L-Leucine

Essential branched-chain amino acid critical for muscle protein synthesis and metabolic regulation

Scientific nameL-Leucine FormulaC6H13NO2 Molecular weight131.17 g/mol CAS61-90-5

95 absorption score

9 safety / 10

25 USD per month

5 documented interactions

Check it against something

Evidence level A

L-Leucine with:

82 out of 100

Works better together Compatible. Timing differs, so follow each supplement's own window.

Check it against your whole stack →

What it does

mechanism and evidence for each claim

Enhanced muscle protein synthesis

Directly triggers mTOR pathway for muscle growth

Activates mTOR signaling cascade and protein synthesis Evidence A-grade 30 minutes to 2 hours post-consumption

Improved muscle recovery

Reduces muscle damage and accelerates recovery

Stimulates protein synthesis and reduces protein breakdown Evidence A-grade 2-6 hours post-exercise

Muscle mass preservation

Prevents muscle loss during caloric restriction or aging

Maintains positive protein balance and muscle anabolism Evidence A-grade 2-4 weeks of consistent use

Enhanced athletic performance

Improves endurance and reduces exercise fatigue

Provides energy for muscles and reduces central fatigue Evidence B-grade 1-4 weeks

Blood sugar regulation

Helps regulate glucose metabolism and insulin sensitivity

Stimulates insulin release and glucose uptake Evidence B-grade 30 minutes to 2 hours

Others take these for the same thing

Same goal, different mechanism — check any of them against this one

Creatine Monohydrate Strength & power enhancement 3-5g A 95% Calcium Bone health & muscle function 1,000-1,200 mg A 30% Magnesium Glycinate Superior absorption sleep & muscle support 200-600 mg A 90% Whey Protein Muscle growth & recovery 25-75g A 95% Creatine HCL Enhanced muscle building 1.5-3g B 98% Oregano Oil Powerful respiratory antimicrobial 100-300mg B 85%

Every pair we checked

12 combinations scored by the engine

L-Leucine + L-Tryptophan High leucine may reduce tryptophan uptake into brain Needs attention 58

L-Leucine + Caffeine No documented interaction either way No known interaction 70

L-Leucine + Creatine Monohydrate No documented interaction either way No known interaction 70

L-Leucine + Vitamin D3 No documented interaction either way No known interaction 70

L-Leucine + Calcium Carbonate No documented interaction either way No known interaction 70

L-Leucine + Lactase No documented interaction either way No known interaction 70

L-Leucine + Bovine Collagen No documented interaction either way No known interaction 70

L-Leucine + DHA No documented interaction either way No known interaction 70

L-Leucine + Turmeric No documented interaction either way No known interaction 70

L-Leucine + Fish Oil (EPA/DHA) No documented interaction either way No known interaction 70

L-Leucine + Magnesium Oxide No documented interaction either way No known interaction 70

L-Leucine + Whey Protein Complete amino acid profile enhances leucine effects Works better together 82

Dosage

Post workout or with meals

Daily2.5-10g

Threshold dose2.5-3g (minimum for effect)

Optimal5-10g daily

Post workout2.5-5g

Divided doses2.5g per meal for muscle preservation

Before you take it

VeganYes

When to expect it

First Effects30-60 minutes for protein synthesis activation

Peak Benefits2-4 weeks for muscle building and recovery

Deficiency DevelopmentDays to weeks with inadequate protein intake

Toxicity OnsetVery rare, mainly digestive upset

How to measure it

$80-150

MarkerPlasma leucine levels

Optimal120-200 μmol/L

Deficient below<100 μmol/L

FrequencyOnly if monitoring protein status or research purposes

What people actually reported to the FDA

CAERS, 1 reports naming it as the suspect

Most reported effects

Diarrhoea 1

Nausea 1

Named as the suspect 1 reports of 2 that mention it at all

Serious outcome 100% hospitalisation, emergency room or worse

Other Serious or Important Medical Event1

Reports are voluntary and unverified — they record what someone experienced while taking something, not what the compound caused. Counts skew towards products people take a lot of, and a serious outcome is more likely to be reported than a mild one, so these are not rates. How we filter them →

What actually reaches you

The label counts the carrier, your body does not

Absorption Very high 95 of 100 · best absorbed form of this compound

Delivered daily 5937.5 mg

Cost per 100 mg delivered $0.01 comparative — uses the absorption score, not a measured percentage

