As a child approaches the age of ten, there is one factor that may help predict how key parts of their brain will have grown and how well they think things through. And it has nothing to do with the school that they go to.

Instead, it’s linked to the make up of their microbiome — the community of bacteria and other microbes that live in the gut.

It is now being recognised that these microbes are not simply bit players in your digestive system: they are involved in every aspect of your health, from your immune defences to your likelihood of putting on weight.

What’s more, early-stage research suggests that your gut microbiome is more intricately tied to your brain health — even the way you think, your memory and reasoning — than was previously thought possible. In fact, new science hints that having certain gut bacteria is linked to how much happiness, gratitude and contentment you feel.

The flip side is that an imbalanced gut microbiome (by which I mean one that may contain a poor or limited mix of microbes) could damage your emotional stability, and there are early suggestions that it may even play a part in the development of both dementia and Parkinson’s disease.

It seems that disruptions in the gut can manifest as symptoms in the brain; and vice versa.

For example, up to 80 per cent of people with Parkinson’s also have constipation, a symptom that can appear as much as 20 years earlier than other key signs of the neurodegenerative condition, such as slowness of movement.

And a third of those with irritable bowel syndrome also have depression and anxiety.

The good news is that by changing what you eat, you can change your gut microbiome — and so boost how you feel right now, with accumulating evidence that your gut bacteria can influence how your brain performs and, in turn, affect your mood, memory and behaviour.

In the long term, feeding your gut bugs the plant-rich diet they love is linked to a lower risk of neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

But before I tell you about what changes you need to make, let me explain how the brain and gut communicate — and just what those gut microbes actually do.

YOUR UNIQUE BUG FINGERPRINT

There are more microbes living just in your gut than there are stars in the Milky Way: 100 trillion or thereabouts. The more varieties of gut bacteria you have (and there can be between 200 and 1,000 different types), the healthier you tend to be.

Your gut microbiome is constantly evolving and, like a fingerprint, is unique to you.

At birth it is influenced by your mother’s microbiome, as she shares some of her vaginal and gut microbiome with her baby during birth.

But as you grow, one of the greatest influences on your gut bacteria is what you eat. They feed on your food, so that they grow in numbers — and, if fed right, they help to encourage, over time, new ‘good’ bacteria to take up residence.

While some good bacteria are bona fide do-gooders, most tend to be a bit situational in whether they are ‘good’ or ‘bad’.

And even the best bacteria, such as Akkermansia muciniphila, which tends to be plentiful in people with great health, can misbehave. Under certain circumstances it is thought to protect against inflammation; but if it is starved of fibre, for instance, it can get ‘hangry’ — and, as a result, it may worsen an allergic response to a food allergy (according to a study in mice).

‘Good’ gut bacteria are those that produce beneficial metabolites (molecules made by breaking down food) that traverse your body like explorers — and research is pointing increasingly to their vital roles in your mood, happiness, emotional stability and how well you think and problem solve.

Certain metabolites can influence your brain directly by crossing your blood-brain barrier, and also indirectly through signals to your immune system and via the extensive network of nerve cells in your gut, known collectively as your enteric nervous system.

They impact mood, behaviour and how you think by altering, for example, levels of brain chemicals such as serotonin — known as the brain’s feelgood chemical.

Of all the metabolites, short-chain fatty acids are particularly important, as these are powerful anti-inflammatory molecules that help fight excess inflammation in the brain.

Levels of short-chain fatty acids can be increased by eating fibre-rich food, as it is this which gut bacteria prefer to feed on. Lower levels have been found in those with conditions such as depression and Parkinson’s (and in mice studies, of those with Alzheimer’s and chronic stress).

‘CHATTY’ GUT TALKS TO BRAIN

The brain and the gut are in constant communication — and a lot of this is done via the vagus nerve, the crucial connector, physically linking both organs.

It’s your gut that’s the chatty one: 90 per cent of the conversation going up and down the vagus nerve is your gut communicating to your brain.

The vagus nerve reaches down from the brain, branching into tendril-like nerves that weave out to your heart, lungs and gut — and specifically to the lining of your intestines, home to your gut microbiome. It helps your brain to know precisely what’s happening in your gut.

This is partly how your brain can influence the make up of your gut microbiome. It can then help control the secretion of digestive juices, the absorption of food and other factors which influence which bacteria thrive or die.

