Leucine

We have talked a lot about the benefits of BCAAs.  I mentioned that Leucine is the most important of the BCAAs.  When it comes to signaling muscle building (protein synthesis), Leucine is king.  It has been shown to increase protein synthesis by up to 145%. In terms of direct effect it may be the single most important amino acid for hypertrophy.

Leucine is one of the branched chain amino acids (BCAA).  Exercise leads to the oxidation of BCAA’s as a primary source of fuel as well as signaling pathways for recovery.  BCAA’s are unique in that they can lead to increased insulin and cortisol depending on the situation. Leucine is the most important of the 3 BCAAs for signaling muscle growth.  Leucine leads to the signaling of a pathway called mTOR, which is the same signaling pathway that is activated by growth factors like IGF-I. In fact Leucine acts in a more direct way on the mTOR pathway than growth factors because it does not require as many intermediate steps.

The mTOR pathway activity increases in response to muscle activity during hypertrophy and decreases in activity during atrophy.  IGF-1 activates AKT, GSK-3beta and mTOR to promote hypertrophy. The mTOR pathway leads to regulation of protein translation, with increased translation acting as a key regulatory point in skeletal muscle hypertrophy.

The activation of the Akt/mTOR pathway and its downstream targets, p70S6K and PHAS-1/4E-BP1, are directly involved in regulating skeletal muscle fiber size.  So if your goal is to grow muscle, you have to activate this pathway, and often.

Leucine works synergistically with the other BCAAs enhancing its effects, but it needs to be within a certain ratio. BCAAs with leucine in a ratio of about 4 to 1 with valine and isoleucine have been shown to be most effective for muscle building. When leucine intake is too far out of balance with the other BCAAs, it can lead to an imbalance in the blood amino acid levels, reducing the anabolic signaling of leucine.