Leucine

The key branched-chain amino acid that triggers muscle protein synthesis.

Leucine is one of the nine essential amino acids and the one with the most direct role in building muscle. Beyond being a building block, it acts as a signal: rising leucine concentration activates mTOR, the pathway that switches on muscle protein synthesis.

That signalling role is why the leucine content of a meal predicts the muscle building response better than the total protein number does. Roughly two to three grams of leucine in a sitting is the commonly cited threshold for a robust response in younger adults, and that is what around 20 to 30 g of a high quality protein typically delivers.

It is one of the three branched chain amino acids, and its reputation launched an entire supplement category. The problem with that category is arithmetic: building tissue requires all nine essential amino acids, so supplying three of them signals the machinery to start a job it lacks materials to finish. BCAA supplements consistently underperform whole protein in trials for that reason.

Older adults appear to need more of it to get the same response, part of the anabolic resistance that makes muscle harder to hold with age, and one of the reasons protein recommendations for older people are higher rather than lower.

Food covers it easily. Dairy and whey are particularly leucine rich, meat, fish and eggs are close behind, and among plant sources soy and pulses do reasonably well while most grains are lower, which is one component of the slightly higher total protein target advised for plant based diets.

There is no case for buying it in isolation if you are eating enough protein. The threshold is met by ordinary meals of a sensible size.

In practice, aim for a decent serving of protein at each of three or four meals and the leucine takes care of itself. Our protein intake calculator sets the daily total.

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