Researchers from the University of Texas Medical Branch (Galveston) discovered that leucine appears to be the real key amino acid of the three BCAAs, as far as protein synthesis goes. Leucine acts much like a key in a lock inside the muscle that instigates the process of muscle protein synthesis. In addition, this amino boosts insulin levels. Insulin is an anabolic hormone that also stimulates protein synthesis, but via a different mechanism than leucine. That means leucine has a dual effect on protein synthesis. Although leucine is definitely the MVP of the BCAA team for stimulating protein synthesis, your best bet is to take leucine along with the other BCAAs. That’s because together, the BCAAs benefit muscle growth by other means as well.
Another way that the BCAAs enhance muscle growth is by facilitating growth hormone. A 2001 study from Italian researchers found that athletes taking BCAAs for one month had higher levels of growth hormone and growth hormone binding protein after exercise. GHBP is important because it acts as a carrier for GH in the blood, taking it to the muscles where it can do its job of instigating muscle growth.
In addition, another hormone that BCAAs affect is the catabolic hormone cortisol. A 2006 study presented at the International Society of Sports Nutrition’s annual meeting found that athletes consuming BCAAs had a significant reduction in cortisol levels during and after exercise as compared to those taking a placebo. This blunting effect on cortisol is critical because cortisol is a catabolic hormone that interferes muscle membrane, which is believed to initiate muscle growth.
Still another way that BCAAs enhance muscle growth is by maintaining higher levels of muscle glycogen, as University of São Paulo (Brazil) researchers discovered. Muscle glycogen is the storage form of carbs in muscle cells. Normally, muscle glycogen drops during a workout because it is used to fuel the muscles. This can compromise muscle size because glycogen pulls water into the muscles, keeping them full and large. Because supplemental BCAAs are readily used for fuel during workouts, muscle glycogen levels are higher after training. By sparing muscle glycogen levels, BCAAs work to maintain muscle size by keeping the muscles full. Fuller muscles place a stretch on the greater strength gains than those taking a placebo. Another study by Australian researchers reported in a 2006 issue of the European Journal of Applied Physiology that six weeks of leucine supplementation not only improved muscle power in outrigger canoeists, but endurance as well.
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