L-Leucine
Research reviewed: Up until 03/2026
L-Leucine is a dietary supplement with 12 published peer-reviewed studies involving 1,650 participants, researched for Muscle Protein Synthesis, Sarcopenia & Muscle Mass in Older Adults, Exercise Recovery.
Evidence at a Glance
Strength is scored by study design, sample size, study type, and outcomes
Muscle Protein Synthesis
ModerateSarcopenia & Muscle Mass in Older Adults
ModerateExercise Recovery
WeakResearch Visualised
Visual breakdown of the clinical data.
Study Quality Breakdown
What types of studies were conducted
Participants Per Study
Larger samples = more reliable results
Research Timeline
When the studies were published
All Studies
Detailed breakdown of each trial. Click to expand.
Muscle Protein Synthesis
To evaluate the effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people.
Study Type
Systematic review and meta-analysis
Purpose
To evaluate the effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people.
Dose
Varied across studies (leucine supplementation)
Participants
Meta-analysis of 9 randomized controlled trials in elderly subjects
Duration
Various
Results
Leucine supplementation significantly increased muscle protein fractional synthetic rate compared to control (standardized difference 1.08, P<0.001). However, no significant differences emerged for lean body mass or leg lean mass between groups.
How They Measured It
Muscle protein fractional synthetic rate (FSR), lean body mass, leg lean mass across 9 RCTs
To determine whether leucine supplementation of a low-protein beverage can stimulate myofibrillar protein synthesis comparably to a high-protein dose.
Study Type
Double-blind, randomized controlled trial
Purpose
To determine whether leucine supplementation of a low-protein beverage can stimulate myofibrillar protein synthesis comparably to a high-protein dose.
Dose
6.25g whey protein + leucine to 5.0g total vs 25g whey protein (3.0g leucine)
Participants
40 young men (mean age 21 years)
Duration
Acute (single session)
Results
A low-protein (6.25g) beverage supplemented with 5.0g total leucine was as effective as a 25g whey protein dose at stimulating muscle protein synthesis. Greatest MPS occurred with 25g whey (~267%) and 6.25g whey + high leucine (~220%).
How They Measured It
Myofibrillar protein synthesis via isotope infusion and muscle biopsies
To compare postprandial muscle protein synthesis after a high whey protein, leucine-enriched supplement vs a dairy-like product in healthy older people.
Study Type
Randomized controlled trial, double-blind
Purpose
To compare postprandial muscle protein synthesis after a high whey protein, leucine-enriched supplement vs a dairy-like product in healthy older people.
Dose
20g whey protein with 3g total leucine vs 6g milk protein (isocaloric)
Participants
19 healthy older adults (9 experimental, 10 control)
Duration
Acute (4-hour measurement period)
Results
FSR was significantly higher after the leucine-enriched supplement (0.0780%/h) vs control (0.0574%/h, p=0.049). Higher postprandial amino acid concentrations appeared to mediate the response.
How They Measured It
Fractional synthesis rate (FSR) via stable isotope infusion (L-[ring-13C6]-phenylalanine tracer)
To determine whether protein leucine content influences shorter- and longer-term muscle protein synthetic responses in healthy older women.
Study Type
Randomized controlled trial
Purpose
To determine whether protein leucine content influences shorter- and longer-term muscle protein synthetic responses in healthy older women.
Dose
Two daily servings of 15g milk protein with 4.2g leucine (LEU) vs 15g mixed protein with 1.3g leucine (CON)
Participants
22 healthy older women aged 65-75 years (n=11 per group)
Duration
Supplementation period with acute and integrated measurements
Results
Acute myoPS increased in response to feeding in both groups, but the increase was greater in the LEU group. Integrated myoPS increased during supplementation in both legs for LEU, but only in the exercised leg for CON.
How They Measured It
Acute myofibrillar protein synthesis (13C6-phenylalanine infusion) and integrated myoPS (deuterated water)
To assess whether leucine-enriched protein enhances integrated myofibrillar protein synthetic response to short-term resistance training.
Study Type
Randomized controlled trial (crossover design)
Purpose
To assess whether leucine-enriched protein enhances integrated myofibrillar protein synthetic response to short-term resistance training.
Dose
Protein bar with 16g protein containing ~3g leucine vs no supplement
Participants
8 untrained young adults (male and female)
Duration
Two 4-day training phases separated by 2-day rest
Results
Training combined with leucine-enriched supplementation enhanced protein synthesis compared to training alone (1.53%/day vs 1.43%/day). Both exceeded rest baseline (1.31%/day). The supplement group showed increased mTOR signalling markers.
How They Measured It
Myofibrillar protein synthesis via deuterated water, muscle biopsies for mTOR signalling
To examine whether leucine-enriched essential amino acid supplementation during moderate steady-state exercise enhances postexercise muscle protein synthesis.
Study Type
Randomized controlled crossover trial
Purpose
To examine whether leucine-enriched essential amino acid supplementation during moderate steady-state exercise enhances postexercise muscle protein synthesis.
Dose
10g essential amino acids with 3.5g leucine (L-EAA) vs 1.87g leucine (EAA) during exercise
Participants
8 adults
Duration
Acute (two separate exercise bouts)
Results
Muscle protein synthesis was 33% greater after L-EAA consumption compared to standard EAA, with enhanced post-exercise signalling markers.
How They Measured It
Muscle protein synthesis via isotopic tracers, intracellular signalling analysis (multiplex and immunoblot)
Sarcopenia & Muscle Mass in Older Adults
To evaluate the effect of leucine supplementation on sarcopenia-related measures in older adults.
