At the turn of the 20th century, prominent physicians who were trying to understand where mental illness comes from seized on a new theory: autointoxication. Intestinal microbes, these doctors suggested, are actually dangerous to their human hosts. They have a way of inducing “fatigue, melancholia, and the neuroses,” as a historical article in the journal Gut Pathogens recounts.
“The control of man’s diet is readily accomplished, but mastery over his intestinal bacterial flora is not,” wrote a doctor named Bond Stow in the Medical Record Journal of Medicine and Surgery in 1914. “The innumerable examples of autointoxication that one sees in his daily walks in life is proof thereof ... malaise, total lack of ambition so that every effort in life is a burden, mental depression often bordering upon melancholia.”
Stow went on to say that “a battle royal must be fought” with these intestinal germs.
Another physician, Daniel R. Brower of Rush Medical College, suspected that the increasing rates of melancholia—depression—in Western society might be the result of changing dietary habits and the resulting toxins dwelling in the gut.
Of course, like most medical ideas at the time, this one was not quite right. (And the proposed cures—removing part of the colon or eating rotten meat—seem worse than the disease.) Your gut doesn’t contain “toxins” that are poisonous so much as it hosts a diverse colony of bacteria called the “microbiome.” But these doctors were right about one thing: What we eat does affect how we feel, and gut microbes likely play a role.