Complete proteinHigh-quality protein

A complete protein supplies all nine essential amino acids in proportions the body can use. Animal sources — meat, fish, eggs, dairy — are complete, as are soy and quinoa among plants. Most other plant proteins are limited in one amino acid: grains are low in lysine, pulses low in methionine.

It matters for muscle because leucine, one of the essential amino acids, is the primary trigger for muscle protein synthesis, and because an incomplete profile caps how much of the protein can be used for building tissue.

In practice the old advice about carefully combining plant proteins at every meal has been superseded: eating a varied range of plant proteins across the day covers the profile perfectly well. Plant-based eaters do generally need somewhat more total protein, since plant sources are typically less digestible and lower in leucine.