HOW MICROBES MAKE YOU HAPPY

You’ve likely heard of serotonin, the ‘happy hormone’. But what you might not know is that 90 per cent of the body’s supply is made in the gut. In the gut, serotonin has a role in helping with digestion, but it can’t cross the protective blood-brain barrier to reach your brain to influence your mood. Instead, ‘happy’ serotonin needs to be made inside your brain.

And the gut microbes appear to help with this by supplying your brain with the building blocks of serotonin, such as tryptophan.

Gut bacteria also oversee how tryptophan is used elsewhere in the body, and imbalances in tryptophan have been seen in depression, irritable bowel syndrome and neurological diseases.

GUT BUGS KEEP MOODS STABLE

Your gut bacteria appear to act like tiny grounding hooks for strong feelings — helping you to return to baseline more easily.

A 2023 study by Harvard University linked certain gut bacteria with having more positive feelings and to being better able to self-regulate emotions, i.e. keeping mood more stable, while those who suppressed how they felt had a lower diversity of gut bacteria.

This could be down to your amygdala, the emotional centre of your brain, which is sensitive to changes in your gut microbiome.

Another study by Oxford University in 2014, published in the journal Psychopharmacology, found that a certain type of fibre, B-galacto-oligosaccharide, that gut bacteria like to feed on (in this case, taken as a supplement) can also help to shift your focus away from the negative and towards the positive.

... AND HELP TO TACKLE STRESS

Your gut bacteria and their metabolites can neutralise stress-related harmful molecules, such as cytokines that promote inflammation, acting a little like that calming rational friend playing mediator in an argument.

But if your microbiome is disrupted — that is, it has more ’bad’ bacteria than ‘good’ — then it’s not able to do this well, if at all.

Again, studies in mice can give us a window into how stress rapidly affects the gut microbiome. Mice that are more resilient to stress have different gut microbiomes from mice that don’t cope well with stress.

When you’re stressed often and for long periods, the ‘good’ bacteria can struggle to thrive, and the ‘bad’ bacteria can start to take over.

Long-term stress weakens the immune system and creates an environment in the gut where it’s easier for bad bacteria to thrive.

A healthy microbiome can recover after a stressful situation, but when it becomes disrupted it slowly loses the ability to bounce back. That means the next time you’re stressed, it isn’t able to help you cope with it as well. And the cycle repeats.

Prolonged high levels of the stress hormone cortisol have been linked to mood disorders and a shrinking hippocampus — the area of the brain that looks after memory and learning.

Repeated stress can also manipulate chemicals in the brain, including your ‘happy’ hormone serotonin, changing your mood and how well you’re able to think.

Meanwhile, a ‘healthy’ diverse gut microbiome, with plenty of ‘good’ gut bacteria, helps your body to handle stress as it can influence levels of the stress hormone cortisol in the blood, acting like mini stress buffers.

An imbalanced microbiome, though, can tip the scales the other way and make the effects of stress worse. So how much stress is too much? In mice, two weeks of stress was enough to significantly change their gut microbiome and make them more anxious.

YOUR GUT MAKES YOU SOCIABLE

There’s a strong connection between anxiety and the gut — so if you have gut issues, you’re more susceptible to feeling low and anxious (and vice versa).

A 2019 review in the journal General Psychiatry showed that changing your gut microbiome (in this case with probiotics — supplements containing live bacteria) can help with anxiety.

While anxiety can be due to many different factors, oxytocin — the cuddle hormone — can influence how strongly you experience it. Oxytocin helps to make the amygdala react less to triggers of anxiety and fear.

Your gut bacteria can communicate with your brain through your vagus nerve to influence how much oxytocin is produced and released by the brain.

Differences in the make up of your gut bacteria have also been linked to personality traits — for example, determining how sociable you are.

LINK TO POOR MEMORY

A number of studies have linked the gut microbiome to cognitive performance — learning and memory, in particular.

In a small study of 26-year-olds, those who had higher levels of certain ‘good’ gut bacteria found it easier to learn new information and problem solve than those with lower levels.

The first insights into how your gut bacteria may influence your brainpower were from mice bred without a gut microbiome — they had memory problems and lower levels of nerve cells in their hippocampus. Other mice infected with ‘bad’ bacteria struggled with their memory under stress, but this was reversed when they were given probiotics.

There have been mixed results from probiotic studies on brain performance in humans, with some showing they are helpful, others having no effect — and, worryingly, some making brain performance worse.

This highlights that there is plenty we still don’t know about how to harness these bacteria in supplement form. So, for now, the best way to look after your gut bacteria is through your diet.

