Common Names

  • L-leucine
  • Leucinum
  • L-alpha-aminoisocaproic acid
  • Branched-chain amino acid (BCAA)

For Patients & Caregivers

How It Works

Leucine is an essential amino acid required for muscle growth and maintenance.

Leucine is an amino acid that is not made in the human body and is required for muscle maintenance. Therefore, it has to be obtained through dietary sources rich in protein such as dairy, soy, and meats. Leucine is also available as a dietary supplement and is used to improve muscle strength and endurance.

Studies evaluating leucine suggest it may be useful in some populations to prevent or treat loss of skeletal muscle mass and strength, known as sarcopenia. Leucine can also increase insulin secretion, but did not improve sugar levels in diabetic patients.

Purported Uses

  • Muscle strength and endurance
    Some studies show that leucine improves muscle strength and endurance, although it is unclear whether these effects are due to leucine, and not to exercise itself.
  • Diabetes
    In one study, leucine did not affect blood sugar levels in diabetic men. Further research is needed.
  • Sarcopenia
    Leucine may be useful in some populations to prevent or treat loss of skeletal muscle mass and strength. More studies are needed as results are mixed across various populations including frail, elderly, obese, and critically ill patients.

Do Not Take If

  • You are taking insulin and other antidiabetic medications: Leucine can stimulate insulin secretion and may further lower blood glucose levels.
  • You have maple syrup urine disease: Leucine can accumulate in blood or urine resulting in dysfunction of nerve cells.
  • You are taking phosphodiesterase 5 (PDE5) inhibitors (sildenafil): In animal studies, leucine increased the effects of these drugs .

Side Effects

  • May lower blood glucose levels
  • Rare: May cause vitamin B3 and vitamin B6 deficiencies with excessive intake
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For Healthcare Professionals

Scientific Name

2-Amino-4-methylpentanoic acid

Clinical Summary

Leucine is an essential amino acid that is not synthesized in the human body and must be obtained from food consisting of plant or animal protein. Leucine is often used together with valine and isoleucine, which are branched-chain amino acids (BCAA), in parenteral forms for nutritional support to maintain nitrogen balance, and to treat cirrhosis and hepatic encephalopathy.

Purified leucine is marketed as a dietary supplement for body building. It is thought to stimulate protein synthesis by activating the mammalian target of rapamycin (mTOR) pathway (1) (2). In various animal models, leucine supplementation improved leptin sensitivity (18) (19), lipid and glucose metabolism (19), exercise tolerance (20) , acquired growth hormone resistance (21), disease-related skeletal muscle dysfunctions (22), and anemia (23).

An oral leucine-enriched BCAA mixture increased muscle protein synthesis and reversed sarcopenic processes in cirrhotic patients (24). However, leucine supplementation alone did not have additional effects on exercise capacity, muscle mass or quality of life when compared with exercise plus leucine supplementation (25). Leucine did not affect glycemic control in elderly diabetic men (5), but in healthy subjects and in the presence of glucose, stimulated insulin secretion and lowered blood glucose levels (8).

Studies with leucine supplementation and exercise show some benefits in older populations. A leucine-rich essential amino acid mixture along with exercise significantly improved muscle mass and strength, and walking speed in community-dwelling sarcopenic women (26). Leucine-enriched whey protein nutritional supplements improved sarcopenia in older and older-obese adults (27) (28). Another study evaluating combined resistance training with high-dose leucine supplementation in older adults suggests moderate benefit on muscle strength and functional status (29).

In athletes, dietary supplementation with leucine can improve performance (3) and upper body strength (4). However, two double-blind RCTs in younger active populations, one of resistance training along with powdered leucine/protein shakes in young men (30), and another evaluating a postsurgical rehabilitation program along with leucine supplementation among athletes (31) did not find improvements attributable to leucine, although it appeared to promote thigh muscle recovery in the latter study (31).

Findings from systematic reviews are mixed. In older sarcopenia-prone individuals leucine supplementation may improve body weight, body mass index, and lean body mass, but not muscle strength (32). Another review concluded that although central leucine injection decreased food intake, this effect was not well reproduced with oral leucine, even though it improved glucose homeostasis (33). Well designed longer-term studies are needed to determine optimal supplementation with leucine across these varied populations.

