Emotions can weaken the immune system

Mental states influence the body, sometimes with long-lasting effects. The brain talks to the gut and can impact the body’s defense mechanisms against pathogens. But the communication goes both ways: The gut tells the brain, for example, what we should eat – without our knowledge!

There is a constant chit-chat between the gut and the brain. Many of its effects can be observed in everyday life; if you have ever had to rush to the bathroom in a stressful situation, you know this all too well. But sometimes, the brain's signals have more serious, longer-lasting consequences. Chronic stress can harm the gut in a number of ways: For instance, inflammations increases and infections become harder to fight off.

Why stress can make you sick 

But how does this work? The gut microbiome, that is, all the microorganisms such as bacteria and fungi that inhabit our intestines, plays a central role. Such a big role, in fact, that many speak of the brain-gut-microbiome axis to describe the intricate relationship between these three components. However, exactly how the brain influences the microbiome is not fully understood.
In the Body-Brain Cybernetics Department, led by Ivan de Araujo, researchers try to shed light on this. For example, the brain uses the vagus nerve to send stress signals directly to the gut. When this happens, the upper intestine secretes less mucus. But the mucus is important for the growth of certain beneficial gut bacteria, and when these bacteria are missing, it is easy to catch a gut infection. Perhaps even worse, the intestinal barrier can get damaged, so that substances from the gut enter the blood stream, to which the body’s immune system responds with inflammation. The good news is that understanding the mechanism can help develop therapies: Vagus nerve stimulation could counteract the detrimental effects of stress on gut health. Alternatively, supplementation with probiotics can go a long way.

How the gut tricks us into eating fatty foods

But that is not the whole story of the gut-brain axis: The gut also sends signals to the brain! Some even call the gut, which has its own nervous system, the “second brain”. And indeed, it seems to make decisions for us – for example, when we choose what and how much to eat. Of course, taste, smell, and mouthfeel play a role, and those are the factors we are consciously aware of. But there is also a hidden, unconscious signal from the gut to the brain, sending a reward each time we eat energy-dense food. This signal can lead some of us to overconsume fatty or sugary meals, increasing the risk of obesity. Surprisingly, this does not even depend on whether we actually like the high-calorie food!
There are countless other examples of what the gut tells the brain: Chemicals released by the gut microbiome can influence brain development from birth on. The gut-brain axis also regulates stress response, anxiety and even memory function and has been linked to a number of mental health disorders including depression, and schizophrenia. Understanding exactly how this works may help inform new therapeutic approaches and help patients with a variety of disorders.