Leucine
Leucine is the single most important amino acid for muscle protein synthesis. Hitting around 2.5 to 3 grams of leucine per meal is the well-studied threshold for maximally turning on muscle building, which is why one scoop of quality whey (about 2.5g leucine) is so effective.
Why It Matters
If your goal is gaining or holding lean muscle, leucine is the one amino acid you should know by name. It activates the mTOR pathway, the molecular switch that tells your body to build muscle protein. Without enough leucine in a meal, the rest of the protein you eat is converted to energy or stored, rather than used to repair muscle tissue.
How to Spot It on a Label
Leucine grams per serving rarely appear on the front of a tub. Look on the back of the label under the amino acid profile. Whey concentrate delivers about 2.2 to 2.5g per 25g of protein; whey isolate is slightly higher; collagen and gelatin protein are very low and not appropriate as your main protein.
Examples from real products
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