Amino Acids & Exercise
Only six of the 20 amino acids are directly metabolized by muscle. These six amino acids are: alanine, aspartate, glutamine, isoleucine, leucine, and valine [21, 57]. These six amino acids are metabolized at accelerated rates during exercise [57]. They are also imtermediates that regenerate the aerobic-TCA energy cycle [57.] During exercise, the carbon atoms from these amino acids are unbidden by protein degradation. The Brain Chain Amino Acids (BCAA) and glutamine are then used to synthesize intermediates for use in the TCA cycle.
This is not good for muscle anabolism because the cellular levels of these amino acids greatly impact growth. Therefore when the supply is depleted, growth significantly suffers. Of these six amino acids, alanine, aspartate, and glutamine are nonessential, but isoleucine, leucine, and valine are the essential branch chain amino acids (BCAA), which serve an even greater role in energy metabolism and muscle growth.
BCAA are of extreme importance. BCAAs are absorbed directly into the circulatory system, bypassing the liver, which allows them to be used for rapid protein synthesis. Studies have shown that BCAA directly supply the nitrogen needed to create and export concentrations of alanine and glutamine produced by muscle [21, 30, 55, 56]. Because of this, BCAA concentrations are lowered from any type of exercise. One study showed that BCAA concentrations were decreased by 30% from aerobic exericise and 8-20% from anaerobic/aerobic exercise [30]. The largest decrease in BCAA concentrations were seen in anaerobic exercise, such as weight training [31]. Of the three BCAA, leucine is of greatest importance during exercise.
Transaminiation of leucine's nitrogen to alanine is doubled during exercise [57]. Leucine is the only amino acid that is capable of being completely oxidized in the TCA-aerobic cycle. Because leucine is an EAA, this oxidation capability is not good for muscle growth, as it can quickly deplete leucine levels. Leucine has been shown to directly stimulate protein synthesis and muscle turnover [43, 53], and without leucine, protein synthesis rates are impaired [11]. To make matters worse, leucine has the shortest half-life of all amino acids in the free pool of 45 minutes. This is compared to the 5-10 hour half lives of the other amino acids [30]. It is constantly being oxidized, leaving little for protein synthesis. Leucine levels need to be increased before protein synthesis can excel.
It should be obvious from examining the above information that the protein source used must contain large amounts of EAA, especially BCAA, and with even more emphasis on leucine. Mero showed that consuming a BCAA supplement, with 30-35% leucine, before or during exercise decreased the rate of protein breakdown, improved both mental and physical performance, and had a sparing effect on muscle glycogen levels [30].
To meet the need for amino acids during exercise, a supplement containing high amounts of EAA and especially BCAA is needed. In order for this protein to be as effective as possible, it needs be a fast absorbing protein. Therefore, protein powders such as casein and milk isolate are discouraged because of their slow digestion rates. Egg protein is another option, but it is still absorbed too slow. These slow digesting protein can create an environment in the intestines that competes with the muscles for blood flow.
