Protein terminology
Amino acids
Proteins are comprised of 21 amino acids, the ‘building blocks’ which make up proteins. This includes 8 (for adults) or 10 (for infants) amino acids which must be obtained directly from food - called ‘essential’ amino acids.
The remaining amino acids are either ‘non-essential’, which can easily be produced by the body, or ‘conditionally essential’, which can usually be made by the body but may be required in the diet under certain circumstances (such as during illness).
These are:
Essential: Isoleucine, Leucine, Valine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine, Selenocysteine.
Non-essential: Alanine, Aspartic acid, Asparagine, Glutamic acid, Serine.
Conditionally essential: Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine.
A diet comprising a variety of plant protein sources will include all the essential amino acids.1
What are ‘complete’ proteins?
A food which contains all the essential amino acids in sufficient quantities to meet the body’s needs is referred to as a ‘complete’ protein.
A few plant-based foods, such as soya, buckwheat, quinoa and amaranth, are complete proteins.
Most plant-based foods do not contain sufficient quantities of all the essential amino acids; instead, the full range of amino acids is provided in the diet across a variety of different foods.
It is not necessary to eat complete proteins in order to obtain all the essential amino acids.
“Protein combining”
You may still encounter the concept of ‘protein combining’, particularly in older literature. This was a concept from some years ago (devised by the author of a book called Diet for a Small Planet published in the 1970s) that it was necessary to eat all the essential amino acids together in the same meal, by eating certain types of foods together such as rice and beans. It is now widely understood that this is not required - a reasonable variety of protein foods consumed over the course of a day is sufficient.7