Complete proteinHigh-quality protein

A complete protein contains all nine essential amino acids in proportions adequate to support the body's needs. Animal sources, meat, fish, eggs and dairy, are complete, as are soya and quinoa among plant foods.

Incomplete does not mean lacking. Most plant proteins contain all nine amino acids and are simply lower in one of them, typically lysine in grains and methionine in pulses, which is what limits how much tissue can be built from that source alone.

The old advice to combine complementary proteins at every meal, rice with beans in the same sitting, was based on an overly rigid reading. The body maintains a circulating pool of amino acids, so the day's total and variety matter rather than the composition of each plate.

What remains true for plant based eaters is a slightly higher total protein target and more attention to variety, because the margin for a limiting amino acid is smaller and plant protein is generally somewhat less digestible.

Protein quality scores exist to capture this, with DIAAS the current standard and PDCAAS the older one. They rank sources by amino acid profile and digestibility, and they matter most at low total intakes and least when someone is comfortably meeting their needs.

Leucine is the amino acid worth watching in particular, because it acts as the trigger for muscle protein synthesis rather than only as a building block, and it is where several plant sources are lower.

In practice, hit an adequate daily total from varied sources and the completeness question largely dissolves. Our protein intake calculator sets the target, adding a little for a fully plant based diet.