You need protein to build lean body mass – no surprise here. Proteins are essentially sequences of amino acids. Your digestive tract breaks down protein from the food you eat and delivers the amino acids to muscle cells, so they can use them for muscle protein synthesis. Amino acids are the basic building blocks from which new proteins are made.
Not all amino acids are created equal. Some your body can make, while others you have to get through diet. One group of essential amino acids called branched-chain amino acids, which includes leucine, isoleucine, and valine, are powerful promoters of muscle protein synthesis, and of these, the branched chain amino acid, leucine reigns supreme.
Leucine is Directly Linked with Muscle Protein Synthesis
What makes leucine such a force? It has more impact on protein synthesis than any other amino acid. In a study carried out at the University of Illinois, leucine blood levels were directly correlated with the degree of muscle protein synthesis. When leucine levels were highest, so was muscle protein synthesis. When leucine levels dropped, muscle protein synthesis also fell.
Leucine has the power to turn on an essential pathway needed for muscle protein synthesis. This pathway, called the mTOR pathway, tells muscle cells that it’s time to build new proteins. Leucine, along with other amino acids, donates the building blocks to do this. Although the mTOR pathway isn’t completely understood, this form of “anabolic signaling” is essential for building muscle proteins. Leucine works hand in hand with insulin to turn on protein synthesis.
Staying in Positive Nitrogen Balance
After a workout, your muscles eagerly suck up the amino acids they need for muscle repair. During long periods of endurance exercise, especially in a carbohydrate-depleted state, your body breaks down muscle protein to use the amino acids to make glucose to fuel your workout when muscle glycogen stores are low. When this happens, your body goes into negative protein or nitrogen balance. Needless to say, this is a catabolic state and not one you want to be in when you’re trying to conserve muscle.
After a resistance training or an endurance workout, you need amino acids, particularly leucine, to maintain a positive protein balance. If your body falls out of positive protein balance, you won’t be able to synthesize new muscle protein and your muscles can’t grow. That’s why many experts recommend consuming protein, in combination with carbohydrates, within an hour of a workout. Carbohydrates not only replenish muscle glycogen stores but transiently increase insulin to help get amino acids into muscle cells where they can be used for muscle growth and repair.
Leucine Helps Preserve Lean Body Mass
Although all amino acids serve as building blocks for proteins, leucine is a potent activator of muscle protein synthesis through the mTOR pathway. As mentioned, leucine also has anti-catabolic effects. When you supply your body with leucine and other branched chain amino acids after endurance exercise, it helps prevent muscle breakdown.
Leucine also counters the anti-catabolic effects of calorie restriction. If you’re eating too few calories because you’re trying to lose weight, getting enough dietary leucine may prevent or reduce muscle breakdown. Of course, it’s not a good idea to go on a very low-calorie diet, even if you have lots of weight to lose. Doing so will slow our metabolism and ultimately make it harder to shed those extra pounds.
Age-related loss of muscle tissue puts older people at risk for falls and destroys their ability to do many of the things they enjoy. Of course, they could hang on to more muscle tissue if they trained with weights, but not everyone is willing to do that. To make matters worse, some older people don’t get enough protein through diet. Research shows leucine supplements in older people jumpstarts muscle protein synthesis.
Leucine may truly be the “king” of amino acids. In a study published in Biochemical and Biophysical Research Communications, researchers quote: “A strong case can be made that proper leucine concentration in compartments in the body is critically important for maintaining body protein levels beyond simply the need for this essential amino acid for protein synthesis.” Shows its importance, doesn’t it?
Best Dietary Sources of Leucine
As you might expect, meat is a good leucine source with beef topping the list. A 5-ounce serving of beef has around 4 grams of leucine. Chicken and fish have slightly less with a serving supplying between 3 and 3.5 grams of leucine. Dairy products have more modest amounts of leucine with a serving of cheese or yogurt supplying about a gram and a cup of milk slightly less.
Surprisingly, vegetarian foods have fairly high quantities of leucine, although some of these foods lack one or more essential amino acids. A cup of beans or lentils has 3.5 grams of leucine and a cup of soybeans between 5 and 6 grams.
For convenience, some bodybuilders consume a whey supplement after a workout to supply their body with post-workout protein. A 20-gram scoop of whey has about 3 grams of leucine, while casein and soy contain less. It isn’t necessary to use whey to get enough leucine to optimize protein synthesis. As you can see real foods contain adequate quantities of this anabolic amino acid. If you take a whey protein supplement for convenience, do your research first. Some whey protein supplements, when tested by third parties, were found to contain contaminants including heavy metals.
Since your body can’t store protein, it’s best to spread your amino acid and leucine intake out evenly over the day rather than getting a disproportionate amount at one meal, for most people, that meal is dinner. You can only absorb so much protein at one time, between 20 and 30 grams, so make sure you’re not getting large amounts at one time.
The Bottom Line
Leucine plays a dual role in muscle protein synthesis – it supplies building blocks for muscle growth and it turns on protein synthesis by acting as a signaling molecule. It’s an example of an amino acid your body can’t make and one that you have to get through diet. Ideally, you should consume between 2 and 3 grams of leucine after a workout and with each meal. Don’t forget that leucine is key to turning on protein synthesis. Make sure you’re getting enough of it.
References:
Journal of Nutrition; June 2009, Vol. 139, No. 6, 1103-9.
Am J Clin Nutr. 2011 Sep;94(3):809-18. doi: 10.3945/ajcn.111.017061. Epub 2011 Jul 20.
BMC Physiology 2011, 11:10 doi:10.1186/1472-6793-11-10.
Med Sci Sports Exerc. 34(5):828-37.
Am. J. Physiol. Endocrinol. Metab. 288(4): E645-653. (2005)
Nutraingredients-USA.com. “Leucine supplements may combat muscle loss in older people” (2012)
Biochemical and Biophysical Research Communications. 313, no. 2: 391-396. January 2004.
United States Department of Agriculture Agricultural Research Service. “Leucine”
Nutraingredients-USA.com. “Leucine: Where whey protein gets its magic”
Consumer Lab. “Protein Powder and Drinks Review”
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