Works better with

3 documented

Isoleucine Valine

Other BCAAs work together for optimal muscle effects

Balanced BCAA profile prevents competition and imbalance · level A

Whey Protein

Complete amino acid profile enhances leucine effects

Provides all essential amino acids for protein synthesis · level A

Carbohydrates

Enhance leucine uptake and anabolic signaling

Insulin response improves amino acid uptake · level A

What not to take with L-Leucine

2 documented

Tryptophan

High leucine may reduce tryptophan uptake into brain

Competition for large amino acid transporter · mild significance

Other Large Amino Acids

High leucine may compete with phenylalanine, tyrosine

Shared transport mechanisms · mild significance

Medication interactions

Discuss these with a pharmacist before combining

diabetes-medications Minor

May enhance glucose lowering effects

What to do: Monitor blood glucose

diazoxide Minor

Leucine may counteract diazoxide effects

What to do: Monitor for drug effectiveness

Available forms

bioavailability compared

L-Leucine (Free Form) 95

L-Leucine (Free Form): Direct mTOR activation, Fast absorption, Well-researched, Highly effective

Food sources

before you supplement

Whey protein 2.5-3g per 25g protein · 95

Lean meats (chicken, beef) 1.7-2.2g per 100g · 90

Fish (tuna, salmon) 1.6-1.9g per 100g · 90

Eggs 1.0g per 100g · 95

Too little

deficiency signs

Muscle wasting and weakness
Poor exercise recovery
Fatigue and reduced endurance
Slow wound healing
Loss of muscle mass with aging
Impaired immune function

Too much

toxicity signs

Very rare with oral supplementation:
Digestive upset and nausea
Amino acid imbalance
Potential hypoglycemia
Generally excellent safety profile

Who benefits most

athletes-muscle-building

Essential for muscle protein synthesis and recovery

2.5-5g post-workout

aging-muscle-preservation

Prevents age-related muscle loss (sarcopenia)

2.5-5g with meals

weight-loss-muscle-sparing

Preserves muscle mass during caloric restriction

2.5-5g between meals

endurance-athletes

Reduces fatigue and improves performance

2.5-5g pre/during/post exercise

recovery-from-illness

Supports muscle protein synthesis during recovery

2.5-5g with meals

Where to buy it

compare forms first

Look for L-Leucine at 2.5-10g per serving. The form that matters here is L-Leucine (Free Form).

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What else affects it

Beyond other supplements

Carbohydrates

Enhance leucine uptake and anabolic signaling

Insulin response improves amino acid uptake

How it works

Directly activates mTOR (mechanistic target of rapamycin) pathway
Stimulates S6K1 and 4E-BP1 for protein synthesis initiation
Reduces protein breakdown via autophagy inhibition
Provides direct fuel for muscle tissue during exercise
Stimulates insulin release for nutrient uptake
Enhances satellite cell activation for muscle repair

Key findings

what the evidence supports

Essential trigger for muscle protein synthesis (extensive research)
Effective for muscle building and preservation (clinical trials)
Reduces exercise-induced muscle damage (sports medicine studies)
Important for maintaining muscle mass during aging (geriatric research)

Sources: NIH Examine.com · data reviewed 2026-08-02

Compiled by Denis, who maintains this database and holds no medical qualification — every figure here is sourced or derived, never asserted. How we calculate it →

Common questions

What are the benefits of L-Leucine?

Documented effects: Enhanced muscle protein synthesis; Improved muscle recovery; Muscle mass preservation; Enhanced athletic performance. Directly triggers mTOR pathway for muscle growth

How much L-Leucine should you take per day?

The common daily range is 2.5-10g. By goal — threshold dose: 2.5-3g (minimum for effect); optimal: 5-10g daily; post workout: 2.5-5g; divided doses: 2.5g per meal for muscle preservation.

When is the best time to take L-Leucine?

Post workout or with meals. First effects are typically reported within 30-60 minutes for protein synthesis activation.

What are the side effects of L-Leucine?

At excessive doses: Very rare with oral supplementation:, Digestive upset and nausea, Amino acid imbalance, Potential hypoglycemia, Generally excellent safety profile. Safety rating at normal doses is 9 out of 10.

What should you not take with L-Leucine?

Separate it from Tryptophan, Other Large Amino Acids by two to four hours. High leucine may reduce tryptophan uptake into brain

Does L-Leucine interact with medications?

Documented interactions with diabetes-medications, diazoxide. Discuss these with a pharmacist before combining.

How do you know if you need L-Leucine?

The marker is Plasma leucine levels, optimal range 120-200 μmol/L. Common deficiency signs: Muscle wasting and weakness, Poor exercise recovery, Fatigue and reduced endurance, Slow wound healing.

Taken for

43 Performance

Caffeine · Creatine Monohydrate · Whey Protein