Gut health and mental health - is there a link?

Ever wondered if there is a link between your gut and how you feel? We look at whether gut health affects your mental health.

Have you been constipated and felt a bit… ‘bleurgh’? What about if you eat a load of takeaways in a row?

What’s making you feel that way? Are you sick, tired, or could you have an issue with your gut bacteria? 

Does gut health affect your mental health?

Well, this is not new thinking. Over 100 years ago, a physician called George Porter Phillips noticed that patients with melancholia often suffered from general clogging of the metabolic process. In other words, severe constipation. 

People had often assumed that the low mood caused constipation. George thought otherwise and reversed the equation. He changed the diet of 18 patients – cutting out most meats and introducing kefir, fermented milk with friendly bacteria. In doing so, 11 patients were cured completely. 

Was this the scientific breakthrough of the century? Alas, no. We pretty much side-lined the notion for the next few decades. But in recent years, the friendly microbe has come to the forefront of our attention once more. 

How many bacteria are in your gut?

Every year we discover more species of bacteria, but right now, we estimate that your gut is probably home to over 36,000 different types of them. 

You probably have about 30-50 trillion bacteria - more than the number of human cells you have in your body. Put together from a genetic point of view, that probably means there are about twenty million bacterial genes inside you right now. How many of your genes do you have? About twenty thousand. 

From a genetic point of view, you could say you’re over 99% bacteria. It would be madness to think that all these organisms weren’t part of our natural biome, contributing to our normal homeostasis, day to day functioning and even mental health. Quite a few people now describe your microbiota as one of your organs. 

The number and type of bacteria inside different people vary widely. Different diets, having antibiotics, being old or young, all contribute to the ecosystem. No two people will have the same make-up of bacteria – it’s as unique to you as your fingerprint.   

Microbes are resilient 

If you kiss someone passionately, you will have probably exchanged over a billion of the blighters. But by morning, everything will be back to normal. That said, they can be disrupted and changed. Some bacteria are super strong and fight out the locals. Some can starve. And of course, antibiotics can wipe out whole populations of them, which explains why some people get terrible diarrhoea after some medications. 

Sometimes when the microbiome is challenged significantly, it can change, leading to increased intestinal permeability, which can cause bacteria and metabolites to leak into your circulation – a condition called a leaky gut syndrome. 

The gut-brain axis

This research is young, fascinating, and exciting. Slowly but surely, we are beginning to find and fit together different bits of this massive jigsaw. 

We now call the bi-directional communication between your central nervous system and gut biota the gut-brain axis (GBA). This relationship starts before you are born, as you get your initial introduction to the bacteria through your mum’s placenta. 

Then we see differences between gut microbiota depending on whether you were a traumatic delivery or a normal one. Breastmilk vs formula also makes a difference, but the biggest changes happen when you move onto solid foods. This begins to build a relationship for life. 

If you get an infection, your immune system kicks in and releases a variety of inflammatory mediators. This tends to be localised, which is why if you get a nasty scratch, the area around it gets a bit warm. 

If the infection spreads systemically around your body, then the mediators also act on your whole body. This makes you have a temperature and makes you feel run down. In the short term, this is fantastic, it’s supposed to happen. But if this goes on for a long time, this constant feeling of rubbishness can lead to low mood, depression, and anxiety. 

This brings us back to the theory behind the leaky gut syndrome. If your gut bacteria are disrupted long-term, it could lead to chronic low-level inflammation and mental health issues. 

What do gut microbes do?

Gut microbes digest a bunch of our food. You might think that’s what your digestive enzymes are for. After all, you have 20 of them. But your bacteria produce ten…thousand. 

Gut microbes have a massive impact on your digestion if things go wrong – they strongly influence how we metabolise the precursors of neurotransmitters like serotonin and dopamine, which are responsible for our wellbeing. 

Diversity is key for gut microbes and we’re beginning to see that those regular processed foods, full of minimal nutritional value, can harm your bacteria and shrink the diversity of your colony. 

There may also be a direct line of communication to the brain via the vagus nerve, which lines our GI tract, keeping digestion in check. The bacteria can stimulate the nerve, which has a direct influence on the brain. This can lead to feelings of anxiety or calm depending on the stimulus. 

Can you improve gut health?

Improving your gut health - you will hear this a lot and it’s a field full of bad research and pseudoscience. However, we are beginning to see some genuine connections. 

It’s difficult to get into specifics because:

  • We don’t fully understand gut health 
  • Everyone is unique

However, if you feel like you have an element of mental ill-health, then there is no harm in trying a couple of simple things. 

Before we talk about this, it’s important to point out that mental health is a complex issue with many factors contributing to it - not just your gut bacteria. 

It goes without saying that if you have depression, anxiety, or another condition, then you can see your GP or visit the Hub of Hope to get important and timely help.

How do probiotics affect mental health?

Probiotics are living organisms – typically yeasts or bacteria, which you can eat or drink. 

In theory, probiotics provide a neuroprotective role by preventing stress-induced nerve changes, and in rats, there was a measurable decrease in stress hormones after two weeks. 

We’ve seen similar reductions in anxiety and depressive symptoms in humans, with patients feeling happier and calmer in less than a month. They have potentially similar impacts on cortisol levels as antidepressant medications - in non-depressed people, that is.

Can food affect mental health?

Research has found that certain foods can trigger symptoms of anxiety and even depression after consumption. Common culprits include caffeine, alcohol, processed foods, and processed carbohydrates which can cause blood sugar levels to spike and drop. The sudden spike and drop in blood sugar levels is also known as Reactive Hypoglycaemia and causes a release of adrenaline in the body which triggers anxiety-like symptoms such as apprehension and a fast heartbeat.

The Mediterranean diet and mental health

The Mediterranean diet, a diet rich in whole foods, plants, oils, nuts and some seafood, with little to no red meat, seems to have a positive impact.

You can find more information on the Mediterranean diet in our everything you need to know about the Mediterranean diet blog.

One study in Spain showed that people eating the Mediterranean diet were roughly half as likely to be diagnosed with depression over four years. It also increases the diversity of gut bacteria. 

Take this research with a pinch of (unrefined) salt because there’s still confounding factors when it comes to diet and mental health, but you can at least begin to appreciate that there’s something there. 

Can you measure gut health?

Not really. But you can do some experiments. Having probiotics or changing your diet, especially if you already know your diet isn’t great, is a simple experiment to implement. 

As we’ve seen, it’ll probably take a few weeks to work and you can’t have a cheat day, otherwise, it’ll reset. Try it and see how you feel. If don’t feel better or you feel worse, stop, and try something else. 

You can also look at your blood. You cannot measure your bacteria, but you can see the impact of dietary changes by tracking your cholesterol, sugar levels and vitamin and mineral reserves. If you want to get scientific, you can even monitor your stress hormones. 

To check out the impact of any dietary changes, we recommend the Nutrition Blood Test.


  1. Journal of Mental Science, Volume 56, Issue 234, July 1910, pp.422 – 430 DOI:
  2. The Body, Bill Bryson, Chapter 3. 2019, Penguin Books. 
  3. Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S.Gut microbiota's effect on mental health: The gut-brain axis. Clin Pract.2017;7(4):987. Published 2017 Sep 15.
  4. Ups and downs in the nervous system. Gaia Vince.2015.
  5. How your belly could heal your brain. David Robson, 2019.
  6. Hypoglycemia - Jackson Siegelbaum Gastroenterology September, 2018.

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