Leucine from dietary sources is generally considered safe. However, excessive intake may result in hypoglycemia (8) and may also cause vitamin B3 and B6 deficiencies (9) (10). Amounts exceeding ∼39 g/d may pose a health risk (17). Animal pancreatic cancer models suggest that it may also increase growth of pancreatic tumors (34).

Food Sources

Plant and animal proteins

Purported Uses

  • Muscle strength
  • Endurance
  • Diabetes
  • Sarcopenia

Mechanism of Action

Leucine is among the essential branched-chain amino acids (BCAA) associated with skeletal muscle growth and maintenance, energy production, and generation of neurotransmitter and gluconeogenic precursors (35). It stimulates protein synthesis by activating the mammalian target of rapamycin (mTOR) pathway (1) and is thought to enhance muscle anabolic signaling (11).

Animal models indicate that BCAA metabolism-related gene expression is regulated during adipocyte differentiation and influenced by nutrient levels, and leucine supplementation induced Bcat2 and Bckdha genes during early and late differentiation (35). It improved insulin sensitivity in high-fat diet-fed mice by decreasing adiposity, rather than through direct action on peripheral target organs (36). Leucine supplementation also improved lipid and glucose metabolism and restored leptin sensitivity in previously obese animals (19). At the same time, in already-obese rats, leucine supplementation worsened adiposity by encouraging hypothalamic gene expression that favored fat accumulation (37). In another animal study, it did not reduce food intake or induce an anorectic pattern of hypothalamic gene expression (38). Co-ingested leucine and glycine markedly attenuated glucose response, with only a modest increase in insulin response, suggesting their effects on glucose metabolism are partially insulin-independent (39). Leucine along with subtherapeutic levels of a PDE5 inhibitor altered lipid metabolism from storage to oxidation, improved glycemic control, and reversed hepatic steatosis induced by high-fat feeding (40).

Leucine supplementation speeds connective tissue repair and muscle regeneration by attenuating transforming growth factor-beta type I receptor and activating Smad2/3 (41). Antiatrophic effects were not mediated by its metabolite, beta-hydroxy-beta-methyl butyrate, and did not occur in dexamethasone-treated rats (42).

The ability of leucine to improve anemia in ribosomal protein-deficient cells occurs independently of TP53 (23). In Diamond-Blackfan anemia patients, leucine modulates protein synthesis by enhancing translation leading to improved hemoglobin levels (7). In the skeletal muscle of cirrhotic patients, an oral leucine-enriched BCAA mixture increased autophagy and reversed impaired mTOR1 signaling (24).


  • Patients with maple syrup urine disease should not consume leucine and other branched-chain amino acids as they can accumulate in blood or urine causing neuronal dysfunction (15).

Adverse Reactions

  • May lower blood glucose levels (8).
  • Rare: Excessive intake of dietary leucine may cause vitamin B3 and vitamin B6 deficiencies (9) (10).

Herb-Drug Interactions

  • Insulin and other antidiabetic medications: Leucine can stimulate insulin secretion and may have additive hypoglycemic effects (8) (43).
  • Vitamin B3 and vitamin B6: Leucine can interfere with synthesis of these vitamins (9) (10).
  • Phosphodiesterase 5 (PDE5) inhibitors (sildenafil): Animal models indicate leucine may have synergistic effects (40).

Herb Lab Interactions

  • May lower blood glucose levels (8).

Dosage (OneMSK Only)


  1. Thomson JS, Ali A, Rowlands DS. Leucine-protein supplemented recovery feeding enhances subsequent cycling performance in well-trained men. Appl Physiol Nutr Metab. Apr 2011;36(2):242-253.

  2. Crowe MJ, Weatherson JN, Bowden BF. Effects of dietary leucine supplementation on exercise performance. Eur J Appl Physiol. Aug 2006;97(6):664-672.

  3. Leenders M, Verdijk LB, van der Hoeven L, et al. Prolonged leucine supplementation does not augment muscle mass or affect glycemic control in elderly type 2 diabetic men. J Nutr. Jun 2011;141(6):1070-1076.

  4. Virgilio M. Treatment of Zebrafish Models of Ribosomopathies (Diamond Blackfan Anemia (DBA) and 5q- Syndrome) with L-Leucine Results In An Improvement of Anemia and Developmental Defects: Evidence for a Common Pathway? Paper presented at: American Society of Hematology Annual Meeting2010; Orange County Convention Center.

