There are many causes of depression. Hormone deficiencies, neurotransmitter imbalances, sugar addiction, and nutritional imbalances are the most common. But there's another cause of depression that most aren't aware of -- even though the evidence goes back almost a century.
The authors of a paper entitled "Intestinal microbiota, probiotics and mental health: from Metchnikoff to modern advances," discuss what at first seems an unlikely cause of mental disorders — intestinal bacterial imbalances. They start off by quoting neuropathologist Armando Ferraro and clinical psychiatrist Joseph E. Kilman of the New York Psychiatric Institute.
Together, in 1933, they wrote the following in an article published in Psychiatric Quarterly entitled, "Experimental toxic approach to mental illness." They write "It is far from our mind to conceive that all mental conditions have the same etiological factor [cause], but we feel justified in recognizing the existence of cases of mental disorders which have as a basic etiological factor a toxic condition arising in the gastrointestinal tract." Now how in the world is a "toxic condition" in the intestines going to cause depression?
First of all, although the intestines are supposed to prevent the absorption of toxins into the bloodstream, they often fail. Many people have alterations in the way their intestines protect them from toxins. The condition is loosely referred to as "leaky gut syndrome." When these toxins get into the bloodstream, they go to the brain. And, in certain susceptible people, they can cause depression and other mental symptoms. In addition, Ferraro and Kilman reported that gut-derived toxins produced brain toxicity at unusually low doses because of the way they work together. To further add to the problem, intestinal toxin levels are increased by antibiotics (prescribed and in our foods), infections, stress, alterations in liver and/or kidney function, and unhealthful diets. The connection is further made in an article entitled, "Major depressive disorder: probiotics may be an adjuvant therapy."
In that article, the authors point out that the cause of major depression is often complex. They go on to say that "Emerging research suggests that nutritional influences on major depressive disorders are currently underestimated." Then they list the reasons. It's because patients with major depressive disorders have "elevated levels of pro-inflammatory cytokines, increased oxidative stress, altered gastrointestinal function, and lowered micronutrient and omega-3 fatty acid status." All of these factors are affected by our diets.
Additionally, there's the issue of an imbalance in our intestinal bacteria. Stress is a known factor in major depression. And stress is known to alter intestinal bacteria by lowering the levels of the healthy bacteria lactobacilli and bifidobacterium. And why is this important to our mental function?
It's because research suggests that bacteria in the intestinal tract can communicate with the central nervous system, even in the absence of an infection in the classic sense. Mice show signs of systemic inflammation of the intestinal tract after exposure to stress.
This is why probiotics (found in raw fermented foods) can correct stress-induced intestinal bacterial imbalances and thereby lower systemic inflammatory cytokines, decrease oxidative stress, and improve nutritional status. And, in this way, probiotics may be an important part of the way we treat and prevent depression. According to the authors, "It is our contention that probiotics may be an adjuvant to standard care in major depressive disorder."
Bested AC, Logan AC, et al. Intestinal microbiota, probiotics and mental health: from Metchnikoff to modern advances: Part II – contemporary contextual research. Gut Pathog. 2013; 5: 3.
Logan AC, Katzman M. Major depressive disorder: probiotics may be an adjuvant therapy. Med Hypotheses. 2005;64(3):533-8.