Complete vs. Incomplete Proteins: What's the Difference?

Protein is made up of amino acids, and the terms 'complete' and 'incomplete' describe whether a food contains all nine essential amino acids your body can't produce on its own and must get from food.

Essential amino acids explained

There are 20 amino acids used to build protein in the body, and your body can synthesize 11 of them. The remaining 9 are 'essential' and must come from your diet — protein synthesis, from building muscle to repairing tissue, requires all nine to be present.

What makes a protein 'complete'

A complete protein contains all nine essential amino acids in meaningful amounts. Most animal proteins — meat, poultry, fish, eggs, and dairy — are complete. A few plant sources are complete as well, including soy, quinoa, and buckwheat.

Common incomplete protein sources

Most individual plant proteins, like beans, lentils, rice, nuts, and seeds, are missing or low in at least one essential amino acid, which classifies them as incomplete on their own.

Complementary proteins solve the gap

Pairing two incomplete proteins that are low in different amino acids — like rice and beans, or hummus and whole-grain pita — creates a combination that supplies all nine essential amino acids, similar to eating a complete protein.

Does it matter if you eat a varied diet?

For most people eating a varied diet across the day, it isn't necessary to pair complementary proteins in the same meal — your body maintains a pool of amino acids and can combine what you eat across breakfast, lunch, and dinner. This distinction matters most for people eating a very narrow range of plant proteins.