L-Leucine - the indispensable amino acid for your muscles
David Janitzek
2026-03-23
3 min. read
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Ever since bodybuilding gained popularity and sculpting aficionados discovered the significant impact food has on our appearance, the question of the optimal protein amount in one meal has arisen.
"What is leucine?"
Leucine is a member of the family of essential amino acids, which means that the human body is not capable of producing it itself, yet it is indispensable for life. It can also be categorized in the group of branched amino acids, which includes isoleucine and valine. These three branched amino acids are characterized by their unique structure, hence their name. The structure of leucine imparts numerous specific properties to it. Following absorption in the digestive tract, leucine reaches the liver, however, unlike other amino acids, it is not metabolized by the liver. In its fully unaltered state, it enters the bloodstream and moves towards the muscle tissue, where it will be fully utilized.
MTOR anabolic kinase
The contact of leucine with muscle cells can be likened to the arrival of Christmas – many good things happen, and the best part is that we can have these holidays every day, not just once a year. Leucine is detected by muscle cells through mTOR kinases (mammalian Target of Rapamycin). mTOR is a protein activated by leucine but also by other anabolic signals such as HMB, insulin, or IGF-1. This mTOR pathway is considered the main mechanism responsible for anabolic efficiency in muscles. It is activated during and after the performance of strength training. It leads to further activation of other building proteins such as p70S6K and eIF4g, resulting in an increase in muscle protein synthesis and the construction of new muscle fibers. What does this mean? Leucine is found in every food that contains protein, but not all products have the same leucine concentration. A lower percentage of leucine in a product indicates a lower protein quality. Dr. Layne Norton has shown in his research that products with different leucine content can have different effects on anabolism, despite equal protein amounts.
Leucine threshold pertaining to muscle protein synthesis
Upon identifying the correlation between leucine, mTOR kinase and muscle protein synthesis, a further groundbreaking discovery was presented: low-dose leucines do not meet this requirement. There is a specific threshold that needs to be surpassed in order to achieve an optimal anabolic response. Dr. Layne Norton suggests that this amount is between 3.2 and 4.4 g of leucine, although other authors propose different approximative values. This information is key, as consuming a smaller dose, e.g. 1.9 g of Leucine, instead of providing half the effect, will likely result in a complete absence of mTOR stimulation. In this manner, the concept of the 'threshold' that needs to be surpassed to achieve a significant change in muscle anabolism arises. Each food product has a different protein and leucine level, therefore various amounts need to be provided to surpass the leucine threshold.
Practical recommendations regarding protein consumption
Based on the information provided, we can draw a few conclusions that are worth implementing in daily life: Firstly, it is advisable to aim for balanced protein consumption throughout the day, ensuring that each meal provides around 40 g of full-value protein in order to exceed the leucine threshold. Smaller amounts may not trigger any anabolic reactions. As the main protein source in the diet, it is recommended to choose protein-rich products such as protein supplements, eggs, meat, and dairy. Although plant-based products contain all amino acids, including leucine, they contain less of it proportionally. Therefore, vegetarians and vegans need to consume more protein to exceed the leucine threshold.
Leucine-abundant BCAA supplement
For individuals who solely consume plant-based protein sources, a potential alternative may be to supplement their diet with leucine. The best and most accessible solution that works efficiently in this case is a leucine-abundant BCAA supplement. Research findings suggest that such a dietary supplement, which typically contains a small amount of protein, can support the proper muscle protein synthesis.
