Bacterial cells make up 56% of the entire cell count in our body, matching up to human cells on pretty much a 1:1 ratio. Most of these are concentrated in our intestinal track and aid our digestion, forming the gut microbiome. The make up of this extensive ecosystem is extremely malleable to our environment, influenced by our genetics, diet, geography, and mental health.
Humans and our microbiomes exist in what is called a symbiotic relationship: they can influence us just as much as we can them. The gut microbiome has been found to be involved in the development of cardiovascular diseases and the regulation of immune function. The brain-gut microbiota axis describes a direct line of communication our guts have to our brains. Scientists are only beginning to understand how this axis can influence our mood.
Like most medical research, a lot of the work on the gut microbiome has been done on rodents, specifically a group called ‘germ-free mice’. These animals exist with completely sterile guts, providing the optimum chance for researchers to investigate the results of manipulating the microbiome environment. Germ-free mice were found to exhibit higher levels of stress reactivity due to the increased secretion of certain steroid hormones from the adrenal glands. Dysregulation of this pathway has been associated with depressive episodes in humans. Less microbiome diversity is hence thought to contribute to certain types of depression.
It’s not just a lack of microbes; the type of microbes we have in our gut can influence our mood as well. In another study, scientists transferred the gut microbiome of humans with major depressive disorder into healthy mice and monitored their behaviour. The rodents started exhibiting depression-like behaviour (such as increased immobility) compared to health controls, indicating that depression is associated with changes in the make up of our gut microbiome. The microbiome ecosystem is incredibly malleable, and its composition can be altered by a variety of environmental factors including diet. Previous links have been made between diets rich in fat and sugar and the onset of depression. One hypothesis is that these diets are altering our gut microbiota for the worse and increasing our risk of depression as a result.
But can we fix it? Manipulating the gut microbiota is nothing new. Companies like JUVIA offer personalised gut health supplements for a better functioning digestive track. However, in terms of treating depression, the simple answer is that we’re not sure yet. Some have suggested that the use of ‘psychobiotics,’ probiotics that influence the brain-gut-microbiota axis, could be beneficial to patients. But as of yet there are no published studies evidencing these benefits in practice. It’s also important to note that depression is a complex condition brought about by a variety of genetic, environmental, and physiological factors, so what works for one patient may not for the next. Despite this, gut microbiota research has the potential to completely shift the way we think about our health.