This is a FASCINATING explanation for why Exercise physiologically alleviates depression. 

In our brain, we convert the amino acid, tryptophan, into serotonin - the feel-good neurotransmitter that signals contentment, happiness, and a sense of ease.  Turn off the lights and this serotonin gets converted to melatonin, which induces a deep, restful, and restorative sleep. 

BUT during times of inflammation in the body (think food sensitivities, environmental toxins, poor diet), tryptophan gets derailed like a railroad switch toward a different fate.  It instead gets converted to a compound called Kynurenine.  

Kynurenine itself isn’t necessarily bad, but it can get converted into one of two compounds: Kynurenic acid (good guy) or Quinolinic acid (bad guy).  

Quinolinic acid is thought to have developed as an evolutionary protective mechanism to kill brain infections.  It acts as a neurotoxin, killing the infection, but also doing us considerable damage as well.  As levels rise, our neurons are destroyed and we develop depression.

Guess what helps push tryptophan toward the serotonin pathway?  EXERCISE!  Exercise helps tryptophan get into the brain, supports serotonin production, AND it helps prevent the formation of quinolinic acid.  

BONUS!

Exercise also increases a compound called BDNF - Brain Derived Neurotrophic Factor.  This supports neurogenesis - the growth of new neurons in the brain.  It also supports plasticity - the ability for existing neurons to change and create new connections.  Why do children learn like sponges and pickup languages so quickly?  Neurogenesis & Neuroplasticity!