Essential Amino Acids, Leucine, and BCAAs

Proteins are built from 20 amino acids; 9 are essential (EAAs) and must come from the diet (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine). The EAAs — not the non-essential ones — are what trigger muscle protein synthesis (MPS) (see muscle-hypertrophy). The ISSN notes that roughly 10 g of EAAs, whether free-form or within a 20–40 g protein bolus, maximally stimulates MPS.

Leucine is the key signaling amino acid: it activates the mTOR pathway that switches on MPS. An effective dose contains about 700–3000 mg of leucine (see protein, protein-distribution). This is why leucine content is a marker of protein quality and why whey (leucine-rich) is effective post-workout (see whey-protein-powder).

BCAAs (branched-chain amino acids) — leucine, isoleucine, valine — are sold as a standalone supplement. The evidence is underwhelming: when total protein intake is already adequate, BCAA supplements add little or nothing, because building muscle requires the full set of EAAs, not just three. Leucine alone or BCAAs can transiently raise MPS signaling but cannot sustain net muscle building without the other EAAs. Whole protein or complete EAA blends are superior and usually cheaper per useful gram (see supplements-skeptical-note). BCAAs are largely a redundant product for anyone eating sufficient quality protein.

• 9 essential amino acids; EAAs (not non-essential) drive MPS.
• ~10 g EAAs or a 20–40 g protein bolus maximally stimulates MPS.
• Leucine is the trigger (mTOR); effective dose ~700–3000 mg leucine.
• BCAAs (leu/ile/val) add little when total protein is adequate.
• Full EAAs/whole protein beat isolated BCAAs for building muscle.

• protein — EAAs within total protein needs.
• protein-distribution — leucine threshold per dose.
• whey-protein-powder — leucine-rich complete protein.
• supplements-skeptical-note — why BCAAs are usually redundant.