Your gut isn’t just a digestive system, it’s the body’s second brain.

Running from our mouth all the way down to our bum, it houses a nervous system that controls our digestion but also plays a role in our mental health.

While our gut isn’t writing poetry or solving maths problems, similar to the brain, it contains a wealth of nerve cells. It also works more independently from the brain than our other organs. The gut communicates with the brain through the vagus nerve (the gut-brain communication highway), the gut microbiome (a community of bacteria and other microbes) and through the immune system, hormones and neurotransmitters.

All of this means that our gut and our brain are intricately connected, in ways that we’re still only just understanding. But, what we have learned is that there are simple habits that can both transform our gut health and enhance our brain power – here are a handful of them.

Eat more fibre ‘big hitters’ to boost brain function

We talk about fibre as being something that just keeps bowel movements regular, but actually, it’s Clark Kent (secretly Superman). It’s the indigestible component in food that our gut bacteria feasts on, making it central for gut health.

It’s also vital for our brain. One review found that each five gram increase in fibre was linked with a 5 per cent drop in the risk of depression. Others from my own research department at King’s College London, have shown that over-60s who took a daily prebiotic fibre supplement (a type of fibre that feeds the “good” gut bacteria) performed better in memory tests that are used to detect early signs of Alzheimer’s. A review of 14 studies found that prebiotic fibre supplements can improve your mood and provide both quick and lasting benefits for your memory, particularly your ability to recall words and events.

Fibre is the closest thing we have to a “superfood”. In the UK, 30g of fibre per day is the recommended starting point. While it’s thought that more may be better, most people in the UK are eating just 18 to 20g per day.

To get more in your diet, focus on eating fibre “big hitters”. These include beans (8g of fibre per half can) and legumes, such as chickpeas (8.3g per half can).

Swap from white bread to pumpernickel rye bread (6g per slice). I also keep a jar of nuts and seeds next to my kettle, so I can add them to my breakfast in the morning (one tablespoon of chia seeds has 5.1g). Forget the idea that you need to eat lots of bland salad to boost your fibre intake. Lettuce has just 1.5g of fibre per 100g (though still contains many beneficial phytonutrients for our health).

If we can get more fibre in our diet, it’s going to make us feel better. Eat in a way that is joyful, non-stressful and doesn’t take up too much time or energy.

Two portions of oily fish a week for healthier brain cells

Omega-three fatty acids are really important for keeping our brain cells healthy. Because our bodies cannot synthesise omega-three fatty acids we need to get them from our diet, and oily fish is one of the best sources.

Eating more oily fish is one of the strongest dietary protective factors for looking after our brain in later life. One study found that people who ate more fish had healthier blood vessels, which may reduce the risk of Alzheimer’s and vascular dementia.

Another found that higher levels of omega-three in the blood was linked to better thinking skills and healthier brain structures, compared to people with low levels. Additionally, omega-threes also feed the gut bacteria and increase the diversity of healthy gut bacteria.

If you’re not sure what counts as oily fish, remember SMASH – salmon, mackerel, anchovies, sardines and herring. The late Michael Mosley popularised that acronym, it’s brilliant.

Have the occasional sweet treat to avoid stress

Enjoying a burger, cookie or slice of cake will do diddly squat to your health. But it’s better for your gut and brain to allow an occasional treat, rather than stressing over doing so.

Consistent long-term stress can affect the health of your gut and make it harder for your “good” gut bacteria to flourish. If you have the perfect diet but are constantly stressed, that’s not the best approach.

Savouring your sweet treat without guilt or stress is key. In one study, volunteers were offered doughnuts. Half were told to be self-compassionate about eating the doughnut, while the others were only told to eat it. The latter group ate much more at their next meal, compared to those who focused on enjoying their snack.

Another study involved asking some chocolate lovers to deprive themselves of it for a week. They had more chocolate cravings than those who didn’t deprive themselves of chocolate and, when they did get the chance to eat chocolate again, they ate double the amount.

So, if we just allow a little bit of deliciousness into our day, it’s going to be easier to maintain a balanced diet and it won’t harm our gut (though a very high-sugar diet will).

Five colours every day for better mental health

Some scientists and nutritionists say we should be eating 30 different plants a week, but the evidence for this is surprisingly weak. Instead, focus on eating five different colours a day. Think red tomatoes, purple aubergines and yellow capsicums.

Eating an assortment of vegetables ensures we’re consuming plenty of fibre and a variety of polyphenols, which are a group of antioxidants.

Polyphenols encourage the growth of “good” bacteria in our gut. They have been linked with a lower risk of depression, a slower rate of cognitive decline, less problems with mental fatigue and improved brain performance.

To help you eat five colours a day, opt for mixed bags of stir-fried vegetables, choose frozen fruits of the forest over frozen raspberries and switch up the salad leaves you eat.

One to three coffees each day for a cognitive boost

Coffee is strongly linked to a diverse, healthy gut microbiome. This is thought to be down to its high polyphenol content (accounting for most of the 800 to 1,600mg that people consume per day).

The drink is also great for our brains. It is the original cognitive booster and one to three cups a day has been shown to protect against cognitive decline.

It doesn’t matter whether you drink supermarket instant coffee or splash out on a barista-made mug every day – studies show both have plenty of polyphenols.

However, coffee can worsen digestive problems, such as irritable bowel syndrome and inflammatory bowel disease, in some people. In this case, choose decaf or tea.

Also, it’s important to stop drinking caffeinated coffee at midday. Caffeine floats around in our bloodstream long after the buzz has worn off and can lead to a lower quality of sleep, even for those who have no problem getting to sleep after an evening espresso.

Two fermented foods a day (cheddar and olives count) for better memory

Kombucha (black tea drink), kimchi (cabbage) and kefir (milk drink) have become increasingly available due to interest in the health benefits of fermented foods.

This is with good reason. One study has linked consuming kefir with improved relational memory (the ability to remember faces and where you left the keys). This could be down to the drink helping the gut create metabolites, which have an anti-inflammatory effect on the brain and protect cognitive health.

Another paper linked daily kefir drinking with improved emotional resilience. This may be a result of microbes being produced in the gut that help produce a calming neurotransmitter called GABA.

However, you don’t need to drink kefir every day. Fermented foods include some cheeses, yoghurt, fresh olives, sourdough bread and relish – though most of the evidence is around yoghurt and kefir. Aim for at least one or two portions of fermented food a day.

Have an early dinner to boost mood

Time-restricted eating has gained a lot of attention in recent years. It’s just a fancy way of referring to our feeding window – the time we have our first meal or drink of the day and the time of our last (just remember that coffee starts the clock).

Your gut bacteria like having a feeding window between eight to 10 hours. This could look like having breakfast at 9am and dinner at 7pm.

A King’s College London study found that most people spread their eating over 12 hours a day. After swapping to a 10-hour window for four months, they reported an 11 per cent increase in mood, a 22 per cent jump in energy and an 11 per cent reduction in bloating.

Make sure you don’t eat dinner too close to bedtime, as it means the body is trying to digest food at the same time as sleeping, which inevitably leads to worse sleep quality. A shorter eating window has also been linked with higher diversity of gut bacteria, better digestion and lower hunger levels.

As told to Emily Craig

Genius Gut, The Life-Changing Science of Eating for Your Second Brain by Dr Emily Leeming is out now.

The Telegraph, London