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 acidleucine – 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!

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.