Leucine Supplementation Does Not Alter Insulin Sensitivity in Prefrail and Frail Older Women following a Resistance Training Protocol.
Jacob KJ, Chevalier S, Lamarche M, Morais JA.
J Nutr. 2019 Jun 1;149(6):959-967. doi: 10.1093/jn/nxz038.

Leucine-rich repeat kinase 2 (LRRK2) inhibitors differentially modulate glutamate release and Serine935 LRRK2 phosphorylation in striatal and cerebrocortical synaptosomes.
Mercatelli D, Bolognesi P, Frassineti M, Pisanò CA, Longo F, Shimshek DR, Morari M.
Pharmacol Res Perspect. 2019 May 27;7(3):e00484. doi: 10.1002/prp2.484.

A 9-year neuropsychological report of a patient with LGI1-associated limbic encephalitis.
Zangrandi A, Gasparini F, Marti A, Bhalla R, Napoli M, Angelini D, Ghidoni E, Rizzi R.
J Clin Exp Neuropsychol. 2019 May 30:1-11. doi: 10.1080/13803395.2019.1617836.

Leucine co-ingestion augments the muscle protein synthetic response to the ingestion of 15 g protein following resistance exercise in older men.
Holwerda AM, Paulussen KJM, Overkamp M, Goessens JPB, Kramer IF, Wodzig WKWH, Verdijk LB, de Groot LCPGM, van Loon LJC.
Am J Physiol Endocrinol Metab. 2019 May 21. doi: 10.1152/ajpendo.00073.2019.

Leucine-rich Repeat Neuronal Protein-1(LRRN1) Suppresses Apoptosis of Gastric Cancer Cells through Regulation of Fas/FasL.
Liu B, Zhang Y, Fan Y, Wang S, Li Z, Deng M, Li C, Wang J, Ma R, Wang X, Wang Y, Xu L, Hou K, Che X, Liu Y, Qu X.
Cancer Sci. 2019 May 13. doi: 10.1111/cas.14042.

Thirteen weeks of supplementation of vitamin D and leucine-enriched whey protein nutritional supplement attenuates chronic low-grade inflammation in sarcopenic older adults: the PROVIDE study.
Liberman K, Njemini R, Luiking Y, Forti LN, Verlaan S, Bauer JM, Memelink R, Brandt K, Donini LM, Maggio M, Mets T, Wijers SLJ, Sieber C, Cederholm T, Bautmans I.
Aging Clin Exp Res. 2019 May 2. doi: 10.1007/s40520-019-01208-4.

A meta-analysis of leucine-rich repeat kinase 2 (LRRK2) polymorphisms in Alzheimer's disease.
Fatahian R, Bagheri SR, Sadeghi M.
Folia Neuropathol. 2019;57(1):1-5. doi: 10.5114/fn.2019.83825.

Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears.
Zheng R, Huang S, Zhu J, Lin W, Xu H, Zheng X.
Cell Tissue Res. 2019 Apr 24. doi: 10.1007/s00441-019-03021-x.

Glucocorticoid-Induced Leucine Zipper: A Novel Anti-inflammatory Molecule.
Bereshchenko O, Migliorati G, Bruscoli S, Riccardi C.
Front Pharmacol. 2019 Mar 27;10:308. doi: 10.3389/fphar.2019.00308.

Efficacy and Safety of Leucine Supplementation in the Elderly.
Borack MS, Volpi E
J Nutr. 2016 Dec;146(12):2625S-2629S. Epub 2016 Nov 9.

Veterinary medicine

Effects of leucine supplementation on muscle protein synthesis and degradation pathways in broilers at constant dietary concentrations of isoleucine and valine.
Zeitz JO, Käding SC, Niewalda IR, Machander V, de Paula Dorigam JC, Eder K.
Arch Anim Nutr. 2019 Apr;73(2):75-87. doi: 10.1080/1745039X.2019.1583519.

Leucine alone or in combination with glutamic acid, but not with arginine, increases biceps femoris muscle and alters muscle AA transport and concentrations in fattening pigs.
Hu C, Li F, Duan Y, Kong X, Yan Y, Deng J, Tan C, Wu G, Yin Y.
J Anim Physiol Anim Nutr (Berl). 2019 May;103(3):791-800. doi: 10.1111/jpn.13053.

L-leucine is the α-isobutyl-substituted amino acid of proteins that the human body cannot produce, but which must be assumed with the intake of foods such as wholemeal flour, dried fruit and others.

It is an α-amino acid and Leucine isomer.

What it is used for and where

Amino acids play a key metabolic function in the human body and are constituents of proteins. 

Food

Ingredient included in the list of European food additives as E641 with the function of flavour enhancer to improve the taste of food and beverages.

Food safety: amino acid α generally considered safe.

Cosmetics

Amino acids together with their salts are used in cosmetics as conditioning agents for both hair and skin (e.g. as moisturisers and other similar functions). Moisturisers are different in nature: the best are the natural ones that exploit the mechanism of integration between the ingredient and the skin by moisturising the horny hydrolipid film, i.e. the thin protective layer that covers the epidermis protecting it from harmful external microbes, keeping the skin moisturised and supple and its pH or acidity value between 4 and 6.  Then there are the occlusive moisturisers, usually derived from petroleum (Paraffinum, Paraffinum liquidum and others), but also triglycerides, lanolin oil, natural or synthetic waxes, fatty acid esters and others that create an artificial occlusive layer on the stratum corneum of the skin with the advantage of accelerating the protective process but with the disadvantage of preventing the skin's natural transpiration.

