At a physical level, "the human body" is not a single organism. A human being operates a body which is a co-operative enterprise consisting of human cells plus bacteria, viruses, even parasites, each part of which contributes to the survival of all. Science has discovered that disruptions in the ecosystem have major effects on "mental health".
In addition to that physical aspect of a person–the various molecules and cells–there is the mind, which consists of a form of organized energy, and which interacts with the body in many ways. Body imbalances can influence mental states, causing anxiety, depression and other issues labeled as "mental illness", for which the usual treatments are (often deadly) drugs.
Exposure to antibiotics, chlorine and other chemicals in air, water and food can suppress the probiotics (friendly bacteria) in the gut, disrupting gut balance and health. The result is mental stress of various kinds.
Have you ever had to take antibiotics for a while and felt sort of "off" or "strange" or depressed or anxious? I know many who have experienced that. Some people feel depressed or unhappy all the time because they have never really enjoyed good gut health in the first place!
Recent research has revealed a definite connection between gut health and mental health. Multiple strains of probiotics work together to improve mental states, and deficiencies of these probiotics worsen mental states. This research merely confirms my own decades of direct observations in this field.
Many of our great grandparents fermented and consumed their own foods, such as sauerkraut. The probiotics in such foods were actually maintaining their gut health. The incidence of "mental illness" in those days was very low. Now it's an epidemic! The toxic environment is quite literally driving the world mad!
I created my Rest Easy and Inner Garden probiotics blends to help restore gut health. I find that consuming these keeps me in good shape, ready to handle the other stresses of life more effectively.