L-Leucine: Amino Acid, Daily Requirement and State of Research

What is L-Leucine?

L-Leucine is one of the nine essential amino acids. “Essential” means the body cannot produce it itself and must obtain it through food. Together with isoleucine and valine, leucine belongs to the group of branched-chain amino acids, commonly abbreviated as BCAAs.

All amino acids serve as building blocks for muscle protein. In research, leucine is additionally described as a “trigger” of muscle protein synthesis (MPS) (Phillips, 2017).

What role does leucine play in the mTORC1 signalling pathway?

Research associates leucine with the activation of the mTORC1 signalling pathway (mechanistic target of rapamycin complex 1) in skeletal muscle (Phillips, 2017). In the literature, this pathway is considered central to the regulation of muscle protein synthesis.

Whether muscle protein synthesis after a meal only rises markedly once a certain amount of leucine is reached (leucine threshold) is assessed differently in research (Zaromskyte et al., 2021; Wilkinson et al., 2023).

How much L-Leucine do you need per day?

The basic requirement for leucine, according to the WHO/FAO/UNU, is 39 mg per kilogram of body weight per day (Joint WHO/FAO/UNU Expert Consultation, 2007). These values refer to healthy adults without significant physical activity:

What is the leucine threshold?

The leucine threshold refers to the amount of leucine per meal at which, according to this concept, muscle protein synthesis rises markedly. The International Society of Sports Nutrition specifies 700 to 3,000 mg of leucine for a single serving of protein (Jäger et al., 2017).

The International Society of Sports Nutrition recommends ideally spreading protein servings evenly across the day, every 3 to 4 hours (Jäger et al., 2017).

You can find more context on how protein and amino acids should be distributed across the day in the article How much protein you really need.

What about leucine as a standalone supplement?

Pure L-leucine powder or BCAA supplements are marketed in the sports world. The evidence for isolated leucine supplementation is mixed. A review considers the claim that dietary intake of BCAAs increases muscle protein synthesis in humans to be unwarranted (Wolfe, 2017). The International Society of Sports Nutrition points out that a supplement should not consist of leucine alone (Jäger et al., 2017).

The International Society of Sports Nutrition advises getting enough leucine at every meal by choosing high-quality protein sources (Jäger et al., 2017). For more on which supplements are useful and which are not, see the article Supplements Guide.

Sources:

• Joint WHO/FAO/UNU Expert Consultation (2007). Protein and amino acid requirements in human nutrition. World Health Organization Technical Report Series, 935, 1-265.
• Wolfe RR (2017). Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition, 14, 30. DOI: 10.1186/s12970-017-0184-9
• Phillips SM (2017). Current concepts and unresolved questions in dietary protein requirements and supplements in adults. Frontiers in Nutrition, 4, 13. DOI: 10.3389/fnut.2017.00013
• Jäger R, Kerksick CM, Campbell BI et al. (2017). International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition, 14, 20. DOI: 10.1186/s12970-017-0177-8
• Zaromskyte G, Prokopidis K, Ioannidis T, Tipton KD, Witard OC (2021). Evaluating the leucine trigger hypothesis to explain the post-prandial regulation of muscle protein synthesis in young and older adults: a systematic review. Frontiers in Nutrition, 8, 685165. DOI: 10.3389/fnut.2021.685165
• Wilkinson K, Koscien CP, Monteyne AJ, Wall BT, Stephens FB (2023). Association of postprandial postexercise muscle protein synthesis rates with dietary leucine: a systematic review. Physiological Reports, 11(15), e15775. DOI: 10.14814/phy2.15775