It’s widely accepted that Depression is the result of your brain & digestive tract making insufficient serotonin, hence SSRIs used to treat it. There are proven ways to increase serotonin naturally: Light therapy, daily exercise, increasing tryptophan, b12, & fiber, adding pre & probiotics, limiting sugar, getting better sleep, getting back to nature, and if none of this helps, for those who haven’t been over diagnosed, Ketamine. Serotonin is mostly created in the stomach (90-95%) So I believe the sugar lobby is becoming a large player in countering the serotonin theory, just as they pushed to prove fats were worse than sugar in the 80s & 90s. The recent answer “we don’t know what causes depression”. Due to mass produced processed foods packed with sugar, and chemical spraying, our diets objectively make us sick. Combine that with inactivity and we aren’t getting sufficient blood to the digestive tract. And when you aren’t sleeping, you aren’t digesting food at an optimal level. For those who truly have an imbalance, and not an over diagnosis to sell drugs, small amounts of ketamine are shown to help.