GLOSSARY

Protein Types can be grouped by origin, function, structure or processing, depending on the question. Categories overlap: one source may be plant-based, isolated and nutritionally suitable at once.

• Classification: Most proteins use combinations of 20 common amino acids, with sequence and quantity varying between proteins. Essentiality, side-chain structure and metabolic fate answer different classification questions. One amino acid can be both essential and branched-chain; these categories overlap rather than necessarily replacing one another or identifying different dietary substances.
• Structure: Sequence, local chain shapes, a chain's three-dimensional arrangement and assembly of subunits are different structural descriptions. Not every protein has multiple subunits. These are not four food types; after digestion, amino acids can build new proteins with different sequences and functions rather than reproduce the original food protein intact.

Nutritional protein quality concerns essential-amino-acid pattern and usability relative to requirements. It does not rate all food properties such as fibre, fat, salt or price. A tiny amount of a highly scored protein may not meet daily needs, while a lower score does not make a food nutritionally useless.