What is leucine?

Leucine is an essential amino acid; it is one of the 3 branched-chain amino acids (BCAAs) [1]. In foods, leucine is incorporated into proteins.

Leucine abbreviation (symbol): Leu

Leucine Functions in the Human Body

Leucine is [1]:

  • Important for the synthesis of hemoglobin, other proteins and hormones, regulation of blood sugar levels, growth and repair of bones and muscles and wound healing
  • Ketogenic amino acid — it can be converted into ketones [15]
  • Not a glucogenic amino acid — it cannot be converted into glucose [15]

Foods High in Leucine

  • ANIMAL FOODS: beef, pork, chicken, turkey, fish, cheese [2]
  • PLANT FOODS: cereals, legumes, nuts, seeds, baker’s yeast and spirulina [2]

Foods low in leucine: fruits and vegetables [2]

L-Leucine Supplements

Oral L-leucine supplements without prescription (over-the-counter) are available.

Leucine Supplements Benefits

Leucine and Muscle Growth

Leucine supplements are POSSIBLY EFFECTIVE in enhancing protein synthesis and promoting muscle growth [8]:

  • In 3 studies [9,11,12] and 2 systematic reviews of studies [13,14] consuming protein with added leucine after exercise, increased protein synthesis in young and older men.
  • In one 2009 study, consuming whey with added free leucine by young males did not result in greater muscle anabolic effect than consuming whey alone [7].

Leucine supplements are PROBABLY INEFFECTIVE in:

  • Increasing short-term physical performance [3] or strength [14]
  • Increasing long-term physical performance or preventing muscle loss in various illnesses [4], including muscle loss during immobilization [10]
  • Reducing blood glucose levels in diabetes type 2 [5]

Leucine Safety: Side Effects and Toxicity

The safety of leucine supplements is described under branched-chain amino acids.

Who should limit leucine intake?

Individuals with the following conditions should limit leucine intake from foods and supplements:

  • Isovaleric acidemia [6]
  • Maple syrup urine disease [6]

Isovaleric Acidaemia (Aciduria)

Isovaleric acidemia (aciduria) is a hereditary disorder of the leucine metabolism in which isovaleric acid builds up in the blood, urine and brain [16].

Symptoms, which develop in the first days after birth, include shaking, hypothermia, poor feeding, vomiting and, if not treated, coma and death; in a chronic form, recurrent attacks with vomiting, extreme tiredness and the smell of sweaty feet can occur [16].

Treatment includes a life-long low-lysine (low-protein) diet and carnitine and glycine supplements [17].

  1. L-Leucine  PubChem
  2. List of foods high in leucine  US Deparment of Agriculture
  3. Pitkännen HT et al, 2003, Leucine supplementation does not enhance acute strength or running performance but affects serum amino acid concentration  PubMed
  4. Balage M et al, 2010, Long-term effects of leucine supplementation on body composition  PubMed
  5. Leenders M et al, 2011, Prolonged leucine supplementation does not augment muscle mass or affect glycemic control in elderly type 2 diabetic men  PubMed
  6. van Vliet D et al, 2014, Single amino acid supplementation in aminoacidopathies: a systematic review  PubMed Central
  7. Tipton KD et al, 2009, Stimulation of muscle anabolism by resistance exercise and ingestion of leucine plus protein  PubMed
  8. Rowlands DS et al, 2015, Protein-leucine fed dose effects on muscle protein synthesis after endurance exercise  PubMed
  9. Koopman R et al, 2006, Co-ingestion of protein and leucine stimulates muscle protein synthesis rates to the same extent in young and elderly lean men  PubMed
  10. Baptista IL et al, 2010, Leucine attenuates skeletal muscle wasting via inhibition of ubiquitin ligases  PubMed
  11. Koopman R et al, 2005, Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects  PubMed
  12. Pasiakos SM et al, 2011, Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances postexercise muscle protein synthesis  PubMed
  13. Xu ZR et al, 2014, The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis  PubMed
  14. Komar B et al, 2015, Effects of leucine-rich protein supplements on anthropometric parameter and muscle strength in the elderly: a systematic review and meta-analysis  PubMed
  15. Amino acid  ChemPep
  16. Isovaleric acidemia  Genetics Home Reference
  17. van Vliet D et al, 2014, Single amino acid supplementation in aminoacidopathies: a systematic review  PubMed Central

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