In my postdoc, I had identified there were vitamin D response elements in genes that convert tryptophan into serotonin in the brain ... If you don't have enough vitamin D, then you can't convert the amino acid tryptophan into serotonin in the brain. Serotonin is important for mood. It's important for executive function. It's important for impulse control. So people that don't have enough serotonin, they have a lot of problems with long-term planning and controlling their impulses.