Leucine

Leucine is one of nine essential amino acids and one of three branched-chain amino acids (BCAAs), along with isoleucine and valine. Its central role in nutrition science is disproportionate to its share of dietary protein: leucine is the principal nutritional signal that triggers skeletal muscle protein synthesis, independent of energy status or insulin.

Molecular mechanism

Cellular leucine concentration is sensed by two complementary mechanisms. Sestrin2, identified by Chantranupong and colleagues at the Whitehead Institute, binds leucine directly and releases its inhibition of GATOR2, permitting activation of the mammalian target of rapamycin complex 1 (mTORC1). Leucyl-tRNA synthetase (LeuRS) additionally functions as a GTPase-activating protein for RagD, further promoting mTORC1 recruitment to the lysosomal surface where its kinase activity phosphorylates downstream targets p70S6K and 4E-BP1, driving cap-dependent translation initiation.

The leucine threshold

Work by Phillips, Churchward-Venne, and others established a dose-response relationship between per-meal leucine content and the muscle protein synthesis response. In young healthy adults, ingestion of 20-25 g high-quality protein (providing approximately 2.5-3.0 g leucine) maximally stimulates MPS for 2-3 hours. In older adults (>65 years), anabolic resistance blunts this response, and higher per-meal leucine (3-4 g, typically from 30-40 g protein) is often recommended to achieve comparable MPS activation. This is the mechanistic basis for the 2013 ESPEN / PROT-AGE consensus recommendation of 1.0-1.2 g/kg/day protein for healthy older adults.

Leucine content of common protein sources

Per USDA FoodData Central amino acid tables, leucine content as a percentage of total protein: whey protein isolate 10-12%, whole egg 8.5%, beef (cooked) 8%, chicken breast 7.8%, soy protein isolate 7.8%, casein 8.5%, wheat gluten 6.8%, rice protein 8.2%, pea protein 8%. Whey's high leucine content and rapid digestion kinetics explain its persistent use as the reference protein in MPS research.

Isolated leucine supplementation

Free-form leucine added to suboptimal protein boluses can elevate MPS transiently, but evidence suggests the response is not sustained without the full complement of essential amino acids — consistent with the "leucine trigger, EAA substrate" conceptualization. Isolated leucine supplementation is therefore not generally recommended when adequate whole-protein intake is feasible.

Metabolism and safety

Leucine is catabolized by branched-chain aminotransferase and branched-chain alpha-ketoacid dehydrogenase, yielding acetyl-CoA and acetoacetate (ketogenic fate). Plasma leucine is a marker of BCAA status and has been associated with insulin resistance in some observational studies, though the causal direction remains debated. No upper intake level has been formally established; chronic intakes up to 500 mg/kg/day have been studied without adverse effect in healthy adults.