Leucine: Build Muscle With This Anabolic Amino Acid

Leucine, a branched chain amino acid, is a protein stimulus gunslinger. Find out how it works and pull the trigger to build more muscle!

Look around the gym and you'll see as many different body types as there are dumbbells. There are giants roaming the weight room, skinny guys on the treadmill, and maybe even some ripped sprinters hustling hard. One reason for these different body types? Different training methods. Different forms of exercise have different training effects, and they affect protein synthesis in different ways.

Protein synthesis is the creation of new skeletal muscle proteins. When this occurs on a large scale, it is known as skeletal muscle hypertrophy (growth), and it's the process by which our muscles get bigger. Before we dig into how to maximize protein synthesis and the unique role of leucine (a branched chain amino acid), let's cover a few basics.

Protein, Exercise And You

Endurance exercise affects skeletal muscle protein turnover by decreasing the rate of skeletal muscle protein synthesis and increasing the rate of protein degradation (muscle breakdown) 1. Thus, endurance training can eventually build the super-skinny or overweight pavement-pounders who run 40 miles per week.

Resistance exercise is unique because it actually elevates skeletal muscle protein synthesis AND increases the rate of skeletal muscle protein degradation. The overall effect, in both cases, is a negative net protein balance 2. During resistance exercise (i.e. training with weights, bands, and/or other torture devices) your body goes into a catabolic condition; at rest, your muscles repair and increase in size. Kapow: Arnold arms!

That is, if you get enough post-workout protein. In order for protein balance to become positive post-workout, dietary protein - and specifically the amino acid leucine - must be consumed.

In fact, protein balance will remain negative until protein is consumed 3,4.

The Power of Protein

Anyone who has ever read a muscle magazine or talked to a meathead long enough knows that consuming a diet high in protein is one of the major ways to maximize muscle gain. That's because protein is essential for muscle growth. Protein is made up of amino acids, the ingredients that stimulate muscle protein synthesis. There are over 20 amino acids that occur in the dietary protein we eat, but not all of them have the same impact on muscle growth.

The amino acid leucine is the king of stimulating muscle protein synthesis!

Leucine, I'm Home!

Leucine is one of the three branched chain amino acids (BCAAs), and is unique in its ability to stimulate skeletal muscle protein synthesis. In fact, leucine has a several fold greater impact on protein synthesis than any other amino!

Leucine activates a major complex in the anabolic (muscle-building) pathway called mTOR 5-7. Think of mTOR as the amino acid sensor of a cell. mTOR is sensitive to leucine concentrations. Decreasing leucine concentrations signal to mTOR that there is not enough dietary protein present to synthesize new skeletal muscle protein, so mTOR deactivates. Increased leucine levels signal to mTOR that there is sufficient dietary protein to synthesize new skeletal muscle protein, so mTOR "turns on."

Though researchers are not sure exactly how leucine activates mTOR, it has been shown that mTOR is sensitive to leucine concentrations and activation of mTOR is strongly associated with increased protein synthesis, which means more muscle growth.

mTOR increases protein synthesis through two different mechanisms 8. It increases the rate that skeletal muscle proteins are translated (process by which genetic information is converted into protein), and it increases the production of different complexes in the protein-synthetic pathway, thereby increasing the body's capacity to stimulate skeletal muscle protein. So, not only does mTOR stimulation cause the body to create new muscle tissue faster, but it also increases the amount that can be produced in a period of time! Sweet!

Making Sense of mTor

Did I lose you? Here's an analogy: The contracting company mTOR is building a protein skyscraper. The machines (bulldozers, cranes, etc.) are the protein synthesis pathway components, and leucine is the cash needed to make the project work. When enough cash is available (increasing leucine concentrations), the contracting company can start building the skyscraper (synthesizing muscle protein) and also purchase more machines (increased synthetic components). This increases the capacity and speed at which mTor can construct the skyscraper (the muscle protein being synthesized).

To build protein skyscrapers (muscles) in our bodies, it's clear that leucine is a necessary component. The question, though, is whether it's beneficial to supplement with extra leucine, or if we get enough from a high protein diet? There is some evidence that supplemental leucine may be beneficial, even if one ingests ample protein.

