Gut Health: A Plain-English Guide
What does a healthy gut actually look like, and why does it seem so hard to get right when the advice sounds so simple?

How digestion works, from top to tail

Digestion starts before you have taken a mouthful. The sight and smell of food gets your saliva flowing, and saliva carries the first digestive enzyme, amylase, which starts breaking down starches while you chew, so taking your time over a meal and chewing well really does give the rest of the gut a head start.

In the stomach, acid does far more than people realise. It kills off many of the microbes that come in with our food, it switches on pepsin so protein can be broken down, and it is needed to release vitamin B12 from food so it can be absorbed further down, along with helping the absorption of minerals like iron and calcium. When the food leaves the stomach, its acidity is one of the signals that tells the pancreas and gallbladder to release their enzymes and bile into the small intestine.

The small intestine is where most of the nutrients from our food are absorbed. Pancreatic enzymes break down proteins, fats and carbohydrates, bile helps us absorb fats and fat-soluble vitamins, and enzymes on the surface of the gut lining finish the job, breaking sugars like lactose down into pieces small enough to pass through. Whatever is left, mostly fibre, water and a few things we cannot digest ourselves, moves on to the large intestine, where the bulk of our gut microbes live and get to work on it.

None of this would work without movement. The gut is lined with muscle that squeezes in waves, a bit like squeezing a tube of toothpaste from the bottom, moving everything along at the right pace. This is called peristalsis, and it can be sped up or slowed down by stress, hormones, hydration, fibre, movement and some medications.

When things move too slowly, two things happen. The large intestine keeps drawing water back out of the stool, so the longer it stays, the harder and drier it gets and the more difficult it is to pass. Our microbes also keep fermenting what is there, which means more gas and that tight, bloated feeling. Once they run short of fibre, they start breaking down protein instead, and a Danish study found that people whose food moved more slowly through the colon had higher levels of these less helpful protein by-products.

There is also a cleaning crew that only comes out when the kitchen is closed. Between meals, roughly every 90 to 120 minutes, a strong wave of contractions sweeps through the stomach and small intestine, clearing out leftover food, cells and bacteria and moving them on towards the large intestine. This is the migrating motor complex, and it is often the rumbling you hear when you are hungry. Eating switches it off, so leaving some proper space between meals, rather than grazing all day, gives it the chance to do its job.

How often should you poo?

Regular bowel movements are one of the simplest signs of how your gut is doing, and they are easy to overlook. This is how the body clears its waste, and it is also one of the ways the body clears hormones it has finished with.

The usual "normal" range is anywhere from three times a day to three times a week. However, a 2024 study of more than 1,400 healthy adults found the most favourable health markers in people who went once or twice a day. Those who went only once or twice a week had higher blood levels of microbial by-products linked to strain on the kidneys, even though they were otherwise well. It shows a link rather than proof, but it suggests that going comfortably most days matters more than many of us assume. Eating more fruit and vegetables, drinking enough water and moving regularly were what the once-or-twice-a-day group had most in common.

Rest and digest

Your gut also needs you to be in the right state to digest. The nervous system has two broad settings. In "rest and digest" mode, largely run through the vagus nerve, it encourages saliva, stomach acid and enzymes and keeps the gut moving steadily. In "fight or flight" mode, digestion is not a priority: blood is sent to the muscles and stomach emptying slows down.

Ongoing stress means more time spent in that second mode, which is one reason stress so often shows up as constipation, bloating or indigestion, although for some people it does the opposite and speeds things up. Stress may also shift the balance of gut microbes. This is clear in animal studies, but in people the findings are mixed, so it is a possibility rather than a given.

Simple things help give your gut the signal that it is safe to get on with its job: sitting down to eat, eating slowly and away from a screen, taking a few slow breaths before a meal, or going for a short walk afterwards.

Fibre for gut health, and why the type matters

Fibre is the part of plant foods we cannot digest ourselves, which is exactly what makes it so valuable, because it passes through to the large intestine where our gut microbes can use it. You will often see it split into two types, and both earn their place.

Soluble fibre dissolves in water and forms a soft gel, a bit like porridge oats left to soak. It slows down how quickly sugars from a meal reach the blood, helps carry cholesterol out of the body and keeps stools soft. You find it in oats, barley, beans, lentils, chickpeas, apples, pears and ground flaxseed.

Insoluble fibre does not dissolve. It works more like a broom, adding bulk and helping things keep moving through the gut. You find it in wholegrains, wheat bran, nuts, seeds and the skins of fruit and vegetables.

