L-Leucine, Unflavored by Primaforce is a high-purity supplement providing 5 grams of L-leucine per serving, an essential branched-chain amino acid (BCAA) with a well-established role in initiating muscle protein synthesis (MPS) via activation of the mTOR signaling pathway. Unlike complete protein sources, this product delivers leucine in isolation, allowing for targeted dosing to amplify the anabolic response to meals or resistance exercise. Evidence indicates that 5 g of leucine is sufficient to maximize myofibrillar protein synthesis after resistance training, particularly when consumed with low-protein meals [1].
As a supplement, L-leucine is most effective in contexts of suboptimal protein intake, aging, or muscle-wasting conditions such as sarcopenia or chronic liver disease. Clinical trials support its use in older adults, where it helps overcome anabolic resistance, improving lean mass and strength when combined with resistance training [2][3]. However, isolated leucine without adequate total protein intake may not yield optimal results, as other essential amino acids are required as building blocks for sustained MPS. This product is unflavored and free of additives, supporting flexibility in use.
Quick Summary
Full transparency: exact amount of L-Leucine (5 g) is clearly listed with no proprietary blends or hidden ingredients.
High efficacy for primary indication: stimulating muscle protein synthesis. The 5 g dose is clinically validated for enhancing MPS, improving lean mass in older adults, and supporting resistance training adaptations. Efficacy is context-dependent and best realized with adequate total protein intake and exercise.
L-Leucine in powder form has high bioavailability and rapid absorption. Free-form amino acids bypass digestion, allowing quick entry into circulation—ideal for acute stimulation of MPS. Score reduced slightly as co-ingestion with protein or carbs may further enhance utilization.
Generally safe at 5 g/day, but potential GI discomfort and long-term safety concerns with isolated high-dose leucine exist. Contraindicated in maple syrup urine disease and may interact with levodopa.
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, particularly after resistance exercise. A 5 g dose has been shown to maximize myofibrillar protein synthesis in young men following low-protein meals [1].
- Enhances Response to Low-Protein Meals: Adding 5 g of leucine to a low-protein beverage significantly boosts post-exercise muscle protein synthesis, helping overcome anabolic resistance in older individuals [1].
- May Benefit Chronic Liver Disease: Leucine and other BCAAs may improve malnutrition and sarcopenia in patients with chronic liver disease due to impaired protein metabolism [5].
- Supports Pre-Cachexia States in Cancer: Preliminary evidence suggests leucine can promote protein anabolism in advanced cancer patients before refractory cachexia develops [7].
Who Should Take This
Potential Side Effects
Ingredient Breakdown
This product contains only one ingredient: L-Leucine. It is classified as a primary ingredient due to its presence in the product name, high dose (5 g), and central role in the product’s intended physiological effects.
Primary Ingredients
L-Leucine
Essential amino acid, mTOR activator, muscle protein synthesis stimulator
Dose of 5 g aligns with upper end of clinical range (3–5 g/day) for sarcopenia and resistance training support. High purity, unflavored powder allows for flexible dosing. Classified as primary due to product naming, dose, and mechanism.
L-Leucine is a branched-chain essential amino acid that serves both as a substrate for protein synthesis and as a signaling molecule that activates the mTOR pathway, thereby initiating muscle protein synthesis. It is particularly effective in older adults and those with low protein intake.
Practical Recommendations
- Combine With Resistance Training: For optimal results, take 5 g of L-Leucine shortly before or after resistance exercise to maximize muscle protein synthesis and strength gains, especially in older adults.
- Pair With Protein-Rich Or Low-Protein Meals: Use with low-protein meals to enhance anabolic response, or with complete protein sources to further support muscle maintenance in aging or recovery.
- Avoid In Maple Syrup Urine Disease: Do not use if diagnosed with MSUD, a rare genetic disorder impairing BCAA metabolism.
- Separate From Levodopa Dosing: If taking Parkinson’s medication like levodopa, administer leucine at least 2–3 hours apart to avoid competition for absorption.
Is it Safe?
L-Leucine is generally safe at the 5 g dose used here, but caution is advised in specific populations. It should be avoided in maple syrup urine disease and used carefully in those on levodopa therapy. No additives or excipients are present, reducing risk of adverse reactions from fillers.
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.
- Adverse Effect: High doses may cause gastrointestinal discomfort in sensitive individuals.
Reports for Similar Products
The Science
The efficacy of this product is driven entirely by its 5 g dose of L-Leucine, which falls within the clinically studied range of 3–6 g/day for sarcopenia, resistance training support, and protein metabolism enhancement. This dose has been repeatedly shown to stimulate muscle protein synthesis and improve body composition outcomes in targeted populations.
Adding leucine to a low-protein beverage enhanced myofibrillar protein synthesis in young men post-exercise; 5 g maximized response.
Leucine supplementation showed beneficial effects on sarcopenia criteria, including lean mass and physical performance.
Meta-analysis found leucine increases muscle protein synthesis and lean body mass in older adults.
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.
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