L-Leucine is an essential branched-chain amino acid (BCAA) that plays a pivotal role in regulating muscle protein synthesis (MPS), primarily through activation of the mechanistic target of rapamycin (mTOR) signaling pathway. Unlike other amino acids, leucine acts not only as a building block for protein but also as a key signaling molecule that triggers anabolic processes in skeletal muscle. This dual function makes it a central focus in interventions targeting sarcopenia, muscle wasting, and age-related anabolic resistance.
NOW Foods Sports L-Leucine Powder delivers 5,000 mg (5 g) of pure L-leucine per serving—the dose most consistently associated with maximizing myofibrillar protein synthesis, especially when consumed around exercise or with low-protein meals. Clinical evidence supports this dosage for improving lean mass, strength, and physical performance in older adults and untrained individuals, particularly when combined with resistance training. The product is free from proprietary blends, allowing precise dosing for targeted nutritional support.
Quick Summary
Full transparency: exact amount of L-Leucine is disclosed with no proprietary blends, allowing accurate dosing and clinical alignment.
High efficacy due to 5 g clinical dose of primary ingredient L-Leucine, which aligns with evidence for enhancing muscle protein synthesis, lean mass, and strength—especially in older adults and with resistance training.
Free-form L-Leucine powder is rapidly absorbed and bioavailable, achieving peak plasma concentrations quickly—ideal for triggering acute anabolic signaling. High-absorption form earns partial credit under supplement guidelines.
Generally safe at recommended dose, but potential interactions and contraindications exist. NOW Foods' GMP certification supports quality control, though long-term safety of isolated leucine is not fully established.
Suggested: 1 serving/day to reach clinical midpoint
Why Take This Product
Key Benefits
- Stimulates Muscle Protein Synthesis: L-Leucine activates the mTOR pathway, a key regulator of muscle protein synthesis. A 5 g dose has been shown to maximize myofibrillar protein synthesis after resistance exercise, especially in low-protein feeding conditions [6].
- Enhances Response to Low-Protein Meals: Adding 5 g of leucine to a low-protein beverage significantly increases postprandial muscle protein synthesis, helping overcome anabolic resistance in older individuals [6].
Who Should Take This
Potential Side Effects
Ingredient Breakdown
The formulation contains a single, highly concentrated ingredient—L-Leucine—provided in its free-form, rapidly absorbable state. This allows for precise dosing and rapid plasma availability, which is critical for triggering muscle protein synthesis.
Primary Ingredients
L-Leucine
Essential amino acid, mTOR activator, muscle protein synthesis stimulator
L-Leucine is a branched-chain essential amino acid that serves as both a substrate and signaling molecule for muscle protein synthesis. It activates the mTOR pathway, making it critical for muscle maintenance and growth, especially in aging or catabolic states.
Practical Recommendations
- Optimal Timing: Consume 5 g of L-Leucine shortly before or after resistance exercise, or add to low-protein meals (e.g., plant-based meals under 20 g protein) to enhance anabolic response.
- Combine With Protein: For best results, use leucine as a complement to, not replacement for, adequate total daily protein intake (1.2–2.0 g/kg/day), ensuring availability of all essential amino acids.
- Monitor Long-Term Use: While short-term use is well-supported, long-term isolated leucine supplementation should be periodically evaluated, especially in older adults or clinical populations.
Is it Safe?
NOW Foods is a reputable supplement manufacturer with third-party testing and Good Manufacturing Practice (GMP) certification, supporting product quality and purity. However, isolated amino acid supplementation requires awareness of metabolic and drug interaction risks.
Safety & Considerations
- Contraindication: Do not use if you have maple syrup urine disease (MSUD).
- Drug Interaction: May interfere with levodopa absorption; separate dosing by at least 2–3 hours in Parkinson’s patients.
- Gastrointestinal: High doses may cause nausea or bloating in sensitive individuals.
Reports for Similar Products
The Science
The efficacy of this product is driven entirely by its primary ingredient, L-Leucine, delivered at a 5 g dose that matches or exceeds the clinical midpoint (4 g) for sarcopenia, resistance training, and low-protein meal augmentation. Evidence confirms this dose effectively stimulates muscle protein synthesis and supports muscle mass when used appropriately.
Leucine supplementation improves sarcopenia-related parameters in older adults, including muscle mass and strength.
L-leucine and BCAA supplementation may improve malnutrition and sarcopenia in chronic liver disease.
5 g/day leucine with resistance training increased fat-free mass and strength in untrained midlife women.
Leucine acts as a signaling molecule to promote protein synthesis in cancer patients before refractory cachexia.
Leucine supplementation did not improve insulin sensitivity beyond resistance training in prefrail/frail older women.
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