When it comes to feeding the microbiome, what matters most is how fermentable a fibre is, which is not quite the same thing as soluble or insoluble. Many soluble fibres are fermented easily, but resistant starch, the starch that forms when cooked potatoes, rice or pasta are cooled, or that you find in slightly green bananas, also feeds our microbes well, while psyllium is soluble yet only lightly fermented. Most whole plant foods contain a mix of fibres anyway, which is one more reason variety matters more than chasing a single "best" fibre.

How much is enough?

The UK recommendation for adults is 30g of fibre a day, and most of us fall well short, with the average adult eating somewhere around 16 to 18g and only around 4% of adults reaching 30g. It is worth knowing that "5 a day" refers to portions of fruit and vegetables, not fibre, and five portions on their own will not get you to 30g, so wholegrains, beans, lentils, nuts and seeds all need to be part of the picture.

A helpful way to think about it is variety rather than counting grams. In a large citizen science study, the American Gut Project, people who ate more than 30 different plant foods a week had a more diverse gut microbiome than those eating 10 or fewer. That study shows an association rather than proof of cause, but it fits with everything else we know, and herbs, spices, nuts, seeds, wholegrains, beans and lentils all count towards the 30.

If you are eating very little fibre at the moment, build up gradually and drink enough water alongside it, as a sudden jump can leave you bloated and uncomfortable while your gut adjusts.

Your gut microbiome: why diversity matters

The gut microbiome is central to a healthy gut and to the body as a whole. Trillions of bacteria, viruses and fungi live together in the gut, mostly in the large intestine, fermenting the fibre we cannot digest and producing beneficial compounds that support the gut lining, the immune system and systems right across the body.

I find it helps to think of it like a garden. A garden with lots of different plants copes much better with a dry summer or a bout of pests than one planted with a single crop, and a gut with a wide range of microbes is more resilient in the same way, because different species do different jobs and can cover for each other. This range is what people mean by diversity, and a more diverse microbiome is consistently linked with better health, while lower diversity has been linked with conditions like type 2 diabetes and obesity.

No two microbiomes are the same. Yours started forming at birth and has been shaped ever since by how you were born and fed, where you live, what you eat, the medicines you have taken and how you live, so it is almost as individual as a fingerprint. That is also why researchers still cannot say what a single "ideal" microbiome looks like. Different people can have quite different mixes of species that do much the same jobs, so the aim is a well-fed, diverse community rather than a perfect list of bacteria.

Among the most important things our microbes make are short-chain fatty acids, mainly butyrate, acetate and propionate, which they produce when they ferment fibre. These feed the cells of the gut lining, help to calm inflammation and send signals that reach well beyond the gut, which is a big part of why what we feed our microbes ends up mattering for the whole body.

Here is something I find fascinating. The Hadza, one of the last hunter-gatherer communities in Tanzania, eat far more fibre than we do and have a far more diverse gut microbiome, and researchers have found that some bacterial species shared by traditional communities on different continents are missing altogether from people living in industrialised countries.

The gut lining (and what people call "leaky gut")

The gut lining is only one cell thick, yet it has the job of letting nutrients in while keeping almost everything else out. Think of it like a border with a security checkpoint, where the cells sit side by side and the gaps between them are held closed by proteins that act like the gates. These are called tight junctions, and on top of the cells sits a layer of mucus that keeps microbes at a safe distance from the lining itself.

This lining is constantly being renewed, with the cells replaced every few days, which makes it one of the hardest working tissues in the body and means it needs a steady supply of fuel and nutrients. Butyrate, the short-chain fatty acid our microbes make from fibre, is the main fuel for the cells lining the colon, and research shows it helps to keep the tight junctions firm and the mucus layer healthy. So when we eat plenty of fermentable fibre, we are in effect feeding the lining through our microbes.

When the lining is under strain, from a low-fibre diet, alcohol, ongoing stress, some medications or infection, those gates can loosen and more passes through than should. This is known as increased intestinal permeability, and because the immune system sits right underneath the lining, it tends to respond, which is where the link between the gut lining and wider inflammation comes from.

The gut and your immune system

Your gut is in contact with more of the outside world than any other part of the body, so it makes sense that it is also where the immune system does a great deal of its learning. Just beneath the gut lining sits a network of immune tissue, including small clusters called Peyer's patches, that is constantly sampling what passes by and deciding what is safe, like food and friendly microbes, and what is a threat. This network is called gut-associated lymphoid tissue, or GALT, and the gut is home to the majority of the body's antibody-producing cells, which is why it is often described as the largest immune organ in the body.

The microbiome is part of that conversation. The short-chain fatty acids our microbes make help the immune system stay calm and tolerant rather than reactive, which is one more way a well-fed, diverse microbiome supports the body beyond digestion.

