. Embracing a diverse array of plant-based foods ensures adequate intake of all amino acids without needing to combine them in every meal.

For more in-depth information on plant amino acid metabolism, a review published in Frontiers in Plant Science offers excellent insights into the regulation and functions of amino acids in plants.

Frequently Asked Questions

No, this is a misconception. Your body maintains a pool of free amino acids from which it can draw to synthesize proteins as needed. As long as a varied diet of plant foods is consumed throughout the day, all essential amino acid needs will be met.

The myth originated from research in the 1970s that highlighted the lower concentrations of certain essential amino acids in individual plant foods compared to animal proteins. This was misinterpreted to mean that specific amino acids were entirely absent.

Essential amino acids are those that an organism, like a human, cannot synthesize on its own and must obtain from its diet. Non-essential amino acids can be synthesized internally. Plants synthesize all of them.

Plants have complex biosynthetic pathways that produce amino acids from simple precursors. These processes use intermediates from core metabolic cycles, such as glycolysis and the citric acid cycle, along with nitrogen from the environment.

Taurine is an amino acid-like compound not found in plants. However, it is not a proteinogenic amino acid, meaning it is not used to build proteins. Its absence does not affect the 'completeness' of plant protein for building tissue.

Some plant foods that contain all essential amino acids in higher proportions, closer to human requirements, include soy, quinoa, buckwheat, and hemp seeds.

A limiting amino acid is one that is present in the lowest amount relative to a person's dietary needs. For example, lysine is often limiting in grains, while methionine is often limiting in legumes.