HOW TO KEEP YOUR GUT HAPPY

Eat more fibre: If there were just one change you make that has the biggest impact on the health of your gut and gut microbiome, it’s to eat more fibre.

Fibre is essential for a healthy gut and a thriving gut microbiome, and certain types of fibre have been shown to support your mood and how well you think. Every 5g of fibre you eat is related to a 5 per cent lower likelihood of depression, according to a review published in the journal Nutritional Neuroscience in 2023.

People who eat more fibre tend to perform far better in cognitive tests on memory and problem-solving. We need at least 30g of fibre a day for health, if not more, yet most of us are barely scratching the surface.

At best we’re reaching just over half of what we need, averaging 15g in the U.S. and 18g in the UK — which is less than the recommended amount for a five-year-old. The key is to regularly eat high-fibre foods such as beans, which contain up to five times more fibre than lettuce.

Other high-fibre foods include wholegrains (think rye bread and wholegrain pasta), nuts and seeds like chia and flaxseeds.

Make half your plate fruit and veg: Fruit and vegetables contain the double-whammy of fibre and polyphenols — a group of antioxidants that act as rocket fuel for your gut bacteria.

They’re nutrient powerhouses packed with vitamins, minerals and plant biochemicals that are vital for your gut and your brain.

Not only that, but the bacteria naturally present on fruits and vegetables make their way to your gut and contribute to your diversity of gut bacteria. Did you know that an apple alone contains 100 million microbes?

Fruit and veg can make you happier, too. A 2022 study of people who upped their intake of fruit and veg were happier than the group who didn’t change what they ate — and also happier than they’d been before the study began, and in only eight weeks.

Psychologists have been able to predict how happy someone is purely based on how much fruit and veg they’d eaten the day before. Having two or more servings of veggies a day is linked to having the cognitive age of someone five years younger as you get older, with leafy green veg shown to offer the strongest protection.

Consistency matters, too. Those who eat veggies daily are 56 per cent less likely to have cognitive problems later in life than those who rarely or never eat them.

Potatoes tend to be excluded from many ‘eat more veggies’ guidelines because they aren’t nutrient dense. Yet they can still make a significant contribution to our fibre intake, particularly if you keep the skins on, which doubles the amount of fibre they provide.

Have a regular bedtime: Going to bed and waking up at regular times helps both your sleep and your gut bacteria. Varying the time you go to bed or wake up by 90 minutes is linked to more ‘bad’ gut bacteria and poorer health compared with those who have a regular sleep schedule.

Your gut bacteria also seem to be early birds: going to sleep earlier is related to a more diverse gut microbiome.

Get your hands dirty: A teaspoon of soil contains more microbes than there are humans on Earth.

For a small 2018 study published in Future Microbiology, a group of adults rubbed their hands in a bucketful of soil for 20 seconds three times a day — and this simple action increased their diversity of gut bacteria in only two weeks. It seems changing their skin microbiome had a beneficial effect on their gut.

Turns out all those mud pies children make are very likely great for their gut bacteria. And spending more time in the garden is great for yours, too.

Have a coffee or two: I co-authored a scientific paper a few years ago where we found that of all the food and drinks we looked at, coffee was the most strongly related to the gut microbiome.

And those who drank the most tended to have a more diverse gut microbiome. Coffee is particularly high in polyphenols and contains some fibre, too.

One 240ml cup can contain up to 1.8g of fibre — that’s more fibre than orange juice. For gut and brain health, stick to one or two cups and have them before noon to avoid disturbing your sleep.

Exercise regularly: Exercise increases the blood flow to your gut and your brain, providing fresh oxygen and nutrients.

For your gut microbiome, this acts a bit like an interior designer, giving their home — your gut — freshly painted walls, arranging the furniture nicely and puffing the cushions. It helps your gut to work well, moving food through efficiently, maintaining the health of the gut barrier lining — and enabling your gut bacteria to thrive.

The extra blood flow to your brain also helps you immediately feel sharper — and can last for at least two hours afterwards. Every time you move your body in a way that raises your heartbeat, feelgood chemicals such as serotonin get released into your brain.

Genius Gut by Dr Emily Leeming (Penguin, £18.99) is published on July 25. © Emily Leeming 2024. To order a copy for £17.09 (offer valid to July 21, 2024; UK P&P free on orders over £25) go to mailshop.co.uk/books or call 020 3176 2937.