Where histamine comes in

Histamine is best known for its part in allergies, but it is also a normal messenger in the gut, involved in stomach acid production, gut movement and immune signalling. It reaches the gut from three places: our own immune cells make it, some gut bacteria make it, and some foods are naturally high in it, particularly aged, cured, fermented and leftover foods, alcohol and some fish.

Most of the histamine in our food is broken down in the gut by an enzyme called diamine oxidase, or DAO, which is made by the cells of the gut lining. When more histamine arrives than DAO can clear, for example when the lining is under strain or the balance of microbes shifts towards histamine-producing species, some people react to histamine-rich foods with flushing, headaches, a blocked nose or digestive upset.

This is usually called histamine intolerance, but the name is worth questioning. Histamine is something our own bodies make and rely on every day, so we are not really intolerant to it. What seems to be going on is a mismatch between how much histamine is coming in or being made and how well it is being broken down, which is why some prefer to talk about histamine excess or reduced histamine breakdown. Seen this way, the places to look are the total load from food, the health of the gut lining and the balance of gut microbes, rather than histamine itself. The research is still developing and there is no well-validated test yet, so it is something to explore carefully rather than assume.

Why our guts have changed

Our gut microbiome has changed a great deal over the last few generations, and it is not down to any one thing. The way we eat, the medicines we take, how much time we spend outdoors, how we are born and fed as babies, how stressed we are and how well we sleep all play a part, but diet is the biggest one we can actually do something about.

Most of us now eat far less fibre than our ancestors did, and far more ultra-processed food, the packaged, ready-to-eat products made largely from refined ingredients and additives that you would not find in a home kitchen. A large review published in the BMJ in 2024, pulling together studies covering almost 10 million people, linked higher intakes of ultra-processed food with a higher risk of 32 health outcomes, including heart disease, type 2 diabetes, anxiety and depression and early death. These studies show links rather than proof of cause, but the pattern is consistent. Part of the problem is simply what these foods crowd out, the fibre and variety our microbes rely on, and some additives, like certain emulsifiers and sweeteners, have been shown to affect the microbiome and the gut lining, although most of that research so far has been in the lab and in animals.

There is a striking experiment in mice that shows why this might matter over the long term. When mice carrying human gut microbes were fed a low-fibre diet over several generations, each generation lost more microbial diversity, and by the fourth generation many species could not be brought back by returning to a high-fibre diet. We cannot assume people work exactly the same way, but it does make you think about what we pass on.

The food we grow

You may have heard that our fruit and vegetables are less nutritious than they used to be. There is some truth in it: a well-known study comparing data from 1950 and 1999 found declines in some minerals and vitamins in many vegetables. The researchers put most of this down to crops being bred for size, speed of growth and yield rather than nutrients, rather than to the soil simply running out. Either way, vegetables remain some of the most nutrient-rich foods we can eat, and eating more of them, and more different kinds, makes far more difference than any small change in their content.

Prebiotics, probiotics and fermented foods

These three get mixed up a lot, so it is worth separating them out.

Prebiotics are the food for your microbes, types of fibre that particular beneficial bacteria thrive on. Onions, garlic, leeks, asparagus, Jerusalem artichokes, slightly green bananas, oats, beans and lentils are all good sources, and cooked and cooled potatoes and rice add resistant starch to the mix.

Probiotics are live microbes, usually taken as a supplement. Their effects depend very much on the specific strain, so one product cannot be assumed to do the same as another, and most of them pass through rather than settling in for good. They can have a place, but they work best alongside a fibre-rich diet rather than instead of one.

Fermented foods include live yoghurt, kefir, sauerkraut, kimchi, kombucha and miso. In a small but well-designed study at Stanford University, healthy adults who gradually built up to around six servings of fermented foods a day saw their microbiome diversity increase and 19 markers of inflammation in their blood go down. Interestingly, the group who ate more fibre in the same study did not see their diversity change over that period, which may simply mean fibre works more slowly or depends on which microbes you have to start with, and is a good argument for including both.

If fermented foods are new to you, start small and build up. They are also naturally high in histamine, so if you know you react to histamine-rich foods, they may not suit you.

How to improve your gut health: the foundations

None of this needs to be done perfectly or all at once, and I know these things are not always easy to fit into real life, but these are the foundations everything else is built on.

Eat as many different plant foods as you can, and count the herbs, spices, nuts and seeds too.

Build your fibre up slowly towards 30g a day, and drink enough water alongside it.

Include some fermented foods if they agree with you.

Choose foods that are as close to how they were grown as you can, and let ultra-processed foods be the occasional extra rather than the everyday.

Leave some space between meals so your gut can do its housekeeping.

Slow down and chew your food.

Move your body every day, in whatever way you enjoy.

Look after your sleep and find ways to bring your stress down, because your gut feels both.

So feed the garden, and it will look after you.