The branched-chain amino acid L-Leucine appears to be the primary nutritional regulator of muscle protein synthesis via direct activation of the nutrient-sensitive mTORC1 signaling pathway. L-Leucine is the primary branched chain amino acid known for the ability to support protein synthesis and help slow the catabolic process. Leucine is probably the most well-researched amino acid to date for increasing protein synthesis.
For example, leucine, but not isoleucine or valine, can stimulate an increase in muscle protein synthesis through activation of the mTOR pathway. mTOR is such a powerful activator of muscle growth; scientists have reported that if you block mTOR with the drug rapamycin, muscle growth is completely inhibited despite muscle overload.
Leucine is like the gas pedal for stimulating protein synthesis; now that you know that mTOR is needed for muscle building and leucine acts as a stimulator of mTOR, here are a few things that inhibit mTOR:
• Protein Restriction
• Calorie Restriction/ Intermittent Calorie Restriction
• Cortisol/Glucocorticoids
Leucine Increases Anabolic Activity of Whey Protein
The good thing about leucine is that you don’t need a big dose to get an anabolic effect. In a previous study, researchers wanted to examine how adding leucine to a protein drink would affect muscle protein synthesis. Researchers assessed the effect on muscle protein synthesis at rest and after resistance exercise.
The men completed leg extensions before the ingestion of various intakes of whey protein, BCAAs and leucine:
The subjects received either:
-High Protein/Low Leucine: 25 grams of whey protein (which contained 3.0 grams leucine)
-Low Protein/Low Leucine: 6.25 grams of whey protein (which contained 0.75 grams leucine)
-Low Protein/Moderate Leucine dose: 6.25 grams whey protein supplemented with leucine (3.0 grams total leucine)
-Low Protein/High Leucine dose: 6.25 grams whey protein supplemented with leucine (5.0 grams total leucine)
-Low Protein/ BCAA combination: 6.25 grams whey protein supplemented with leucine, isoleucine, and valine (5.0 grams total leucine)
One would expect that the 25 grams of whey protein group would have the biggest impact on muscle protein synthesis, but that’s not what the researchers found. The fascinating finding was the low protein (6.25 grams) with high Leucine (5 grams) mixed beverage was as effective as a high-protein whey (25 grams) shake at stimulating increased muscle protein synthesis rates when supplemented with a high amount of leucine (five grams).
These results, indicating that high leucine can have enhanced muscle anabolism effects, are important for athletes who are on a strict diet and cutting back on total calories.
In a new 2015 study published in Clinical Nutrition, researchers found that taking two supplements drastically enhanced lean muscle mass while on a calorically restricted diet. 60 overweight subjects, aged >55 years, participated in the study, The subjects engaged in a 13-week period of voluntary weight loss in which they ate 600 kcal less than their estimated requirement for weight maintenance (as measured by reference to their resting energy expenditure calculated with indirect calorimetry), while also engaging in a program of resistance training.
The strength training program comprised three sets of 20 repetitions of the following exercises: Lat pull down, biceps curl, high row, shoulder press, horizontal row, chest press, triceps extension, knee extension, leg curl, and leg press. During the program, the subjects consumed a whey protein supplement containing 150kcal per serving (20g of whey protein and 3g leucine).
At the end of the study, the researchers observed that the subjects lost a mean 3.1 kg of body weight over the 13-week period.
They noted that individuals consuming 1.2g per kg of bodyweight of protein lost much less muscle than those consuming only 1.0g per kg of bodyweight. The odds of increasing muscle mass during the weight loss period were significantly (5.2 times) higher with protein intakes >1.2g per kg of bodyweight and even higher (6.2 times) with protein intakes of >1.9g per kg of bodyweight.
The researchers concluded that obese, elderly subjects could enhance the odds of increasing muscle mass while engaged in both caloric restriction and resistance training by consuming protein of >1.2g per kg of body weight and preferably 1.9g per kg. Be sure to incorporate a good whey based protein and some extra leucine to retain muscle mass while dieting.
For bodybuilders looking to remain anabolic, one may consider taking some leucine throughout the day, even if it’s a small amount. The amount of leucine necessary to optimize performance and muscle growth is unknown, but recent research in humans points to a plateau effect (at least on muscle protein synthesis) occurring around 3 grams; there is a ceiling effect, so more is not going to have better results.
Leucine Prevents Catabolism
Leucine is such a powerful stimulator of anabolism that it can even reduce catabolism caused by sleep deprivation. In a recent study, researchers examined sleep deprivation-induced muscle loss (i.e. atrophy) and the muscle-specific fiber types affected and to determine the effects of leucine supplementation on atrophy and pertinent portions of the pathways of muscle protein synthesis and degradation in rats.
A total of 46 rats were distributed into four groups:
• control,
• leucine supplementation,
• sleep deprivation,
• and leucine supplementation + sleep deprivation
The animals were sleep deprived for 96 hours and were administered a high dose of leucine (1.35 grams of Leucine/kg/daily). At the end of the study, sleep deprivation led to the atrophy of IIa and IIb muscle fibers; however, leucine supplementation prevented muscle loss and type IIb fiber atrophy.
3 Grams of Leucine After Exercises Increases IGF-1 Levels in Muscle
To demonstrate the potent muscle building properties of leucine, 9 trained men performed 3 lower-body resistance exercise sessions involving 4 sets of 8–10 repetitions at 75–80% one repetition maximum on the angled leg press and knee extension. Immediately following each session, participants orally ingested:
– 3 g cellulose placebo or
– 3 grams L- leucine
Blood samples were obtained pre-exercise and at 2 and 6 hours post-exercise. Muscle biopsies were also obtained pre-exercise and at 2 and 6 hours post-exercise
At the end of the study, supplementation did not induce increases in serum IGF-1; however, skeletal muscle IGF-1 concentrations were significantly increased at 2 and 6 hours post-exercise in response to the leucine supplementation.
This suggests that leucine can increase muscle anabolism after exercise.
Key Points:
Leucine Increases Muscle IGF-1 levels and can provide potent anabolic effects when taken after training.
3 grams of L-Leucine seems to be the optimal dose.
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