Generally, when we talk about mental health, we think only about the brain. However, mental health does not only involve the brain. The gut (or intestine) also plays a significant role in mental health. Multiple studies have found an association between brain diseases, including depression, anxiety, autism, and disturbance in gut microbiota — the depletion of friendly bacteria and proliferation of pro-inflammatory bacteria.
In any human body, there are about 30 trillion cells, but the number of estimated microbes is 36 trillion, including bacteria, viruses, and fungi living mainly in the human gut. Together, these microbial species are known as the microbiome, and constitute up to 1.8 kilograms of our body mass, which is the approximate weight of the human brain.
Similarly, the human body has around 25,000 genes in each of the cells. The microbiome, however, holds 100 -150 times as many genes as in the human genome. These striking statistics suggest the tremendous potential and power that the microbiome holds.
The microbiome predates human existence, and it is believed that the microbiome and humans co-evolved, which has led to cooperative interaction and homeostasis. Humans receive most of the microbiome in the body (two thirds) from their mothers, usually during birth. We also ingest around a million microbes in our food. Similarly, we get others directly from the environment, places we visit, and people we interact with.
However, each human may have a different microbiome composition as the nature of the diet, consumption of antibiotics, and lifestyle habits - including sleep, exercise, stress, and hygiene paranoia - can directly impact the content of the human gut microbiome. For example, a person on a fiber-rich diet will have a drastically different microbial composition to a person on a high-fat diet.
The microbiome plays a major role in many body functions. These include activating human genes involved in absorbing nutrients; digesting toxins and sugars; differentiating harmless bacteria from life-threatening pathogens; generating an immune response against pathogenic species; and even releasing neurotransmitters. Some of the microbiome transfer from mother to baby is essential. In the absence of the microbiome, the baby will not be able to digest one specific sugar in mother’s milk – sialylated oligosaccharides. This sugar is digested only through the microbiome to form sialic acid, which is essential for brain development.
Because of various genetic or environmental factors, a person may have an imbalanced gut microbiome composition and diversity. Dysbiosis - an imbalance of the gut microbial community - is usually associated with the depletion of anti-inflammatory bacterial species and proliferation of inflammation, thereby increasing bacterial species.
This pattern of microbiome imbalance is also observed in patients with major psychiatric disorders, including depression, anxiety, schizophrenia, and autism. These patients usually have a low number of microbial species in their gut compared to healthy control numbers.
Nutritional factors greatly influence the microbiome and its diversity, including ingesting prebiotics and probiotics (food containing beneficial live bacteria). Vegetables and fruits constitute prebiotic factors and enhance the composition of good microbiota because of their anti-inflammatory properties. Probiotics such as yogurt, cheese, and pickles rebalance the microbial dysbiosis and ensure the sustenance of good mental health. Similarly, lifestyle habits, including sleep patterns, stress, and exercise, affect the microbial composition. An example is the Hadza tribe in Tanzania - the members of which have unprecedented rich diversity in their gut microbiome which are 30-40% higher than the average human. These tribes eat almost 600 plant and animal species and have huge seasonal variations. Scientists have found almost none of the major diseases - including mental diseases, heart diseases, or cancer - within the members of this tribe.
There are other methods through which a good microbiome can be achieved, and some of these methods are used by clinics while others are based on cultural practices. The clinical approach is fecal matter transplantation (FMT), in which fecal matter of a healthy donor is transplanted into the colon of a patient to reset the microbial imbalance. There is very strong evidence of FMT's success in treating many brain diseases, including depression and anxiety.
Another method ties into healthy eating habits. Eating from the same plate as a healthy person - a habit prevalent in Arab and many Asian countries - may transfer good microbes from the healthy individual to the other person.
Whether and how changes in diet or alteration in the gut microbiome or its metabolites influence mental health, and whether these good bacteria can directly be cultured and mixed in probiotics like yogurt to treat common mental diseases, are emerging and exciting concepts in mental health disorders. More research needs to be done to learn about the mechanism through which different types of bacteria and bacterial products exert their effects on mental health.
Dr. Mohammad Farhan is an assistant professor at Hamad Bin Khalifa University. His laboratory at the College of Health and Life Sciences is taking the lead on understanding the molecular mechanisms through which microbiome changes are associated with brain diseases such as autism. Dr. Farhan recently organized an international symposium - “Gut-Brain Axis and Mental Health” - discussing the implications of gut and microbiome in human health with various international scientists.