Mental disorders comprise a broad range of problems, with varying symptoms. Depression is one such mental illness that is recognised around the world and affects approximately one in ten people, with >320 million people currently diagnosed with the disorder. It is recognised as a long lasting low mood disorder, which affects your ability to do everyday things, feel pleasure or take interest in activities.
Depression can lead to an increased risk for developing several diseases including cancer, heart disease and suicide. Unfortunately, effective therapies to treat depression are not universally available as regrettably approximately one-third of patients do not respond to standard antidepressants. There is thus a need to reduce the increasing public burden of depression and the development of novel methods for the treatment of depression and strategies that promote mental health.
Is the Gut Microbiome Involved in Depression?
The gut microbiome, commonly known as intestinal flora, consists of trillions of microorganisms. These microorganisms are mainly bacteria, but eukaryotes, eukaryote viruses and cells, genes and metabolites from them can also be found. The gut microbiome aids digestion and the absorption and synthesis of nutrients. Furthermore, recent evidence suggests that the gut microbiome interacts with the host central nervous system for the gut-brain axis, and there is also evidence of bi-directional cross-talk occurring between the gut and the lung of the host, which has been termed the gut-lung axis.
The gut-brain axis is now actively being studied to determine the role of the gut microbiome and how it may effect brain development, brain function and behaviour of the host. Additionally, several studies have shown that a disruption in the gut microbiome may be implicated in neuropsychiatric disorders such as bipolar disorder, depression and schizophrenia. In fact, it is hypothesised that people with depression have distinct gut microbiomes in comparison to healthy controls. Therefore, it stands to reason that the gut microbiome may be a potential novel target for antidepressant agents.
The Role of Diet and Nutrition
Diet plays an important role in determining the composition of the gut microbiome, where gut microorganisms help to assimilate dietary nutrients, which are otherwise indigestible. Changes in dietary patterns can have an effect on the gut microbiome and lead to the development of intestinal disorders, such as diabetes, obesity, colon cancer and inflammatory bowel disease. Furthermore, diet and nutrition have received increasing attention for their significant roles in regulating mental health, and current research has shown that they may be linked to mental disorders such as aggression, anxiety and depression. It is known that nutritional intervention can be utilised to support well-being and manipulate mental health problems. A tryptophan-rich diet is known to reduce stress-related mood disorders by regulating serotonin levels, and a diet rich in ω-3 polyunsaturated fatty acids and flavonoids appears to lower the risk for depression. As such, diet and nutrition are convincing alternatives to drugs in the treatment of mental disorders.
Probiotics are live microorganisms that are promoted as having various health benefits by restoring the natural balance of bacteria in your gut. They’re usually added to yoghurts or taken as dietary supplements, and can be described as friendly, or good, bacteria. Recent research is emerging to suggest that probiotics may have anxiolytic and antidepressive properties. In animal studies, the consumption of probiotics has shown to increase the level of particular neurotransmitters, such as γ-aminobutyric acid, serotonin and its precursor, and brain-derived neurotrophic factor, which are all implicated in depression. A further study has shown that rodents treated with probiotics had a reduction in proinflammatory cytokines, which are also all implicated in depression, resulting in lower anxiety and depressive-like behaviour. Another study, this time in the clinic, where patients with depression were given probiotic supplementation resulted in the patients having a significantly reduced severity of depression in a self-reported mood test.
Korean Nationwide Study
A nationwide cross-sectional study in Korea sought to conduct a well-controlled, large population-based study for better understanding of the regulatory roles of probiotics on depression. The scientists aimed to investigate the hypothesis that probiotic food consumption is negatively associated with the prevalence or severity of depression. This study included over 26,000 people aged 19-64 years of age and included a food frequency questionnaire to assess probiotic food consumption. Depression status was determined by two different methods, which included a Patient Health Questionnaire and self-reported clinical diagnosis.
The group that consumed the highest amount of probiotic food had significantly lower results of depression severity in both the Patient Health Questionnaire and the self-reported clinical diagnosis, in comparison to the group consuming the lowest amount of probiotic food. Surprisingly, this study has shown that there was a significant effect of probiotic food consumption on lower self-reported clinical depression in men but not in women. Possible biological factors that resulted in these differing results may include genetic predisposition, hormonal differences and psychopathology. It is interesting to note that a higher BMI and energy intake were observed in the individuals with a higher consumption of probiotic foods, as well as healthier lifestyles and lower cigarette use. The study does show that the beneficial probiotic effects remained in lowering the risk for depression after an adjustment was made for health status and lifestyle.
It is hypothesised that the gut microbiome may have a fundamental role in the modulation of mood disorders and behaviour via different mechanisms. Furthermore, the gut microbiome modulates systemic and gut inflammation, which has also been linked to psychiatric disorders. Therefore, it stands to reason that improving the gut microbiome can lead to a suppression of inflammation, resulting in improved mood and behaviour.
It has been hypothesised that probiotics may have beneficial effects on a variety of health problems including immunologic diseases, metabolic disorders and mental disorders. Ongoing research is showing the potential of probiotics to improve brain function in humans through the gut–brain axis and that probiotics have the potential to manipulate the gut microbial composition and hence play a role in the treatment of mental disorders.
A Korean nationwide study has shown that probiotic food intake from fermented dairy products and vegetables was inversely associated with depressive symptoms in the general population, and that probiotic food has shown a decisive effect on lowering the risk for self-reported diagnosis of clinical depression. It is thus hypothesised that probiotic food consumption may have beneficial effects on depression, particularly in men.
Despite the limitations of this study, it is the first nationwide, large population-based study showing that probiotic food consumption is significantly associated with lower severity and prevalence of depression. These results provide pertinent information showing that probiotic food may have a role in lowering the risk of depression in adults. Further research, included randomised controlled trials, is required to identify the mechanisms of actions and the link between probiotics and depression with comprehensive investigations of the relations between gut microbiome and moods.
Kim CS et al. (2019) Probiotic food consumption is associated with lower severity and prevalence of depression: A nationwide cross-sectional study. Nutrition. Jul-Aug;63-64:169-174
Nutritional Insight In-Depth Insight: Links Between Microbiota and the Gut-Lung Axis. https://www.nutritional-insight.co.uk/in-depth-links-between-microbiota-and-the-gut-lung-axis/