  5. Jaako P. Bone Marrow Failure in RPS19-Deficient Mice Is Partly Caused by p53 Activation and Responds to L-Leucine Treatment. Paper presented at: American Society of Hematology Annual Meeting2011; San Diego Convention Center.

  6. Kalogeropoulou D, Lafave L, Schweim K, et al. Leucine, when ingested with glucose, synergistically stimulates insulin secretion and lowers blood glucose. Metabolism. Dec 2008;57(12):1747-1752.

  7. Bapurao S, Krishnaswamy K. Vitamin B6 nutritional status of pellagrins and their leucine tolerance. Am J Clin Nutr. May 1978;31(5):819-824.

  8. Glynn EL, Fry CS, Drummond MJ, et al. Excess leucine intake enhances muscle anabolic signaling but not net protein anabolism in young men and women. J Nutr. Nov 2010;140(11):1970-1976.

  9. Cortiella J, Matthews DE, Hoerr RA, et al. Leucine kinetics at graded intakes in young men: quantitative fate of dietary leucine. Am J Clin Nutr. Oct 1988;48(4):998-1009.

  10. Yudkoff M, Daikhin Y, Nissim I, et al. Brain amino acid requirements and toxicity: the example of leucine. J Nutr. Jun 2005;135(6 Suppl):1531S-1538S.

  11. Yudkoff M, Daikhin Y, Grunstein L, et al. Astrocyte leucine metabolism: significance of branched-chain amino acid transamination. J Neurochem. Jan 1996;66(1):378-385.

  12. Kasinski A, Doering CB, Danner DJ. Leucine toxicity in a neuronal cell model with inhibited branched chain amino acid catabolism. Brain Res Mol Brain Res. Mar 30 2004;122(2):180-187.

  13. van Loon LJ. Leucine as a pharmaconutrient in health and disease. Curr Opin Clin Nutr Metab Care. 2012 Jan;15(1):71-7.

  14. Pencharz PB, Elango R, Ball RO. Determination of the tolerable upper intake level of leucine in adult men. J Nutr. 2012 Dec;142(12):2220S-4S.

  15. Yuan XW, Han SF, Zhang JW, et al. Leucine supplementation improves leptin sensitivity in high-fat diet fed rats. Food Nutr Res. 2015;59:27373.

  16. Binder E, Bermudez-Silva FJ, Elie M, et al. Leucine supplementation modulates fuel substrates utilization and glucose metabolism in previously obese mice. Obesity (Silver Spring). Mar 2014;22(3):713-720.

  17. de Moraes WM, Melara TP, de Souza PR, et al. Impact of leucine supplementation on exercise training induced anti-cardiac remodeling effect in heart failure mice. Nutrients. May 2015;7(5):3751-3766.

  18. Shemesh A, Wang Y, Yang Y, et al. Suppression of mTORC1 activation in acid-alpha-glucosidase-deficient cells and mice is ameliorated by leucine supplementation. Am J Physiol Regul Integr Comp Physiol. Nov 15 2014;307(10):R1251-1259.

  19. Narla A, Payne EM, Abayasekara N, et al. L-Leucine improves the anaemia in models of Diamond Blackfan anaemia and the 5q- syndrome in a TP53-independent way. Br J Haematol. Nov 2014;167(4):524-528.

  20. Roman E, Torrades MT, Nadal MJ, et al. Randomized pilot study: effects of an exercise programme and leucine supplementation in patients with cirrhosis. Dig Dis Sci. Aug 2014;59(8):1966-1975.

  21. Liu KA, Lashinger LM, Rasmussen AJ, et al. Leucine supplementation differentially enhances pancreatic cancer growth in lean and overweight mice. Cancer Metab. 2014;2(1):6.

  22. Binder E, Bermudez-Silva FJ, Andre C, et al. Leucine supplementation protects from insulin resistance by regulating adiposity levels. PLoS One. 2013;8(9):e74705.

  23. Iverson JF, Gannon MC, Nuttall FQ. Interaction of ingested leucine with glycine on insulin and glucose concentrations. J Amino Acids. 2014;2014:521941.

  24. Pereira MG, Silva MT, Carlassara EO, et al. Leucine supplementation accelerates connective tissue repair of injured tibialis anterior muscle. Nutrients. Oct 2014;6(10):3981-4001.

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