mTOR: A Protein That Controls How Large Your Muscle Grow

There’s a protein you may not be familiar with that plays a vital role in building lean body mass. It’s referred to as mammalian target of rapamycin or mTOR. mTOR is involved in cellular growth and proliferation as well as protein synthesis and breakdown. Why is this so important? When mTOR is “turned on,” it stimulates muscle protein synthesis so you can reap the full benefits of a resistance training workout. Understanding what turns mTOR on and off can help you better understand what you need to do to build lean body mass.

What Turns mTOR On?

This powerful protein inside muscle cells is very sensitive to energy status. When you’re adequately fed and consuming enough protein AND calories, it turns mTOR on. This enhances muscle protein synthesis so you can build lean body mass.

Of course, you have to stimulate the muscles by doing resistance training or there will be no real stimulus for muscle growth. mTOR is activated when muscles are “stressed,” such as by exercise, so repair of damaged fibers can take place. It’s this repair process that leads to hypertrophy. Lifting heavier weights and generating more power maximally activates mTOR. But if you’re in an energy-deprived state or not consuming enough protein, you won’t turn it on to the same degree and will have a hard time building muscle.

There are other factors that can turn mTOR on or off. For one, mTOR is activated by insulin. This makes sense since insulin levels are higher when you’re fed and not in a low energy state. This is why it’s important to consume enough calories and eat a carb and protein-rich snack after a workout. This creates an environment that activates mTOR to turn on protein synthesis. Many people try to build muscle while restricting calories. One reason this doesn’t work well is that a low energy state inactivates mTOR, creating an environment that’s not optimal for protein synthesis.

Branched Chain Amino Acids Activate mTOR

Branched-chain amino acids, valine, isoleucine, and leucine, are strong activators of mTOR. Of these leucine seems to be the most powerful stimulator of mTOR. There’s also evidence that branched-chain amino acids have other benefits. Some research shows that branched-chain amino acids help to reduce fatigue during a workout, although more research is needed to confirm this. Most importantly, branched chain amino acids boost protein synthesis by activating mTOR. They also help to reduce cortisol, putting a damper on protein breakdown. This works in your favor when it comes to muscle growth.

There’s a balance between protein synthesis and protein breakdown and you tip the balance toward protein synthesis when you get adequate amounts of protein and calories and don’t overdo the endurance exercise. Long periods of endurance exercise, especially combined with a low carb diet, can elevate cortisol levels and favor protein degradation and muscle breakdown. That’s not what you want when you’re trying to maximize muscle growth.

The Special Properties of Leucine

As mentioned, the branched chain amino acids, leucine, isoleucine, and valine, activate muscle protein synthesis through the mTOR pathway. Of these, leucine appears to be the strongest amino acid activator of mTOR. In one study, researchers found that supplementing with leucine in combination with carbohydrate and protein stimulated muscle protein synthesis more than carbohydrate and protein alone. Branched-chain amino acids like leucine also boost metabolism and decrease appetite, making them ideal after a workout. Getting these amino acids within an hour after a workout maximizes their benefits.

Resistance Training and Interval Training over Endurance Workouts for Muscle Growth

It’s important to do exercise that boosts your heart rate for the cardiovascular benefits and for fat burning, but long periods of endurance exercise creates a metabolic environment that’s not conducive to building lean body mass. That’s why shorter, high-intensity interval workouts are a better option if you want more muscle definition. Working intensely during the active intervals maximizes the release of anabolic hormones like growth hormone and testosterone with less release of cortisol.

For even more benefits, make sure you get a combination of protein and carbs after a resistance training workout. For bonus points, make sure you’re getting enough leucine. Surprisingly, one of the best sources of leucine is soybeans. Other natural sources include red meat, lentils, and peanut butter. Salmon is also a good source of leucine. If you can’t get enough leucine through diet, you can get it by taking a branched-chain amino acid supplement after a workout but it’s usually best to get your nutrients from foods rather than supplements.

 The Bottom Line?

Now you know why it’s important to get enough protein and calories when you’re trying to build lean muscle tissue and why shorter, high-intensity cardio sessions are better. Keep this in mind when you plan your diet and workouts.

References:

Nat Cell Biol. 2001 Nov;3(11):1014-9.

Journal of Applied Physiology May 2009 vol. 106 no. 5 1720-1729.

Proc Natl Acad Sci U S A. 2005 October 4; 102(40): 14238–14243.

Am. J. Physiol. Endocrinol. Metab. 288(4): E645-653.

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