Ingredient
Strong evidenceLeucine
The amino acid that flips the switch on muscle protein synthesis.
What it is
Leucine is one of the nine essential amino acids and the most important of the three branched-chain amino acids (BCAAs). It is the single amino acid most directly responsible for switching on muscle protein synthesis, the process that builds and repairs muscle.
How it works
Leucine acts as the trigger that signals the body to start building muscle protein after you eat protein or train. Hitting a meaningful leucine threshold in a meal or shake maximises that signal. This is why a complete protein or a full EAA blend matters: leucine flips the switch, but the other essential amino acids are the building blocks it needs to finish the job.
Effective dose
Around 2 to 3g of leucine per serving is the threshold that robustly stimulates muscle protein synthesis. You get this naturally from roughly 20 to 25g of a quality protein like whey, so most people who hit their protein target do not need to supplement leucine separately.
Safety and upper limit
Leucine and the other amino acids are safe at normal supplemental doses. Taking large amounts of isolated leucine or BCAAs without the other essential aminos is simply wasteful rather than dangerous, because the body cannot build muscle from leucine alone.
Where you find it
Whey and other complete proteins, EAA blends and (less usefully) BCAA products.
Top EAAs picks
The highest-scoring products in the category where Leucine matters most.
Frequently asked
How much leucine do I need?
Around 2 to 3g per serving maximises the muscle-building signal. You get that from roughly 20 to 25g of a quality protein like whey.
Should I supplement leucine on its own?
Usually not. Leucine triggers muscle protein synthesis but needs the other essential amino acids as building blocks, so a complete protein or full EAA blend is better.
Are BCAAs enough for muscle growth?
No. BCAAs provide leucine but lack the other six essential amino acids needed to actually build muscle, so a full EAA blend or whole protein is superior.
Leucine vs ...
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This page is educational and not medical advice. Doses are general guidance from the published research, not a personal prescription. Before starting any supplement that could affect a medical condition or interact with medication, speak to a qualified clinician.