• Antistatic agent. Static electricity build-up has a direct influence on products and causes electrostatic adsorption. The antistatic ingredient reduces static build-up and surface resistivity on the surface of the skin and hair.
• Hair conditioning agent. A significant number of ingredients with specific and targeted purposes may co-exist in hair shampoo formulations: cleansers, conditioners, thickeners, matting agents, sequestering agents, fragrances, preservatives, special additives. However, the indispensable ingredients are the cleansers and conditioners as they are necessary and sufficient for hair cleansing and manageability. The others act as commercial and non-essential auxiliaries such as: appearance, fragrance, colouring, etc. Hair conditioning agents have the task of increasing shine, manageability and volume, and reducing static electricity, especially after treatments such as colouring, ironing, waving, drying and brushing. They are, in practice, dispersants that may contain cationic surfactants, thickeners, emollients, polymers. The typology of hair conditioning agents includes: intensive conditioners, instant conditioners, thickening conditioners, drying conditioners. They can perform their task generally accompanied by other different ingredients.
• Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.

Cosmetic safety: amino acid α generally considered safe when formulated to be non-irritant.

Safety

α-amino acids that have similar physical structures undergo similar changes with regard to solubility in water/ethanol mixtures, and technologies to separate α-amino acids from industrial residues, which may not even be innocuous, are constantly being improved. However, many data on the solubility in water-ethanol and ethanol of some α-amino acids are contradictory or even lacking, and the effects of ethanol on the solubility of amino acids may be different. Overall, the scientific literature considers that α-amino acids do not pose significant problems for human health when taken orally, except in people with certain genetic diseases.

Medical

It was discovered that it produces anabolic effects on muscle protein synthesis and cell signaling (1).

Rotator cuff tears (RCTs), the most common tendon injury, are always accompanied by muscle atrophy, which is characterized by excessive protein degradation. Autophagy-lysosome systems are the crucial proteolytic pathways and are activated in atrophying muscle. Leucine potentially attenuates tendon transection-induced muscle atrophy through autophagy inhibition (2).

The most relevant studies on this ingredient have been selected with a summary of their contents:

L-leucine studies

• Molecular Formula: C₆H₁₃NO₂
• Linear Formula (CH₃)₂CHCH₂CH(NH₂)CO₂H 
• Molecular Weight: 131.175 g/mol
• UNII: GMW67QNF9C
• CAS: 61-90-5    7005-03-0  21675-61-6  25248-98-0  71000-80-1
• EC Number: 200-522-0
• FEMA Number: 3297
• PubChem Substance ID 24896488
• MDL number MFCD00002617
• Beilstein Registry Number 1721722
• eCl@ss 32160406

Synonyms:

• leucine
• (2S)-alpha-2-amino-4-methylvaleric acid
• (S)-2-Amino-4-methylpentanoic acid
• (S)-Leucine
• 2-Amino-4-methylvaleric acid (L)
• 2-Amino-4-methylpentanoic acid (L)
• L-a-Aminoisocaproic acid
• H-Leu-OH
• L-2-Amino-4-methylpentanoic acid
• (2S)-2-amino-4-methylpentanoic acid
• (S)-(+)-Leucine
• Leucin
• (2S)-2-amino-4-methyl-pentanoic acid
• alpha-Amino-gamma-methylvaleric acid
• L-Norvaline, 4-methyl-
• 3h-l-leucine
• H-Leu
• Leucine (USP)
• L-a-Aminoisocaproate
• 4-methyl-l-norvalin
• L-Leucine,(S)
• (S)-2-Amino-4-methylpentanoate
• (s)-2-amino-4-methylvalericacid
• (S)-2-Amino-4-methyl-pentanoic acid
• 2-Amino-4-methylvaleric acid, (L)-
• Leucine (H-3)
• 4-methyl-L-Norvaline
• L-Leucine, homopolymer
• 1-Leucine
• 4-methyl-norvalin
• L-Leucine;
• (L)-leucine
• .alpha.-Amino-.gamma.-methylvaleric acid
• (3H)Leucine
• (S)-2-Amino-4-methylvaleric acid
• L-leucin
• L-alpha-Aminoisocaproic acid
• Leucin [German]
• Leucine (VAN)
• LEUCINE, L-
• leu
• Leucinum
• (2S)-alpha-leucine
• L-(+)-Leucine
• L-(-)-2-Amino-4-methylpentanoic acid
• 2-amino-4-methylvaleric acid
• Leucine [USAN:INN]
• Leucinum [INN-Latin]
• L-Leuzin
• Leucina [INN-Spanish]
• Leucina [Latin,Spanish]
• alpha-Aminoisocaproic acid
• Valeric acid, 2-amino-4-methyl-, (S)-
• Pentanoic acid, 2-amino-4-methyl-, (S)-
• FEMA No. 3297
• 2-Amino-4-methylpentanoic acid, (S)-

References_____________________________________________________________________

(1) Efficacy and Safety of Leucine Supplementation in the Elderly.  Borack MS, Volpi E  J Nutr. 2016 Dec;146(12):2625S-2629S. Epub 2016 Nov 9.

(2) Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears.  Zheng R, Huang S, Zhu J, Lin W, Xu H, Zheng X.  Cell Tissue Res. 2019 Apr 24. doi: 10.1007/s00441-019-03021-x.