Leucine and Protein Synthesis
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.
1. It increases the rate that skeletal muscle proteins are translated (process by which genetic information is converted into protein).
2. 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!
Making Sense of mTor
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.
In a recent study, subjects did resistance training for 45 minutes. After, one group of them supplemented with just 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 group9. 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. 5g of leucine for 30-40g of dietary protein will have a greater stimulatory effect on muscle protein synthesis.
The Take Home Message
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.
Suggestion: Try to get 3-5 g of Leucine with your post workout shake or meal.