Glossary
Leucine.
Direct answer
Leucine is an indispensable branched-chain amino acid that the body cannot make for itself. Leucine serves not only as a building block but also as a signal: a rise in its blood concentration activates the mTOR pathway inside the muscle cell and starts the assembly of muscle protein.
The signal is saturable. Above a certain intake the response no longer grows, and without the other amino acids the building material is simply missing. Leucine on its own therefore builds no protein at all; it gives the starting signal, while the limiting amino acid decides how far construction actually goes.
Most leucine research comes from humans and rodents, using meals built around concentrated protein. The horse takes protein from forage across the whole day, in smaller waves and with different digestive kinetics. Carrying intake patterns from one species to the other is therefore not admissible, however similar the underlying biochemistry appears.
The evidence letter
Once a month: new syntheses, checked studies, and what has changed in the state of the evidence.