The garden of bacteria that rules your gut might be responsible for how your body and brain tick.
There’s a great way to get a snapshot of gut health and it’s called the Blue Poo Challenge. Before I lose you, do read on. Dying our first meal of the day with royal blue dye is a bit like a home-administered barium meal. It was first developed by Professor Tim Spector of the ZOE project, where they “gave blue muffins to thousands of people and discovered that gut transit time - the time it takes for food to travel from mouth to toilet through the gut - can say a lot about a person’s gut microbiome.” I did a blue poo recently, and went from being a pretty smug healthy eater to realising my transit time was slow, and my diet needed some gut-friendly tweaks.
When it comes to learning more about these? There is no better expert to turn to than Spector, the Professor of Genetic Epidemiology at King’s College London. Thirty years ago he founded the UK’s biggest twin study (TWINSUK). TWINSUK led to the understanding that our microbial mix is linked to our genes, as well as diet and environment. Fascinated by the impact of the microbiome on health, Spector launched his British Gut project a decade ago, before microbes became the research topic du jour.
When I spoke to him, he described a frustration that other scientists weren’t listening. “Britain came to the gut microbiome party late. It was considered an over-hyped fad that wouldn’t last by the senior scientists who influenced where science spending went.” The US, Spector said, had a small core of well-funded centres a decade before Britain.
Spector’s work proved that when it comes to diet, one size does not fit all. In 2020, he launched ZOE, an app that uses microbiome profiling and levels of fats and sugars in the blood to help users understand their body’s unique responses to food. Nearly ten years ago, via his TWINSUK studies, Spector had proved that unlike our genes, which are pretty fixed, we can make highly effective tweaks to our microbiome by just changing what and how we eat.
“The aim of ZOE is not to limit food choice but to increase it. Swap your breakfast muesli for yogurt or vice versa. Or swap a banana for a pear. We also want to increase the amount of microbe-friendly foods people eat.”
Those who contribute to ZOE get to learn helpful things about their body while also contributing to one of the most large-scale citizen science studies ever. As such, Spector reappropriated ZOE’s tech early in the pandemic to crowdsource Covid symptoms, and the results were the first to confirm loss of taste and smell as symptoms.
What Is the Gut Microbiome? A Microbiome Definition
A decade ago I wrote a story about an elderly professor at UCLA who was linking the microbiome - or ‘gut flora’ as they called it then - with mental health. I spent unaccounted-for days researching this short article because I just couldn’t grasp how bacteria in our gastrointestinal systems could affect the mind. As someone who is constantly wondering how to optimise my health, the field really interested me. Cut to now and the microscopic worlds that live inside us all are the hottest academic topic of our times.
So: what is the gut microbiome, exactly? In the simplest terms, the gut microbiome is the vast, dynamic ecosystem of microorganisms that live in your digestive tract. We’re talking bacteria, fungi, archaea, viruses, and tiny parasites - in total, as many as 1,000 different species, outnumbering your human cells ten to one. These aren’t passive passengers. They interact directly with your cells, your DNA, and each other, and in the process they produce waste products including B vitamins and short-chain fatty acids that can either boost or undermine human health.
Covid, obesity, IBS, cancer - there is a link between microbes and almost every action in the human body, including mental health. The gut microbiome’s importance cannot be overstated. But how do they do this? That’s a big question. I’ve heard the gut compared to the Amazon rainforest (because it is such a complex ecosystem) and to space (because it is so vast and unexplored). But we know more about space than we do our own gut microbiomes. If we compared the space race to the research rush to navigate our inner universe, we haven’t even put a dog in a rocket yet.
What we do know is that having a healthy gut microbiome will limit inflammation, strengthen immunity, and aid the digestion and absorption of nutrients. Understanding the gut microbiome’s importance is no longer fringe science - it is the frontier of medicine.
The SCFA Mechanism: How Your Gut Bacteria Actually Talk to Your Body
This is the part most articles skip. It’s also the most important part.
