Leucine is one of 20 amino acids that your body uses to make new proteins. The proteins in your muscles, and elsewhere in your body, are constantly undergoing turnover as old proteins are broken down and new ones built. When you strength train, the stress of training tears muscle fibers that must be repaired and rebuilt. You need a steady supply of amino acids to do that.
Your body can make 11 of the 20 amino acids that your body needs, so you don’t have to get these through diet. This collection of amino acids is called non-essential amino acids. However, there are nine amino acids that your body can’t make and you must get through diet. They include tryptophan, threonine, phenylalanine, methionine, histidine, isoleucine, leucine, lysine, and valine. These amino acids are essential amino acids because it’s essential that you have them in your diet.
While you need each of these essential amino acids, one, in particular, has a function that sets it apart from the rest. That amino acid is leucine. What makes leucine so special? Leucine, along with isoleucine and valine, are called branched-chain amino acids, referring to their branched structure. Leucine plays a special functional role that sets it apart. In fact, it’s the “head honcho” with regard to activating muscle protein synthesis.
How Leucine Aids in Muscle Growth
What makes leucine distinctive is its ability to turn on muscle protein synthesis. It essentially acts as a signaling molecule that activates the mTOR pathway. The mTOR pathways, also known as the mammalian target of rapamycin, is the most important pathway for the synthesis of new muscle protein. However, leucine doesn’t act alone. It gets a little help from insulin. The combination of leucine and insulin is a powerful combination for activating mTOR and boosting muscle protein synthesis. In contrast, calorie restriction, where insulin and leucine are at low levels, turns off mTOR.
Now, you know why nutrition and adequate calorie and protein consumption is so important for building muscle proteins. Resistance training, too, turns on mTOR, as long as you exceed a certain intensity threshold. Lifting light weights that don’t fatigue your muscles isn’t enough of a stimulus.
Another benefit of leucine is that it reduces muscle breakdown. If you consume a low-calorie diet or do excessive cardio while dieting, your body goes into a catabolic state and breaks down muscle tissue. The liver can harvest the amino acids from protein breakdown to make glucose. It’s a nifty little mechanism your body has for creating glucose when you need fuel. Leucine helps to keep your body out of a muscle wasting, catabolic state. This makes leucine especially important during times when your body is under stress.
Finally, leucine helps to switch off your appetite when you consume it with a meal. Research shows that leucine interacts with the appetite hormone, leptin. When your leptin level drops too low, you feel hungry and are more likely to snack on unhealthy foods. If a meal contains adequate leucine, your leptin level rises and you feel fuller and more satisfied. So, leucine may curb your appetite when you’re trying to lose weight.
Does leucine have other benefits? Some older studies suggested that leucine might reduce fatigue during endurance exercise. During endurance exercise, your brain takes up tryptophan. The tryptophan is converted to serotonin and the higher level of serotonin can cause fatigue. The idea was that leucine would interfere with the uptake of tryptophan by the brain and thereby reduce fatigue. More recent studies call this idea into question.
Another situation in which leucine might prove beneficial is for preserving muscle in older people. With age, both men and women develop anabolic resistance, where muscles become less responsive to signals that turn on muscle protein synthesis. At least in rodents, leucine helps to restore muscle protein synthesis in elderly rats.
Finally, some studies suggest that branched-chain amino acids may reduce muscle damage related to exercise and may enhance muscle recovery.
Are You Getting Enough?
Exercise itself can deplete your body’s amino acid stores. After strength training, your leucine level drops by as much as 30%. Even endurance exercise can deplete your body of this essential amino acid. One study showed that supplementing with 50 mg. of leucine per kilogram of bodyweight daily helps prevent the depletion in response to exercise. That’s why it’s a good idea to supply your body with leucine before or after a strength workout. Some studies show that consuming a source of this amino acid before a workout is more effective for activating muscle protein synthesis than at other times.
What about the other branched-chain amino acids? Although all of the branched-chain amino acids have some effect on mTOR, leucine is the most powerful activator. To get leucine, you can take a branched-chain amino acid supplement. However, if you’re consuming sufficient protein, you don’t necessarily need to take a supplement. Meat and dairy contain substantial quantities of leucine and the other branched-chain amino acids.
If you eat a vegan diet, you can get leucine by supplementing your diet with pea protein. Soybeans are another non-animal source of this amino acid. Taking a high dose of one or a few amino acids may interfere with the absorption of others. Plus, high doses of leucine can cause a drop in blood sugar.
Since there appear to be benefits to consuming leucine prior to a workout, consider consuming leucine-rich foods or drink a whey protein or pea protein shake to supply your muscles with this essential amino acid. Keep in mind, there’s no evidence that leucine or branched-chain amino acids supplements offer additional benefits for people who consume adequate protein. Where a branched-chain amino acid supplement might be beneficial, beyond an adequate protein diet, is in the elderly where anabolic resistance is a problem.
The Bottom Line
Make sure you’re getting enough protein in your diet and that you have amino acids, like leucine, in your bloodstream prior to your workout. Ideally, aim for 8 grams of leucine daily in divided doses. Resources are available online that will tell you how many grams are in different foods. If you don’t consume animal protein, pea protein is a good alternative as it contains a substantial percentage of branched-chain amino acids. Regardless of how you do it, make sure you’re getting your protein!
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