I mentioned a long time ago about mTOR, the signalling pathway in cells causative of muscle protein synthesis.
I also mentioned a few weeks back about AMPK, a signalling pathway that increases nutrient uptake into cells.
Although they sound like a great pair, they really don't like each other. Not one bit. In fact, if you activate mTOR, then AMPK tends to become immediately suppressed and if you activate AMPK, then mTOR tends to becomes suppressed.
For these reasons, it would probably be good to avoid activating AMPK excessively near a workout if the goal is muscle mass accrual and to avoid activating mTOR needlessly if your goal is fat loss (although leucine can be good for preserving lean mass; use it only if needed)