The Gut-Brain Axis: Mood, Stress and Your Microbiome

Most of us have felt it: the churning stomach before an exam, the loss of appetite when something is wrong, the “gut feeling” that something is off. The feelings have a physical basis. Your digestive system and your brain are in constant conversation, and the scientific name for that two-way link is the gut-brain axis.

Over the last two decades, researchers have asked whether the trillions of bacteria in the gut play a part in that conversation. The field is young, and much of the early work was done in animals, so it pays to be careful about what is established and what is still being explored.

How the gut and brain communicate

There are several routes, and they work together.

Nerves

The vagus nerve is a long nerve that runs from the brainstem to the abdomen and carries signals in both directions. In addition, the gut has its own network of more than a hundred million nerve cells, often called the enteric nervous system. It can manage a good deal of digestion without any instruction from the brain, and it still sends information upwards.

Hormones and the stress response

When you feel stressed, the brain triggers the release of hormones such as cortisol. These affect how fast food moves through the gut, how much acid and mucus it makes and how sensitive the gut is to sensations like gas. Stress can therefore cause diarrhoea in one person and constipation in another.

The immune system

Around much of the gut lining sits a large portion of the body’s immune tissue. Immune signals, including inflammatory ones, travel from the gut to the rest of the body and can influence the brain.

Chemical messengers

Gut bacteria ferment fibre into short-chain fatty acids and make other compounds that can act as messengers. Many people know that most of the body’s serotonin is made in the gut. That serotonin mainly helps to regulate gut movement, and it does not cross directly into the brain, so it is not accurate to say the gut “makes your mood”. Bacteria do, however, influence the raw materials and signals involved in making other neurotransmitters.

What does the research suggest?

Studies suggest that people with depression or anxiety, on average, have a different gut bacteria profile from people without them. Some trials have tested specific probiotic strains, sometimes called psychobiotics, and a few have reported small improvements in mood or stress. Other trials found no effect.

The honest summary is that the connection looks real, but we do not yet know which direction it runs in most cases. Do gut bacteria change mood, or does a low mood, with its effects on sleep, diet and stress hormones, change the gut? Probably both. No test can tell you that your anxiety is “caused” by a particular bacterium, and diet changes are not a replacement for mental health care.

Stress, digestion and a vicious circle

Gut symptoms and stress feed each other. Pain or bloating increases worry, worry increases gut sensitivity and the cycle continues. This is why approaches that calm the nervous system, such as regular sleep, time outdoors, gentle exercise and breathing exercises, often help digestive symptoms too. Cognitive behavioural therapy and gut-directed hypnotherapy have both been studied for irritable bowel syndrome, with encouraging results for some people.

Everyday habits that support both

You do not need a test to start on these:

• Eat a varied, mostly plant-based diet. Different fibres feed different bacteria, and a wider mix of plants is linked in research to a more diverse microbiome.
• Eat regular meals rather than skipping and then grazing late at night.
• Protect your sleep. Poor sleep affects both mood and gut bacteria.
• Move your body, ideally outdoors. A walk is enough.
• Limit alcohol, which can irritate the gut lining and disturb sleep.
• Make time to talk to people. Loneliness is hard on the whole body.

Where a microbiome test fits

A gut microbiome test gives you a snapshot of the bacteria in your bowel. Part of its value is that it lets you look at your gut alongside the other things going on in your life. A report may show diversity, the levels of bacteria that produce short-chain fatty acids and markers linked to inflammation, all of which are areas studied in relation to the gut-brain axis. It cannot diagnose a mental health condition or tell you how you feel.

Used sensibly, it can help you and a practitioner decide where to start with diet and lifestyle, particularly when low mood or stress comes with digestive symptoms, sleep problems or fatigue.

Taking the next step

Our GI Axis test includes a gut-brain axis section among its ten gut health axes, along with sleep and inflammation insights, all from a stool swab you take at home. For a wider look that also screens for fungi, parasites and pathogenic bacteria, there is GI Axis Advanced. Everything we offer is in the shop.

If you are struggling with your mood or feeling overwhelmed, please speak to your GP. In Ireland you can also call Samaritans free on 116 123 at any time.