Understanding Limiting Amino Acids: How to Know if Your Protein Is Complete and Balanced

When choosing a protein powder or evaluating your diet, you have probably heard the term “complete protein.” But what does that actually mean — and how can you tell if your protein source truly provides everything your body needs?

To answer that, we need to understand a key concept in protein nutrition: the limiting amino acid.

---

What Are Amino Acids?

Proteins are made up of smaller building blocks called amino acids.
There are 20 amino acids used by the human body, divided into two main categories:

• Essential amino acids (EAAs) — 9 that your body cannot make, so they must come from your diet.

• Non-essential amino acids — 11 that your body can synthesize.

The nine essential amino acids are:

Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.

To synthesize body proteins — like muscle tissue, enzymes, and hormones — all nine must be present in adequate amounts.
If one is lacking, your body’s ability to build new protein is limited, no matter how much total protein you consume.

---

What Is a Limiting Amino Acid?

A limiting amino acid is the essential amino acid present in the lowest proportion relative to the body’s requirement.

Imagine building a house: even if you have plenty of bricks and wood, construction stops if you run out of nails.
Similarly, if your diet lacks even one essential amino acid, the body cannot efficiently use the rest — the missing one becomes the limiting factor in protein synthesis.

---

Why Limiting Amino Acids Matter

When a protein source is low in one or more essential amino acids, it’s considered incomplete.
This doesn’t mean it’s “bad” — but it means your body can’t fully utilize it unless you combine it with another protein that supplies the missing amino acid.

Common Limiting Amino Acids by Source

Protein Source	Typical Limiting Amino Acid
Pea protein	Methionine
Rice protein	Lysine
Hemp protein	Lysine
Wheat protein (gluten)	Lysine
Corn protein	Tryptophan
Legumes in general	Methionine
Grains in general	Lysine

This is why combining plant proteins — for example, rice + pea — creates a more complete amino acid profile.

---

How to Determine if a Limiting Amino Acid Is Sufficient

To find out whether a protein source provides enough of each essential amino acid, scientists compare its amino acid content to human requirements established by the FAO/WHO (Food and Agriculture Organization / World Health Organization).

FAO/WHO Reference Pattern for Adults 2007

(Amino acid requirement per gram of protein)

Essential Amino Acid	Requirement (mg per g protein)
Histidine	15
Isoleucine	30
Leucine	59
Lysine	45
Methionine + Cysteine	22
Phenylalanine + Tyrosine	38
Threonine	23
Tryptophan	6
Valine	39

---

The Amino Acid Score (AAS)

To assess sufficiency, nutrition scientists calculate the Amino Acid Score (AAS):

AAS = (Amino acid content in the protein / FAO requirement for that amino acid)

You do this for all 9 essential amino acids, then identify the lowest score — that’s your limiting amino acid and the protein’s overall AAS.

• AAS = 1.0 (or 100%) → Sufficient; all EAAs meet or exceed requirements.

• AAS < 1.0 → Limiting amino acid present in insufficient amount.

Example:

Pea protein contains ~15 mg methionine + cysteine per g protein.
Requirement = 22 mg/g → AAS = 15 ÷ 22 = 0.68

Methionine is the limiting amino acid, meaning pea protein alone is incomplete.
If combined with rice protein (rich in methionine), the total AAS rises close to 1.0 — a complete profile