It shouldn’t surprise us that the gut affects the brain. After all, 2,500 years ago Hippocrates claimed that ‘all disease begins in the gut‘.
But the real shocker is that we didn’t appreciate all the microbes residing in our gastrointestinal tract until the 21st century. Bacteria had been found in faeces, but they were generally dismissed as pathogens. It took next-generation gene-sequencing machines to discover that our colon is home to over a thousand different species of bacteria, numbering roughly 100 trillion cells and weighing some 1.3 kilos.
Far from being dangerous germs, this batch of bacteria – called the microbiota – turns out to be symbiotic. In return for a warm environment flowing with nutrients, our microbiota provides protection from the marauding pathogens of the outside world.
Even more than our own immune system, these microbes defend against the continuously shape-shifting bacteria in our air and our food. This arrangement is important enough to pass down from mother to child in almost every animal species known. The birth canal is one of the first microbial exposures provided to the newborn, followed by breast milk which contains both microbes and the prebiotics they need to thrive.
Attainment of a cooperative microbiota early on can spell the difference between robust health and a life of sickness and misery. A healthy microbiota trains the nascent immune system to tolerate the beneficial microbes, preventing continuous inflammation. A good microbiota even affects brain development, encouraging the growth of nerves. Once established at around 1,000 days, gut microbes develop into a tightly knit community resilient enough to last a lifetime.
Through several different avenues, the microbiota can also affect our mood. Pathogens that penetrate the gut lining can produce metabolites that cause anxiety in a matter of minutes. Conversely, microbes that improve mood are known as psychobiotics. Gut microbes communicate with the brain in several ways:
- Some bacteria produce feel-good neurotransmitters like dopamine and serotonin that may directly affect the brain via the vagus nerve.
- Certain microbes trigger cytokines that circulate throughout the body and can lead to mood changes.
- Some beneficial bacteria produce short-chain fatty acids like butyrate that can heal the gut and directly pass through the blood-brain barrier to improve brain function.
- The microbiota communicates hormonally with the brain by means of the hypothalamus, pituitary and adrenal glands – the HPA (hypothalamic pituitary adrenal) axis.
Stressors that activate the adrenal glands end up slowing down the gut and tamping down the immune system. This allows the body to maximise energy stores to fight the stress, but the weakened immune response may allow pathogens to breach the gut lining. No matter how beneficial they are in the gut, bacteria that enter the blood system are pathogenic. Continued stress can lead to a chronic inflammatory response by the immune system that can reach the brain, resulting in depression and anxiety. But good bacteria, communicating via neurotransmitters, cytokines, short-chain fatty acids, and hormones, can turn things around.
There are many reasons for depression, including grieving and excessive rumination on past or current problems. These are addressable by therapy. But psychiatrists would be wise to look into gastrointestinal issues when they enrol a new patient because the odds are good that the microbiota is also involved.
Dr Ted Dinan, an Irish psychiatrist (and the person who coined the term psychobiotic), makes it a habit to examine gut problems with all his patients. He has had good success with psychobiotic treatments – especially with those patients who are trying to avoid drugs or who have tried everything else. Dr Dinan has conducted research with John Cryan, a neurology professor at University College Cork, and together they have broken ground on many of these amazing gut-brain connections.
So what is psychobiotic therapy? It depends on the patient, but it can range from better food choices (a Mediterranean diet, including fermented foods) to probiotic supplements. Exercise, which has a salutary effect on the microbiota, is always involved. Adding more fibre to the diet is important, as it boosts populations of psychobiotic microbes in the gut.
Science shows that there is a lot of variation from person to person, so Dr Dinan recommends that people try different strategies and keep notes on what works best. We are just at the beginning of this new paradigm, but the research is proceeding quickly and none too soon: there are 300 million people around the world suffering from depression. For many of them, a psychobiotic solution could be close at hand.
Scott Anderson is a scientist and writer who researches prebiotics and probiotics for both animals and humans.