Leucine powder is sold as a bodybuilding supplement, meant to be mixed with fluid and consumed twice daily, especially after a workout. Leucine is an amino acid more commonly found in branched-chain amino acid supplements, where it can work together with isoleucine and valine to prevent muscle breakdown during exercise and help stimulate muscle growth. Leucine is also sold separately because it is broken down and absorbed faster than the other BCAAs. Whether it works as well in practice as it does in theory remains to be seen, however, and research is ongoing.
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Leucine stimulates muscle protein synthesis and absorbs quickly, causing leucine levels in the blood to rise quickly and dramatically according to a study in the January 2014 issue of "Nutrition Journal." The more leucine there is to go around, the more protein synthesis can occur, theoretically.
Leucine's effect on protein synthesis is greater than that of other amino acids, according to the August 2010 "Journal of Nutrition." When you lift weights, it causes damage that breaks down your muscles. As protein synthesis occurs, not only do your muscles repair themselves, but they steel themselves for the next onslaught of abuse by making your muscle cells larger. When this occurs frequently over time, the evidence is seen as muscle growth. Leucine's job is to keep your muscles from breaking down, but then to allow them to rebuild at the same rate or more than they would have anyway. In other words, your muscles will rebuild at least the usual amount, but they won't have to compensate for muscle damage at the beginning of the process. This means that all growth that occurs would contribute to increased size.
Leucine has also been explored as a fat-loss drug. When you restrict your calories, some of the weight you lose can come from muscle mass, which can slow your progress. Muscle requires energy to exist, so it burns calories, even when you're sitting still. Fat is just dead weight. So ideally, you want to hold on to as much muscle as possible while shedding fat only. Exercise helps with this, but leucine supplementation may help your muscle survive the calorie restriction better by providing an environment and ingredients for muscle growth. It may also play a role in reducing hunger and fatigue by stabilizing glucose levels. A 2003 study in the "Journal of Nutrition" concluded that BCAAs, but especially leucine, could play a beneficial role in weight-loss programs.
Branched-chain amino acids make up about 15 to 25 percent of the protein you eat, according to a study at Duke University, published at DukeHealth.org. Before you take a leucine supplement you should take note of their findings, which is that too much protein, or BCAAs, if consumed along with fat, may increase insulin resistance and your risk of obesity. If you eat a balanced diet and consistently get enough protein, a supplement may be unnecessary for you. Animal proteins are complete proteins, meaning they provide all the essential amino acids. Meat, poultry and dairy are especially high in leucine. Vegetarians must eat a variety of foods to get all of the amino acids, but soy, lentils and peanuts are all high in leucine; in fact, soy and lentils contain more leucine than beef.