A quick reference for the terminology used throughout this site — useful if a guide or comparison page uses a term you want defined without leaving the page you were on.
Amino acids and completeness
Essential amino acid: One of nine amino acids the human body cannot synthesize on its own and must obtain from food. Complete protein: A protein source containing all nine essential amino acids in meaningful amounts. Limiting amino acid: The essential amino acid present in the lowest relative amount in a given protein, which caps how much of that protein your body can actually use for building new tissue. Leucine: The specific essential amino acid that most directly triggers muscle protein synthesis via the mTOR pathway — see our full explainer.
Quality scoring
PDCAAS: Protein Digestibility-Corrected Amino Acid Score — the older FAO/WHO protein quality standard, capped at 1.0. DIAAS: Digestible Indispensable Amino Acid Score — the newer, uncapped, more precise successor standard. See our full comparison.
Physiology and process terms
Muscle protein synthesis (MPS): The biological process of building new muscle tissue from amino acids; the primary mechanism protein intake and resistance training both drive. Anabolic resistance: The reduced muscle protein synthesis response to a given protein dose seen in aging muscle, requiring a higher per-meal dose in older adults to achieve the same effect. Nitrogen balance: A research method estimating protein adequacy by measuring nitrogen intake versus nitrogen excretion — the basis for the original RDA. Gluconeogenesis: The liver's process of converting amino acids (among other substrates) into glucose, relevant to protein's modest, delayed effect on blood sugar.
Diet and product terms
Whey concentrate/isolate/hydrolysate: Three processing levels of milk-derived whey protein, differing in fat/lactose content and cost — see our full comparison. Casein: The slow-digesting counterpart to whey, also derived from milk. DIAAS-uncapped: Refers to scores above 1.0 possible under the DIAAS system but not PDCAAS. Thermic effect of food (TEF): The energy cost of digesting and metabolizing a food — highest for protein among the three macronutrients.
Trend-specific terms
Protein-maxxing: The broader 2025-2026 social media trend of aggressively prioritizing protein intake, often via fortified processed foods and supplement stacking. Fibermaxxing: The related, earlier trend emphasizing fiber intake — see our guide on why the two actually complement each other, rather than compete.