Leucine is one of the 8 essential amino acids which means they cannot be manufactured by the body so therefore must be ingested.

Due to its molecular structure, Leucine is classified as a “branched chain amino acid” along with valine and iso-leucine. Unlike other amino acids, the branched chain amino acids are metabolized in the muscle and not the liver. Thus, when ingested they are absorbed and transported unchanged directly to the muscle. They make up approximately 1/3 of skeletal muscle and have been shown to have significant anti-catabolic properties.
Leucine has been shown to have a specific role in stimulating protein synthesis. It does this by activating the signaling pathways that are responsible for telling new muscle cells to grow. Thus Leucine can be considered as a “limiting nutrient” in that it dictates how amino acids are used regardless of how much protein you consume. Leucine is present in Whey but free form Leucine has been shown to spike blood plasma levels much quicker. Studies have shown that the highest rates of muscle protein synthesis occur following ingestion of carbohydrate + protein + Leucine. We all know that carbohydrates spike insulin levels which shuttles amino acids to the muscle. Protein provides the complete range of the 20 amino acids needed for muscle growth. Leucine then switches on the whole muscle building process.
Take home message
Leucine is the amino acid that triggers muscle protein synthesis. Adding free form leucine to a Carb/Protein recovery drink will optimize lean muscle mass growth. Recommended quantities are 5-10g of free form Leucine. Specific dosages have been estimated at 0.1g/kg BW (body weight). Some very recent studies have suggested that it is advantageous to keep Leucine plasma levels elevated for several hours post training. Thus, a very effective strategy (and very new!) would be to take leucine immediately post training (with Carbs/Protein) and the same dose with your next meal 1-2hrs post training.

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