Study Type
Systematic review and meta-analysis of 17 RCTs
Purpose
To evaluate the effect of leucine supplementation on sarcopenia-related measures in older adults.
Dose
Varied across studies; subgroup analysis at ≥5g showed greater benefit
Participants
1,418 subjects across 17 randomized controlled trials
Duration
Various
Results
Leucine alone showed no effect on total lean mass, handgrip strength, or leg press. When combined with vitamin D, leucine demonstrated significant improvement in handgrip strength and gait speed. Doses ≥5g showed significant gait speed improvement.
How They Measured It
Handgrip strength, lean mass, gait speed, leg press, physical performance tests across 17 RCTs
To assess the effects of leucine administration on sarcopenia criteria in institutionalized elderly individuals.
Study Type
Randomized, double-blind, placebo-controlled trial
Purpose
To assess the effects of leucine administration on sarcopenia criteria in institutionalized elderly individuals.
Dose
6g/day leucine vs 6g/day lactose (placebo)
Participants
50 institutionalized individuals aged 65 and older
Duration
13 weeks
Results
Leucine was well-tolerated and significantly improved walking time (p=0.011) and lean mass index. Maximum static expiratory force improved significantly (p=0.026). No significant changes in cognitive function, inflammatory markers, or nutritional assessment.
How They Measured It
Walking time, lean mass index, respiratory muscle function, cognitive function, inflammatory markers
To evaluate the efficacy of a novel formulation of L-Carnitine, creatine, and leucine on lean body mass and functional muscle strength in healthy older adults.
Study Type
Randomized, double-blind, placebo-controlled trial
Purpose
To evaluate the efficacy of a novel formulation of L-Carnitine, creatine, and leucine on lean body mass and functional muscle strength in healthy older adults.
Dose
L-Carnitine 1500mg + L-leucine 2000mg + creatine 3000mg + Vitamin D3 10 mcg daily
Participants
42 healthy older adults aged 55-70 years (14 per group across 3 groups)
Duration
8 weeks
Results
The combination treatment showed 1.0 kg increase in total lean muscle mass, 0.35 kg increase in leg lean mass, and 1.0 kg increase in lower leg strength at week 8 vs placebo. mTOR protein expression was significantly increased, suggesting enhanced protein synthesis.
How They Measured It
DXA scans (body composition), dynamometry (strength), 6-minute walk test, muscle biopsies for mTOR protein expression
Exercise Recovery
To investigate whether free leucine supplementation improves muscle recovery from resistance exercise in young adults.
Study Type
Randomized, double-blinded, crossover study
Purpose
To investigate whether free leucine supplementation improves muscle recovery from resistance exercise in young adults.
Dose
6g daily free leucine (two 3g doses) vs placebo
Participants
14 untrained young adults (mean age 23.9 years)
Duration
Two exercise sessions separated by 7-day washout; recovery assessed over 72 hours
Results
No significant difference between leucine and placebo conditions. Leucine supplementation provided no improvement in muscle recovery markers compared to placebo.
How They Measured It
Repetition-to-failure tests, perceived exertion, lactate, creatine kinase, muscle soreness, testosterone, cortisol
To assess whether leucine-enriched essential amino acids improve recovery from post-exercise muscle damage.
Study Type
Randomized, double-blind, placebo-controlled, parallel-group trial
Purpose
To assess whether leucine-enriched essential amino acids improve recovery from post-exercise muscle damage.
Dose
4g LEAAs (containing 1.6g leucine) three times daily vs isocaloric placebo
Participants
20 recreationally active males
Duration
4 days following exercise
Results
Integrated myofibrillar protein synthesis increased ~72% over 96h after exercise with no differences between groups. However, total peak torque was ~10% greater during recovery in LEAA group vs placebo, indicating functional recovery benefit.
How They Measured It
Myofibrillar protein synthesis (deuterium oxide), isometric/isokinetic torque, soreness, Z-band streaming, creatine kinase, IL-6
To evaluate the effects of free leucine supplementation combined with moderate training on muscular fatigue in elderly subjects.
Study Type
Randomized, placebo-controlled, double-blind trial
Purpose
To evaluate the effects of free leucine supplementation combined with moderate training on muscular fatigue in elderly subjects.
Dose
Leucine-rich amino acid supplementation
Participants
Elderly subjects
Duration
12 weeks
Results
Supplementation with leucine resulted in reduced loss of strength at 0 and 3 hours after downhill walking compared with the placebo group. Reduced muscular fatigue observed after 12 weeks.
How They Measured It
Muscle fatigue indices, strength measurements after downhill walking protocol
Frequently Asked Questions
Common questions about L-Leucine research
There are currently 12 peer-reviewed studies on L-Leucine (L-Leucine), involving 1,650 total participants. Research covers Muscle protein synthesis, Anabolic signalling, Recovery and 1 more areas. The overall evidence strength is rated as Moderate.
The evidence is currently rated as "Moderate Evidence". This rating is based on study design quality (randomisation, blinding, placebo controls), sample sizes, study types (10 human studies), and reported outcomes.
L-Leucine has been researched for: Muscle protein synthesis, Anabolic signalling, Recovery, Sarcopenia. Each area has its own body of evidence which you can explore in the study breakdowns above.
Yes, 10 out of 12 studies are human trials. Human trials carry more weight in our evidence scoring system.
Similar Supplements
Other supplements researched for similar health goals
