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L-Leucine is the primary branched chain amino acid known for the ability to support protein synthesis and slow the catabolic process.*





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 in animals. 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. So now that you know that mTOR is needed for muscle building, and that leucine acts as a stimulator of mTOR, here are a few things that inhibit mTOR:

• Protein restriction
• Calorie restriction/ Intermittent Calorie Restriction
• Cortisol/Glucocorticoids

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 really interesting 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.

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, and there is a ceiling effect so more is not going to have better results.

Li F, Yin Y, Tan B, Kong X, Wu G. Leucine nutrition in animals and humans: mTOR signaling and beyond. Amino Acids. 2011 Jul 20.

Anthony JC, Yoshizawa F, Anthony TG, Vary TC, Jefferson LS, Kimball SR: Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. J Nutr 2000, 130:2413-2419.

Escobar J, Frank JW, Suryawan A, Nguyen HV, Kimball SR, Jefferson LS, Davis TA:Regulation of cardiac and skeletal muscle protein synthesis by individual branched-chain amino acids in neonatal pigs. Am J Physiol Endocrinol Metab 2006, 290:E612-E621.

Effects of dietary leucine supplementation on exercise performance. Melissa J Crowe, Jarrad N Weatherson, Bruce F Bowden. Eur J Appl Physiol. 2006 August; 97(6): 664–672.

Bukhari SS, Phillips BE, Wilkinson DJ, Limb MC, Rankin D, Mitchell WK, Kobayashi H, Greenhaff PL, Smith K, Atherton PJ. Intake of low-dose leucine-rich essential amino acids stimulates muscle anabolism equivalently to bolus whey protein in older women at rest and after exercise. Am J Physiol Endocrinol Metab. 2015 Jun 15;308(12):E1056-65.