The Effects of Gut Health on Depression: A Review of The Literature

Brittany Cutler, The University of Georgia

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Major depressive disorder includes the presence of sad, empty, or irritable mood along with cognitive changes that significantly affect an individuals’ capacity to function. In the United States, major depressive disorder has potential onset at any age, but is likely to peak at age twenty. There are higher prevalence rates among females, and eleven percent of youth will experience depression (American Psychiatric Association, 2013; Avenevoli, Swendson, He, Burstein, & Merkangas, 2015). The second leading cause of death among adolescents ages 13 to 18, is suicide (Centers for Disease Control and Prevention, 2013).

The individual will receive a diagnosis of major depressive disorder if five or more symptoms are present for the same two-week period (American Psychiatric Association, 2013). The symptoms include sadness, loneliness, or hopelessness all day or nearly all day. Family or friends may report that the individual appears tearful. The individual has lost interest in almost all activities most or all of the day. The individual has lost a significant amount of weight, experiences insomnia or hypersomnia, fatigue, feelings of worthlessness and diminished ability to think or concentrate. The individual has recurrent thoughts of death, plans of suicide, or has attempted suicide (American Psychiatric Association, 2013). It is evident that psychological effects of major depressive disorder manifests physiologically. Individuals report excessive worry and pain that consist of head-aches, joint, abdominal, or other pains.

Depression is the most common mental disorder among adults. “The World Health Organization (WHO) recently announced that depression will soon become the number one health problem, affecting more individuals worldwide than any other medical condition and causing the greatest health burden” (Goethe, 2010). The high prevalence rates of depression are initiating new advancements in treatments and guiding efforts towards prevention. Research suggest that the microbiome-gut-brain axis effects mood and behavior and an unhealthy diet correlates a risk factor for depression (Dash, 2015). The gut microbiota is associated with not only major depressive disorder, but a plethora of metabolic disorders such as obesity, diabetes mellitus and neuropsychiatric disorders such as schizophrenia, autistic disorders, and anxiety disorders (Evrensel, 2015).

The gut microbiota, also known as gut bacteria, plays a significant role in connecting the gut to the brain. “The term microbiome refers to all microorganisms and their genetic material living in the body, and the term microbiota refers to populations of microorganisms present in the body’s various ecosystems, for example the gut microbiota or skin microbiota” (Evrensel, 2015). The gut microbiota is essential for human health, the immune system, and the development of the brain systems (Evrensil, 2015). New research has found that microbiota could activate the central nervous system and microorganisms in the gastrointestinal tract. This new research is imperative because gut microorganisms produce and deliver serotonin and gamma-aminobutyric acid, which act on the gut-brain axis (Evrensel, 2015). Gut health has a direct link to brain functioning, so maintaining a healthy diet can help the body stay balanced psychologically.

History Behind Gut Health

The connection between gut and mental health may ring innovative, but dates back to over 100 years ago. Scientist in the 19th and 20th century believed that accumulated waste in our colon produced poisons in our gut leading to depression, anxiety, and psychosis (Schmidt, 2015). The patients during that time were treated with colonic purges and bowel surgeries until those practices were believed to be unhelpful.

Current scientists are now circling back to that belief, but implementing a different approach. The focus is no longer about purging the body; it is about replenishing it. Scientists believe that the microflauna in our intestines can impact our minds.

“The gut-brain axis seems to be bidirectional—the brain acts on gastrointestinal and immune functions that help to shape the gut’s microbial makeup. The gut microbes make neuroactive compounds, including neurotransmitters and metabolites that also act on the brain. The microbial compounds communicate via the vagus nerve, which connects the brain and the digestive tract, and microbially derived metabolites interact with the immune system, which maintains its own communication with the brain” (Schmidt, 2015).

