A number of studies have confirmed that gastrointestinal inflammation can play a critical role in the development of depression, and that healthy bacteria may be an important part of the treatment. For example, a Hungarian scientific review published in 2011 made the following observations:

1. Depression is often found alongside gastrointestinal inflammation, as well as autoimmune, cardiovascular and neurodegenerative diseases, and chronic low-grade inflammation is a significant contributing factor in all of these. Thus, "depression may be a neuropsychiatric manifestation of a chronic inflammatory syndrome."

2. A number of clinical studies have shown that treating gastrointestinal inflammation with probiotics, omega-3 fats and vitamins B and D also improves symptoms of depression by attenuating proinflammatory stimuli to your brain

3. Research suggests the primary cause of inflammation may be dysfunction of the "gut-brain axis." The gut-brain connection is well-recognized as a basic tenet of physiology and medicine, so this isn't all that surprising. Your gut acts as a second brain, and is in fact created from the identical tissue as your brain during gestation.

If you consume loads of processed foods and sugars, your gut bacteria will be severely compromised because processed foods tend to decimate healthy microflora. This leaves a void that is filled by disease-causing pathogenic bacteria, yeast and fungi that instead promote inflammation

Other research has shown the probiotic Lactobacillus rhamnosus has a marked effect on GABA levels — an inhibitory neurotransmitter involved in regulating many physiological and psychological processes — in certain brain regions, lowering the stress-induced hormone corticosterone. As a result, anxiety- and depression-related behavior was lessened. Strong connections between the gut microbiome and schizophrenia and bipolar disorder have also been found.