Not only does serotonin regulate neurogenesis, it is also an important regulator of mood, impulse control, sensory gating, executive function and more. This study highlights the importance of exercise in brain aging, normal brain function, and mood. Tryptophan is the precursor to serotonin but it must be transported into the brain in order to produce it. Branched-chain amino acids (like leucine) outcompete tryptophan for transport into the brain; however, exercise causes branched-chain amino acids to be taken up by muscle cells, thus, allowing tryptophan unabated transport into the brain to make serotonin.