Protein Glossary

Glossary

Plain-English definitions of the protein science terms you’ll see across ProteinMD.

Amino acids: The 20 building blocks of protein. Nine are essential: your body can’t make them, so they must come from food.
Anabolic resistance: The blunted muscle-building response to protein seen with aging, inactivity or illness, and a reason seniors need more protein per meal.
Anabolic window: The period after exercise when muscles are especially responsive to protein. Practically: eat 20–40 g of quality protein within about 2 hours of training.
Anti-nutrients: Compounds such as phytates, lectins and trypsin inhibitors in some plant foods that reduce protein or mineral absorption. Soaking, sprouting, fermenting and cooking reduce them.
BCAAs: Branched-chain amino acids: leucine, isoleucine and valine.
Bioavailability: How much of a nutrient is digested, absorbed and used by the body. For protein it depends on digestibility and amino acid profile.
Casein: The slow-digesting milk protein (~80% of milk protein). Forms a gel in the stomach and releases amino acids for hours, ideal before sleep.
CKD & eGFR: Chronic kidney disease is staged by eGFR (estimated glomerular filtration rate). Stages 3–5 not on dialysis usually call for reduced protein (0.55–0.6 g/kg per KDOQI 2020).
Collagen: Structural protein rich in glycine and proline but lacking tryptophan, so it is not a complete protein and scores 0 on PDCAAS.
Complementary proteins: Two proteins whose limiting amino acids differ, so eating them together (or across the day) forms a complete profile, like rice and beans.
Complete protein: A protein that supplies all nine essential amino acids in adequate proportions, e.g. eggs, dairy, meat, fish, soy, quinoa and mycoprotein.
DIAAS: Digestible Indispensable Amino Acid Score. The FAO’s preferred method (2013) that measures digestibility of each amino acid at the end of the small intestine. Not capped: ≥100% excellent, 75–99% high, <75% no quality claim.
Essential amino acids (EAAs): Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
g/kg: Grams of protein per kilogram of body weight per day, the standard way to express protein needs. 1 kg = 2.2 lb.
Leucine: A branched-chain essential amino acid that acts as the main trigger for muscle protein synthesis via mTOR.
Leucine threshold: The ~2.5–3 g of leucine per meal needed to maximally stimulate muscle protein synthesis in adults. Older adults may need slightly more.
Limiting amino acid: The essential amino acid in shortest supply relative to need. It caps how much of that protein your body can use. Legumes are usually limited in methionine; grains, nuts and seeds in lysine.
MPS (muscle protein synthesis): The process of building new muscle proteins. Stimulated by protein intake (especially leucine) and resistance exercise.
mTOR: Mechanistic target of rapamycin, a cellular signaling hub that switches on protein synthesis when it senses leucine, insulin and mechanical load.
Mycoprotein: Protein from the fungus Fusarium venenatum (e.g. Quorn). A complete, high-fiber meat alternative.
Nitrogen balance: The difference between nitrogen eaten (from protein) and excreted. Positive balance means net protein gain, e.g. during growth or muscle building.
PDCAAS: Protein Digestibility-Corrected Amino Acid Score. Rates protein quality from 0 to 1.0 using overall (fecal) digestibility. Capped at 1.0, so top-quality proteins look identical.
PKU (phenylketonuria): An inherited disorder in which the body can’t break down phenylalanine. Managed with a Phe-restricted diet and Phe-free protein substitutes.
RDA: Recommended Dietary Allowance: 0.8 g/kg/day for adults. A minimum to prevent deficiency, not an optimal target for active people.
Sarcopenia: Age-related loss of muscle mass and strength. Adequate protein (1.0–1.2 g/kg for adults 65+) and resistance training help prevent it.
Whey: The fast-digesting milk protein (~20% of milk protein). Very high in leucine; popular post-workout.

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