Gut bacteria and even all bacteria en virusses that compromise the ecosystem that is your body greatly determines not only your physical but even your mental health. I don't really think this is a fad. The first 1000 days of a child (including pregnancy) are the most important for humans to determine their future wellbeing. https://www.psychologytoday.com/us/blog/neuroscience-in-ever... https://www.sciencedaily.com/releases/2018/02/180223092441.h...