Complete vs Incomplete Proteins: What's the Difference?

What Are Essential Amino Acids?

Proteins are built from chains of amino acids — 20 total in the human body. Of these, nine are classified as essential amino acids (EAAs): the body cannot synthesise them and must obtain them from food. The remaining 11 are non-essential, meaning the body can produce them from other compounds when needed.

The nine essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Among these, leucine holds a special significance — it is the primary trigger for muscle protein synthesis, meaning its presence in a meal is the key signal that initiates the muscle repair and growth response.

A food protein's value depends not just on how much protein it contains, but on whether it provides all nine essential amino acids in adequate amounts. This is where the complete vs incomplete proteins distinction becomes important.

Complete vs Incomplete Proteins: The Key Distinction

A complete protein is a food source that provides all nine essential amino acids in amounts sufficient to meet human physiological needs. An incomplete protein is one that is low in or missing one or more essential amino acids — called the limiting amino acid — which restricts how much the body can use the protein for synthesis.

Use the Protein Intake Calculator to determine your daily protein target, then choose a mix of complete protein sources — or complementary plant protein combinations — to meet that target effectively.

Based on: FAO. "Dietary Protein Quality Evaluation in Human Nutrition." FAO Food and Nutrition Paper 92. Rome: Food and Agriculture Organization of the United Nations, 2013. FAO ↗

Are Plant Proteins Really "Incomplete"?

The "incomplete protein" label can be misleading — it does not mean plant proteins are unusable. It means that if you rely on a single plant protein source exclusively, you may be limiting your supply of one or more essential amino acids. In practice, anyone eating a varied diet — including multiple plant protein sources across the day — will easily cover all essential amino acids.

The old idea that you had to combine complementary proteins within the same meal (e.g., rice and beans together) to form a complete protein has been revised. Protein complementation works across the full day — your body maintains an amino acid pool that draws from all proteins consumed, not just those in a single meal. Eating rice at lunch and lentils at dinner provides the same complete amino acid profile as eating them together.

How to Combine Plant Proteins for a Complete Amino Acid Profile

While protein complementation does not need to happen within a single meal, it is still useful to understand which plant protein combinations cover all nine essential amino acids effectively:

• Rice + lentils/beans: Grains are lysine-deficient; legumes are high in lysine. Legumes are methionine-deficient; grains are high in methionine. Together they cover all EAAs.
• Hummus + whole grain bread: Chickpeas (legume) + wheat (grain) = complete amino acid profile.
• Pea protein + rice protein: The basis of most plant-based protein powder blends. Together they achieve an amino acid profile comparable to whey.
• Soy-based foods (standalone): Tofu, tempeh, edamame, and soy milk are complete proteins on their own — the gold standard of plant-based protein sources for completeness.

Protein Quality Scores: PDCAAS and DIAAS

Two standardised scores measure protein quality by accounting for both amino acid content and digestibility:

• PDCAAS (Protein Digestibility Corrected Amino Acid Score): The older standard (WHO, 1991). A score of 1.0 is the maximum and indicates a protein that meets all essential amino acid requirements with high digestibility. Whey, casein, egg, and soy proteins all score 1.0.
• DIAAS (Digestible Indispensable Amino Acid Score): The newer, more accurate standard (FAO, 2013). DIAAS can exceed 1.0 for superior proteins and more accurately reflects digestibility at the small intestinal level. Whey protein isolate has a DIAAS of approximately 1.09; soy protein isolate ~0.90; pea protein ~0.82; wheat ~0.40.

In practical terms: animal proteins and soy have the highest quality scores. Plant proteins (outside of soy) have lower DIAAS scores, meaning you need slightly more total grams per kg to achieve the same anabolic effect. This is why plant-based eaters should target the upper end of protein recommendations (1.8–2.2 g/kg) to compensate.

Based on: Berrazaga I, Micard V, Gueugneau M, Walrand S. "The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review." Nutrients. 2019;11(8):1825. PubMed ↗

Practical Takeaways for Meat-Eaters and Plant-Based Diets

For those consuming animal products, complete protein sources are abundant — virtually all meat, fish, dairy, and eggs qualify. The main focus should be on total daily protein per kg body weight and protein distribution across meals.

For plant-based eaters, the practical strategy is to:

• Prioritise soy-based proteins (tofu, tempeh, edamame) as the most complete and leucine-rich plant proteins.
• Combine legumes and grains across the day to cover the lysine-methionine gap.
• Target 1.8–2.2 g/kg total daily protein to compensate for lower DIAAS scores of most plant proteins.
• Consider a pea + rice protein blend if supplementing, as this combination most closely matches whey's amino acid profile.

Review our Best High-Protein Foods for Fitness guide for a full table of animal and plant protein sources ranked by protein density, including their complete vs incomplete status.