Supplement Scrutiny

In a recent study, subjects did resistance training for 45 minutes. After, one group of them supplemented with carbohydrates, another group with carbohydrates and protein, and the last group supplemented with carbs, protein and leucine.

Researchers found that the carbohydrate/protein/leucine supplement group reduced muscle protein breakdown and increased muscle protein synthesis to a greater degree than the carbohydrate/protein group, and to a much greater degree than the carbohydrate-only group 9. Thus, more leucine might equal more muscle when added to protein and carbs!

As a free-form supplement, leucine gets absorbed into circulation much faster than when it's part of a whole protein. Even with a fast-digesting protein such as whey, it can take hours for the leucine to be liberated from the protein and enter circulation. Therefore, leucine concentrations in the blood never spike to high levels. An isolated leucine supplement, however, would be quickly absorbed into circulation, thus spiking blood leucine levels and drastically increasing intracellular leucine concentrations and activating anabolic pathways.

For those who are dieting but want to maintain maximum muscle mass, a leucine supplement may help alleviate some of the catabolic effects of a calorie-deficit diet. Leucine is more anabolic than other amino acids. While this hasn't been studied in detail, it might be possible to substitute something like 5g of leucine for 30-40g of dietary protein and still have the same or greater stimulatory effect on muscle protein synthesis.

Again, this is a hunch of mine based on current research and my own PhD work. With a leucine supplement, you may be able to maintain high levels of protein synthesis and knock an extra 100 calories off your diet. Can you say, "shredded?" (You don't have to say it. Just be it.)

Leucine Lowdown

In conclusion, it's clear that leucine increases protein synthesis by increasing the activity of mTOR. Leucine has a far greater stimulatory effect on protein synthesis than any other amino acid. Protein synthesis increases similarly in response to a relatively small dose of leucine compared to a whole food meal. Also, adding leucine to a protein-rich meal further increases the rate of skeletal muscle protein synthesis.

  1. Dohm, G. L., Kasperek, G.J., Tapscott, E. B., & Beecher G., R. (1980) Effect of exercise on synthesis and degradation of muscle protein. Biochem. J. 188: 255-262.
  2. Phillips, S., M, Tipton, K. D., Aarsland, A., Wolf, S. E., & Wolfe, R. R. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. (1997) Am. J. Physiol. 273(1 Pt 1): E99-107.
  3. Gautsch, T. A., Anthony, J. C., Kimball, S. R., Paul, G. L., Layman, D. K., & Jefferson, L. S. (1998) Availability of eIF4E regulates skeletal muscle protein synthesis during recovery from exercise. Am. J. Physiol. 274(2 Pt 1):C406-414.
  4. Levenhagen, D. K., Carr, C., Carlson, M. G., Maron, D.J., Borel, M. J., Flakoll, P. J. (2002) Postexercise protein intake enhances whole-body and leg protein accretion in humans. Med Sci Sports Exerc. 34(5):828-37.
  5. Anthony, J. C., Yoshizawa, F., Anthony, T. G., Vary, T. C., Jefferson, L. S., & Kimball, S. R. (2000) Leucine stimulates translation inititation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. J. Nutr. 130: 2413-2419.
  6. Crozier, S. J., Kimball, S.R., Emmert, S. W., Anthony, J. C., & Jefferson, L.S. (2005) Oral leucine administration stimulates protein synthesis in rat skeletal muscle. J. Nutr. 135: 376-382.
  7. Bolster, D. R., Crozier, S. J., Kimball, S. R., & Jefferson, L. S. (2002) AMP-activated protein kinase suppresses protein synthesis in rat skeletal muscle through down-regulated mammalian target of rapamycin (mTOR) signaling. J. Biol. Chem. 277: 23977-23980.
  8. Merrick, W. C., & Hershey, J. W. B. (2000) The pathway and mechanism of initiation of protein synthesis. In: Sonnenberg N, Hershey JWB, Mathews MB, editors. Translational control of gene expression. Cold Spring Harbor Laboratory Press.*
  9. Koopman R, Wagenmakers AJ, Manders RJ, Zorenc AH, Senden JM, Gorselink M, Keizer HA, van Loon LJ. (2005) Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects. Am. J. Physiol. Endocrinol. Metab. 288(4): E645-653.

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