The microorganisms living in the intestines are in contact with the gut and immune system cells and through this contact many neuropsychiatric and metabolic disorders can develop (Evrensel, 2015). The microorganisms in the gut contain more genes than the human genome. The living microorganisms in the gut are transported into the human systematic circulation and lead to activation of inactive genes. This interaction leads to either health benefits or diseases (Evrensel, 2015). The gut contains a plethora of bacteria species and subspecies that produce active metabolites. For example, Eshcerichia and Enteroccus produce serotonin. The gut microbiota can change when under the influence of drugs, diet, and stress (Evrensel,2015). The researchers have done many trials on rodents as well as volunteers to test the effects of diet, drugs and stress on the microbiota in rats.  

Microbiota and Depression

Rodents are utilized to test microbiota and depression. They are given Campylobacter jejuni which induces anxious behavior (Evrensel ,2015). They are also given a bacterium called Bifidobacterium infantis. This bacterium is found in the neonatal intestinal tract and probiotic drugs. Depression can be elevated in rats by reversing this one bacterium (Evrensel, 2015). The bacterium is defined as a “psychobiotic” due to an antidepressant effect. A study conducted by Debonnet, indicated that bacteria called Bifidobacterium infantis leads to relaxation in neural processes (Evrensel, 2015). Another study with healthy volunteers indicated that subjects who took probiotics regularly had a decrease in stress levels and urinary free cortisol levels (Evrensel, 2015). In other clinical trials, the Bacteroides family is associated with depression. Milk fermented with probiotic bacteria showed a positive effect on emotional centers in healthy individuals. Also, another study by Tillisch, utilized functional magnetic resonance imaging and examined the link between probiotics and healthy brain functioning (Evrensel, 2015). Lastly, a study reported that low-level inflammatory condition is present in depression, which may be associated with gut permeability disorders (Evrensel, 2015).

“The microbiota has been suggested to be the key factor in the link between unhealthy diets and depression” (Evrensel,2015). If an individuals’ hormones are out of balance it can be caused by inflammation, and depression is an inflammatory disease. Inflammatory cytokines produce symptoms called “sickness syndrome” which overlap with depressive symptoms such as lethargy, sleep disturbance, decreased social activity, mobility, libido and the inability to experience pleasure from activities. Researchers have reported that patients with higher levels of inflammatory markers are less likely to respond to antidepressants. They are more likely to respond to anti-inflammatories (Brogan, 2015). Anti-inflammatories include primrose oil and fish oil, curcumin and probiotics (Brogan, 2015).

Diet and Depression

The link between unhealthy diets and depression cannot be reiterated enough. A systematic review concluded that a healthy diet is significantly associated with reduced odds for depression (Dash, 2015). Specifically, eating a Mediterranean-style diet was associated with a reduced risk for depression as well as cognitive decline (Dash, 2015). Furthermore, increased intake of fatty foods and sugar increased psychological symptomatology in children (Dash, 2015). “Depressive symptoms prompt the increased consumption of high-fat, sugary foods, although the long-term impact of these dietary habits is toxic. Microbial changes perpetuated by poor diet may drive and exacerbate depressive symptoms” (Dash, 2015). Specifically, eating fiber from fruits, vegetables, and other plants, can alter intestinal microbiota (Miki, 2016).

According to a study with Japanese employees, “higher intake of dietary fiber derived from vegetables and fruits was associated with significantly lower odds of depressive symptoms in Japanese employees even after adjustment for a variety of potential confounders including depression-related nutrients” (Miki, 2016).


Probiotics help the gut and brain communicate effectively. In randomized, controlled trials conducted by the Department of Occupational and Environmental Health, the meta-analysis indicated that probiotics decreased the depression scale score (Huang, 2015). They effected the healthy population as well as patients with major depressive disorder. The research indicates that there is a need for larger samples to solidify their theory, but overall depressive mood was significantly decreased.


As aforementioned, major depressive disorder is becoming an epidemic around the world. Patients are unique so treating them holistically is invaluable. Social workers are to treat the entirety of a patient, taking into account biological, psychological, and social factors. The knowledge and research pertaining to the gut-brain access will guide social workers in creating treatment plans that will address dietary needs.



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