GLOSSARY

Amino Acid Three-Letter Codes Table

Standard codes for the twenty common protein amino acids; one-letter codes are case-sensitive identifiers, not concentration units.

Amino Acid contains amino and carboxyl groups; common alpha forms build proteins. Side chains distinguish types, and not every amino-acid compound belongs to ordinary human proteins.

Common protein amino acids use amino and carboxyl groups in the chain backbone, while side chains create differences. Charge depends on the environment and pH; a neutral textbook drawing is not always the dominant bodily form. A chemical-group name alone establishes neither an ergogenic effect nor a supplement's nutritional value.

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.

Free supplemental leucine and leucine bound in milk protein have different pre-absorption routes. An amino-acid name alone does not require the free form; quantity and dietary adequacy matter.

Stomach and intestinal proteases break protein into smaller peptides and amino acids. Uptake includes free amino acids and di- and tripeptides, with those peptides largely broken down further inside intestinal cells. Absorption does not mean intact food protein travels directly into muscle or retains its original structure there.

Free amino acids in blood and cells form a dynamic pool supplied by food, protein breakdown and synthesis of some types. It is not a large dedicated store like glycogen. Tissue protein also has functional roles, so breaking down muscle is not simply a cost-free withdrawal from stored food.

Amino acids also supply non-protein compounds: arginine participates in nitric-oxide production and tryptophan in serotonin pathways. Such roles do not mean a supplement increases the final product without limit. Enzymes, regulation and other substrates control pathway use, so a mechanism alone does not establish a practical sporting benefit.

Protein sources bring other components: pulses supply fibre and carbohydrate, dairy can supply calcium, and meat or fish types differ in fat. Budget, preferences and meal purpose therefore matter. Foods with equal protein are not necessarily equal in energy or accompanying nutrients, so source selection needs a whole-meal view.

Table references: WHO · 1 · IUPAC Gold Book · 2