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What is Leucine?

In this article, we explain what leucine is and discuss the leucine ‘trigger’ theory and its role in regulating protein synthesis.

Leucine is perhaps the most important amino acid and plays an essential role in regulating the adaptation and recovery of muscles.


What is Leucine Infographic


What are Amino Acids?

There are 20 amino acids that are found naturally in our diets, and these amino acids link together into ‘chains’ to make proteins. Amino acids are like building blocks for protein, and different amino acids have different properties. When protein is eaten from a food or drink, it is digested and broken down, releasing the amino acids which are then absorbed.


Essential and Non-Essential Amino Acids

Of the 20 amino acids, 11 can be produced by the body (known as ‘non-essential amino acids’) whilst 9 cannot be produced in the body and have to be consumed as protein in the diet. Leucine, along with the other BCAAs (see article "What are BCAAs?") are examples of essential amino acids. The essential amino acids are most important for triggering the process of muscle protein synthesis.


What is Muscle Protein Synthesis?

Muscle protein synthesis is the process of building new proteins in muscles. Muscles are made of proteins, which all have different roles. Some provide structure to muscles and are required to generate movement, while others are responsible for producing energy for exercise. Although muscle protein synthesis is often thought of as the process for building larger and stronger muscles when lifting weights, it is also the process for adapting to aerobic exercise and improving endurance exercise performance. Muscle protein synthesis is a short-term process, lasting just a few hours. It is activated by both eating protein and doing exercise, but the combination of exercise and eating protein leads to an even larger increase.


The "Leucine Trigger"

Muscle protein synthesis needs both a ‘trigger’ to start the process, but also a supply of amino acids to be used to build or repair proteins in the muscle. Leucine is thought to act as the biggest stimulus that activates the process, which underpins the ‘leucine trigger’ hypothesis. This is the concept that a certain amount of leucine is required to start the process of muscle protein synthesis, which is approximately 3g of leucine. Leucine has its effect by activating a molecule in the muscle called mTORc1 (the full name is mechanistic target of rapamycin complex 1), which is often described as the ‘master regulator’ of protein synthesis. Because leucine mainly acts as the trigger, other amino acids are needed for the actual building process. Studies have shown that in the short term, 3g of leucine with a small amount of protein (~6g) can stimulate muscle protein synthesis after exercise, but only for a few hours, whilst 25g of protein (also with 3g of leucine) is much better at sustaining it for a longer time (up to 5 hours). Therefore, whole proteins that contain sufficient leucine, but also all other essential amino acids, are the ideal way to stimulate muscle protein synthesis.


Best Food Sources of Leucine

Most proteins contain some leucine, but many contain only small amounts. On average, animal-based proteins have more leucine per serving of protein than plant-based proteins, by around 25%. The milk proteins whey and casein are particularly high in leucine, making them ideal supplemental protein sources. Some plant-based proteins do have large amounts of leucine, such as corn protein, but like most plant-based proteins it is low in other essential amino acids that mean it is not an ideal protein source on its own, unless eaten in larger amounts than animal-based proteins. 3g of leucine can be easily achieved with ~25g of whey protein, whilst ~40g of a plant-based protein like soy needs to be eaten.


Related Articles

What is the Best Protein?

What are Branched Chain Amino Acids (BCAAs)?



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  2. Gorissen SHM, Crombag JJR, Senden JMG, Waterval WAH, Bierau J, Verdijk LB, van Loon LJC. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids. 50(12):1685-95, 2018.

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