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The guys in the gym tell me I should be supplementing with BCAA, especially leucine. What is so special about this supplement?

Researchers have known about leucine for decades, but exciting new research has shed a new and broader light on this important health-promoting amino acid. Leucine is one of the essential amino acids, which simply means our bodies cannot make leucine and must get it from dietary sources. Leucine is also the most abundant of the three branched-chain amino acids (BCAA), the other two being isoleucine and valine. BCAA make up a high proportion of the amino acids in muscle. BCAA are also unique because they are the only amino acids that are burned by muscle as fuel and thus both blood and muscle levels decrease after exercise. Leucine in particular is the only amino acid that increases in direct proportion to dietary intake. Historically, amino acids were simply viewed as substrates for making muscle and other body proteins - sort of like the bricks needed to build a house. In the case of leucine, recent investigations have shown multiple roles of this amino acid that go well beyond simply providing material to build muscle.

Leucine and Protein Synthesis

Whether you are interested in building muscle or preventing muscle loss that normally occurs during aging and weight loss, stimulating protein synthesis is absolutely key. It is probably not news that simply consuming protein induces an increase in muscle protein synthesis, which is why protein supplements are popular to consume after exercise. Researchers have begun to refine their knowledge on what it is about protein that stimulates anabolism in muscle. For example, scientists have shown that it is the level of amino acids in the blood that directly boosts protein synthesis in muscles. Specifically it is the amino acid leucine that is most highly related to protein synthesis. In a series of elegant experiments, researchers directly examined muscle protein synthesis after feeding animals various formulations of amino acids and compared them to glucose ingestion. When a complete protein was consumed (one that contains all the amino acids), protein synthesis was increased. When just essential amino acids were provided and the nonessential ones left out, the same increase was noted indicating nonessential amino acids are not required to stimulate protein synthesis. When just the BCAA were given, again there was the same increase in protein synthesis. Finally when just leucine was consumed, yet again protein synthesis increased to the same magnitude. These findings provided strong evidence that leucine was the driving force behind the ability of dietary protein to stimulate protein synthesis.

A series of cellular studies has now clearly shown that leucine directly activates a critical compound in muscle called the mammalian target of rapamycin (mTOR). It turns out mTOR is like a molecular switch that turns on the protein synthetic machinery in muscle and leucine is one of the major activators of mTOR. Thus, leucine not only provides the building blocks for protein synthesis, it also has a critical role in up-regulating the process. For example, even when an overabundance of amino acids are available to provide the building materials for new muscle, adding extra leucine augments protein synthetic rates further. The application of all this work is that adding additional leucine to typical protein beverages is an effective strategy to maximize muscle anabolism after resistance exercise.

Leucine and Body Fat

A large number of studies have shown that high protein diets help spare muscle loss during weight loss. Researchers at the University of Illinois have conducted studies that examined weight loss diets that contained 10 g/day of leucine and 125 g total protein per day with a minimum of 2.5 g leucine at each meal. In two separate studies, this group demonstrated that the high leucine diets resulted in greater weight loss, greater fat loss, and better preservation of lean body mass. In these studies, the high leucine diets also resulted in better glucose control. Finally, a recent animal study showed that doubling leucine intake prevented diet-induced obesity and improved a host of metabolic disturbances.

Practically Speaking

The ideal amount of leucine to take is a matter of debate among nutritionists and scientists. In acute studies, oral intake of as little as 2.5 g of leucine stimulates protein synthesis. For chronic studies, leucine intakes equivalent to 8 or more grams per day are recommended divided so that at least 2.5 g of leucine are consumed at each meal. The best food sources of leucine include any proteins from animals that naturally contain all the essential amino acids. The protein source with the highest leucine content is whey which contains about 10 g per 100 g of protein. Other protein supplements like casein and soy contain less leucine. Leucine can be purchased as a single amino acid and added to other protein sources like whey or casein. One cautionary note: supplementing with just leucine alone will not result in optimal effects if the background diet is low in protein. This is because the other BCAA (isoleucine and valine) will be preferentially oxidized and lead to a BCAA imbalance that compromises anabolism. Thus it is important to consume sufficient protein or at least supplement with all three BCAA if total protein intake is low.