At the molecular level, there may be a few reasons why intense aerobic exercise gives such a strong benefit. The first has to do with a precursor for a neurotoxic substance called kynurenine, which is formed from the amino acid tryptophan. Some of you may be familiar with tryptophan because of its important role as a precursor for other bioactive compounds, including serotonin, a neurotransmitter that's important for mood and cognition. Exercise actually increases the availability of tryptophan to be transported into the brain, which is a good thing because in the brain it forms serotonin. However, tryptophan is also a precursor for another compound called kynurenine, which can cross the blood-brain barrier and in the brain form a neurotoxin called quinolinic acid.

Quinolinic acid can cause cells to die in the brain. It's also associated with depression and other mental health and neurological disorders. So what determines whether tryptophan will be transported into the brain to form serotonin or whether it will form kynurenine? Well, it turns out the bioavailability of tryptophan for these different biochemical pathways is influenced by a variety of factors ranging from stress to inflammation to exercise. Stress hormones like cortisol can shunt tryptophan away from forming serotonin because it activates enzymes that are involved in converting tryptophan into kynurenine. Similarly, chronic low-grade inflammation can also shunt tryptophan away from forming serotonin because it activates other enzymes that are involved in converting tryptophan into kynurenine.