Leucine is one of three branched-chain amino acids, along with isoleucine and valine. Research has shown that of the three, leucine is the primary and most potent activator of muscle protein synthesis. Cells sense leucine and respond by stimulating protein synthesis through mammalian target of rapamycin (mTOR), a master regulator of protein synthesis. Although leucine is the most anabolic, research shows that all branched-chain amino acids are required to fully activate muscle protein synthesis. We’ll talk more about the benefits of leucine, how it works, and the research results.
L-leucine is an essential amino acid and is considered one of three branched-chain amino acids. Essential amino acids are considered “essential” because they cannot be synthesized on their own and therefore must be obtained from food or supplements.
Together with valine and isoleucine, leucine performs so-called intramuscular signaling.
While most essential amino acids are oxidized in the liver, BCAA and leucine are primarily oxidized in skeletal muscle and other peripheral tissues. The anabolic or muscle-building effects of high-intensity training and resistance training are controlled through changes in signal transduction that control and promote muscle protein synthesis (the process of building muscle).
The signaling network that controls muscle protein synthesis through translation involves phosphorylation of the mammalian target of rapamycin (mTOR). Leucine has a defined effect on this specific process and signaling pathway, thus exerting an anabolic effect on protein metabolism, increasing the rate of protein synthesis and decreasing the rate of protein degradation.
This creates an effect known as positive amino acid balance, which is critical for increasing muscle size and strength.
Leucine, in particular, is critical in building muscle mass and indirectly activates p70 s6 kinase and the eIF4F complex, which is essential for the anabolic effect and recovery of muscle tissue.
When you exercise, your body uses essential amino acids for fuel. When your body is deficient in essential amino acids, it slows down protein synthesis and breaks down muscle tissue, and your body cannot adequately repair itself. When you train hard and deplete those BCAA reserves, your body starts catabolizing, or breaking down muscle for fuel. By supplementing with essential amino acids, you can ensure your body is in a positive balance to help promote muscle growth, repair and reduce post-workout muscle soreness.
First, leucine helps stimulate muscle protein synthesis. This is well documented and established in studies utilizing elderly patients.
As we age, we experience a phenomenon called anabolic resistance. This is when cells become more difficult to turn on the switch for muscle protein synthesis. Supplementation with leucine may partially improve anabolic resistance, as studies have shown that older adults require twice as much leucine to activate a similar MPS compared to younger adults (R).
This phenomenon can partly be explained by increased intestinal retention of orally ingested leucine, which was twice as high as in young adults (R).
However, studies have shown that supplementing with leucine alone does not cause greater anabolic changes to supplementing all BCAAs, EAAs, or protein.
Supplements marketed with leucine ratios higher than 2:1:1 had no greater effect on muscle protein synthesis.
In a randomized, double-blind, placebo-controlled study conducted at Baylor University, 30 subjects took BCAAs, leucine, or a placebo. These supplements are taken in three equal servings pre-resistance training, pre-resistance training, and post-workout. Results showed that supplementation with both leucine and BCAA resulted in higher levels of phosphorylated 4E-BP1 and cellular signaling of the mTOR pathway, a serine/threonine kinase that stimulates protein synthesis through amino acid activation. These findings suggest that the other two BCAAs (isoleucine and valine) may contribute to greater activation than leucine alone (R). Furthermore, the study concluded that supplementing with BCAAs had a greater effect on mTOR than taking more leucine alone.
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Positive net amino acid balance and increases in muscle protein synthesis are also achieved by inhibiting protein breakdown. Inhibiting muscle breakdown will lead to faster recovery times and less exercise-induced muscle soreness.
Resistance training and weightlifting can cause micro-tears in muscle fibers, which can lead to muscle soreness. Soreness can interrupt your regularly scheduled schedule and have a huge impact on peak power, strength and reps.
A study published in the Journal of Nutrition examined the effects of BCAA supplementation on recovery from resistance training. Using a randomized, double-blind, placebo-controlled study design, participants were enrolled in BCAA or placebo groups. At 72 hours after the workout, the BCAA group reported significantly less muscle soreness than the placebo group.(3)
Likewise, leucine supplements are more effective when all EAAs or BCAAs are taken together.
A study conducted at the University of Illinois found that leucine is critical for enhancing or amplifying protein synthesis signals at the peptide initiation level. However, intake of all amino acids, rather than leucine alone, initiates muscle recovery more quickly (R).
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Numerous studies have found that supplementing the diet with leucine can help with weight loss. It is starting to become more common to use macronutrients as a primary weight loss solution. Higher protein intake and lower carbohydrate and fat intake were associated with reduced weight loss.
The mechanism may be an increase in energy consumption due to improved energy efficiency. Although most studies are from animal studies, more evidence from human clinical trials is needed to replicate these results (R).
Animal studies have shown that leucine improves resistance to human growth hormone. HGH decreases as we age due to anabolic resistance and the pituitary reduces the amount of HGH it produces.
Preliminary results suggest that long-term leucine supplementation increases growth hormone levels by promoting IGF-1, reducing IGF-1 degradation and promoting growth hormone resistance synthesis in the liver, and promoting muscle protein synthesis by modulating mTOR anabolic signaling (R).
More research is needed to confirm these results and the use of leucine as a potential treatment for increasing HGH.
Leucine is a powerful essential amino acid and a key signaling molecule in protein synthesis. Studies have shown that the most effective way to utilize leucine is to supplement with protein, essential amino acids (EAA) or branched-chain amino acids (BCAA). Leucine plays a role in intramuscular signaling but is co-dependent with other amino acids to promote muscle growth and recovery.
Look for Supplements That Contain a Healthy Dose of L-Leucine
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