Petra Lundström: How the gut affects the brain and body

Author: Petra Lundström, PhD, IOC Dip Sports Nutr

The Gut and Microbiota – Our Hidden System

We don't really think much about our stomach and gut as long as everything is working as it should.

It mostly operates in the background, doing its job without demanding attention. But as soon as our stomach starts acting up, we quickly notice the significant impact it actually has on our well-being. Pain, diarrhea, constipation, or bloating can affect mood, energy, and concentration, and sometimes completely incapacitate us.

The gastrointestinal tract is one of the body's most underestimated systems. We often talk about the brain and muscles, while the gut is mostly expected to function without drawing attention to itself.

At the same time, it is crucial for far more than just digestion. Besides breaking down food, it protects us from harmful substances and microorganisms and helps us absorb the nutrients and energy we need.

The Gut and the Brain – A Constant Communication

The gastrointestinal tract has its own nervous system with more nerve cells than the spinal cord and is in constant communication with the brain. This communication primarily occurs via the vagus nerve, but also through chemical signals produced by gut cells and bacteria in the gut.

Changes in the gut can therefore quickly affect our mental well-being. The gut constantly informs the brain about what is happening in the body.

At the same time, communication goes the other way. When we are feeling mentally unwell, the brain can send signals that affect both gut motility and its sensitivity. This is one reason why stress and anxiety are so often felt in the stomach.

Microbiota and Gut Barrier

Billions of bacteria, collectively called the microbiota, reside in the gut. They use dietary fiber as nourishment, among other things, and form short-chain fatty acids, with butyrate being particularly significant. Butyrate has anti-inflammatory effects in the gut and helps keep gut cells healthy.

A well-functioning gut barrier ensures that bacteria and irritating substances stay where they belong and do not reach tissues where they can contribute to inflammation, pain, and discomfort.

Butyrate has also been linked in studies to communication between the gut and the brain, as well as to improved cognitive function.

The microbiota also produces other metabolites, including vitamin K and various signaling substances that affect both the immune system and the nervous system.

When the balance in the microbiota changes, the signaling between the gut and the brain can also be affected. The gut barrier can then become more permeable, while the immune system is unnecessarily activated, which in some individuals can contribute to inflammation and increased gut sensitivity.

What Affects the Microbiota?

The bacteria that dominate in the gut are influenced by what we eat, but also by sleep, stress, alcohol, medications, and diseases. Our daily habits can therefore alter the balance in the microbiota and, in the long run, affect our well-being.

Stress – The Body's Survival System

Stress is the nervous system's and hormones' response to a perceived strain or challenge. This can range from training and competition to fear, infections, or insufficient energy intake. Its fundamental purpose is to help us manage threats and challenges.

When the brain perceives something as a danger, stress hormones like adrenaline and noradrenaline are released. The heart beats faster, energy is quickly made available, and blood flow is redirected from, among other places, the gastrointestinal tract to muscles and the brain.

Stress is not just about hormones but also about motivation and the ability to act. Neurotransmitters like dopamine contribute to increased focus, drive, and alertness, allowing us to quickly act and handle the situation.

During short-term stress, the communication between the gut and the brain changes rapidly. This affects blood flow and bowel movements, suppressing the gut's normal activity.

During short-term stress, it is therefore normal for digestion to be temporarily deprioritized. Bowel movements and digestion are inhibited so that the body can instead focus on performance and survival. This is a natural and important part of the human biological defense system.

Pain suppression during acute stress

Stress is an advanced survival system found in all mammals and most other animal species.

An important part of the system is that it temporarily suppresses pain signals while stress hormones are elevated. Pain is still sent from damaged tissues to the brain but does not reach our consciousness in the same way as it does at rest.

For a short period, we can therefore continue to act despite injuries. We can run despite pain, drag ourselves to safety, or continue to fight even when the body is actually signaling that it is injured.

This is also where the stories come from about people who, in extreme situations, perform actions that would normally have been impossible for them.

From a biological perspective, the stress system is the body's way of temporarily releasing resources and capacity that are otherwise held back.

All people have this ability to varying degrees when the situation demands it. Elite athletes also train to consciously reach this state and use it for performance and endurance.

The next section deals with chronic stress, i.e., when the stress system is no longer short-term but remains activated over time.