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Ina Schuppe Koistinen

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Depression and anxiety: what do we know today about the role of the intestinal flora in the development of the disease?

The research on the connection between the gut and the brain, known in English as the “gut-brain axis”, continues to fascinate and baffle us. At the end of February, a Chinese research group published the largest study to this day (link here) on more than 3,200 patients with depression and their gastrointestinal symptoms during depressive episodes. How much impact does the intestinal flora have on our mental well-being? Should psychiatry start investigating and treating gastrointestinal problems in patients with depression and anxiety?

Today we know that an intestinal flora that is out of balance, also called dysbiotic, can affect our mental health. In people with mental illnesses, lower diversity has been observed in the intestinal flora. But causality remains unclear. In microbiome research, the role of the intestinal flora in disease development is usually studied by studying mice that have been bred in a sterile environment and lack intestinal flora. The first and perhaps also unexpected finding was that these sterile mice behaved anxiously and differently.  In other words, the absence of microbes resulted in a behavioral change. When the intestinal flora of healthy people was transplanted into these sterile mice, the behavior normalized. However, when transplanting the intestinal flora from patients with depression into the mice, the mice experienced symptoms similar to depression and anxiety. These experiments show that the intestinal flora plays a biological role in the development of these mental illnesses.  

Even when you get food poisoning or get sick from viral infections, you develop a disease behavior that is not so different from depression. Studies  in mice and humans have shown that the small parts of the cell walls of bacteria, called liposaccharides (LPS), are enough to trigger anxiety and disease behavior. Within just a few minutes, the study participants – who had been injected with LPS – developed a disease behaviour similar to anxiety and depression. In addition, research has long shown that patients with inflammatory bowel disease often suffer from anxiety and depression.

Back to the study from China that shows that the link between intestinal flora and depression may be greater than we think. The researchers examined 3,256 patients with severe depression and found that more than 70% had problems with their stomach and intestines. Most notable was that the patients who reported more severe symptoms from the stomach/bowel also had more severe depressions. 

However, we must not forget that communication between the brain and the intestine goes both ways. Prolonged stress and lack of recovery can affect our brains and gut flora. Improving our intestinal flora through a diet rich in dietary fiber and fermented foods leads to a more resistant intestinal barrier, reduced  inflammation and better mental well-being. Even methods such as CBT and conscious presence (through meditation or yoga) are believed to improve our intestinal flora and have shown a beneficial effect on disease relapse in patients with inflammatory bowel disease or IBS.

There are many causes of anxiety and depression, some known as chronic stress or pain, addiction, infections or genetic vulnerability. Biology is extremely complex and the connections with the thousands of bacteria we have in the gut extend this complexity. The gut-brain axis needs to be studied in more detail and psychiatry needs to pay attention to patients’ gastrointestinal problems so as not to miss an important component. So far, there are only  pieces of the puzzle of knowledge. If you want to read more about how we should evaluate individual research studies, please look at my column  “Before you start worrying about the findings of a new study – read this!”.

This is a guest post. Any opinions expressed are the writer’s own.

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