When your gut bacteria ferment dietary fibre - the plant-based material that your body cannot digest on its own - they produce a class of molecules called short-chain fatty acids, or SCFAs. The three primary ones are butyrate, propionate, and acetate. Think of them as the currency your gut bacteria use to pay the rest of your body for keeping them housed and fed.
Butyrate: the colon’s primary fuel
Butyrate is the most studied and arguably the most powerful. It is the preferred energy source for colonocytes - the cells lining your colon - and it plays a central role in maintaining the integrity of the gut wall. A healthy gut wall is what stops bacterial toxins from leaking into your bloodstream (a phenomenon researchers call intestinal permeability, or ‘leaky gut’). Butyrate also has demonstrable anti-inflammatory properties, acting on immune cells both locally and systemically. Exercise helps produce butyrate; so does the fermentation of certain dietary fibres. It is the reason why Spector has long championed both.
Propionate: the liver’s regulator
Propionate travels via the portal vein directly to the liver, where it plays a key role in regulating blood glucose and suppressing appetite signals. Research from Imperial College London has linked higher propionate production to reduced calorie intake and lower body weight - one mechanism by which a high-fibre diet supports healthy weight management that has nothing to do with calorie restriction.
Acetate: the systemic communicator
Acetate is produced in the largest quantities and travels furthest through the body. It crosses the blood-brain barrier, influences appetite regulation in the hypothalamus, and has been linked to immune modulation. It is the least understood of the three but perhaps the most far-reaching in its effects.
The microbiome’s influence on human health is largely mediated through SCFAs. Feed your bacteria the right fibres, and they produce the right signals. Get this wrong - through a poor diet, antibiotics, or chronic stress - and the downstream effects touch virtually every system in the body.
The Fibre Length Revolution: Not All Fibre Is Created Equal
Here is where microbiome research has quietly undergone a revolution that most popular health writing has not caught up with yet. For years, the advice was simple: eat more fibre. Hit your daily 30g. Job done. The emerging science tells a far more nuanced story.
Different species of gut bacteria are adapted to ferment different lengths of dietary fibre. Short-chain fructo-oligosaccharides (scFOS) are rapidly fermented in the upper colon, feeding the bacteria that live there. Longer-chain fibres such as inulin travel further down the digestive tract before being fermented, reaching and feeding the microbial communities of the lower colon - communities that are often starved on a typical Western diet. Medium-chain fibres occupy the territory in between.
This means that eating only one type of fibre - even in large quantities - is a bit like only watering the front half of your garden and wondering why the plants at the back are dying. The length and structure of the fibre determines which bacteria it feeds, and where.
What the world’s longest-living populations can teach us
Researchers studying Blue Zone populations - the communities in Sardinia, Okinawa, Ikaria, Loma Linda, and Nicoya that produce a disproportionate share of the world’s centenarians - have consistently noted something striking about their dietary fibre intake. It is not just the quantity; it is the extraordinary diversity of plant sources, each contributing fibre molecules of different lengths and structural complexity. Their guts are exposed to a remarkably broad-spectrum fibre profile, feeding bacterial communities across the entire length of the colon. This is not coincidence. It is the nutritional blueprint of extraordinary gut health.
The emerging concept sometimes described as ‘fibermaxxing’ has begun to capture this idea - not simply eating more fibre, but eating smarter fibre: diverse in source, varied in chain length, matched to the full ecology of your gut. Institutions from UCLA to Tufts are now publishing on this shift, and the evidence base is growing quickly. The question is no longer how much fibre you eat. It is which fibres, in what combination, and whether they’re reaching the communities that need them most.
Gut Microbiome and Mental Health: The Brain-Gut Microbiome Axis
Back to that professor at UCLA. The link between the gut and mental health - once considered fringe - is now one of the most active areas of microbiome research. The brain-gut microbiome axis is the bidirectional communication network between the central nervous system and the enteric nervous system (the ‘second brain’ embedded in your gut wall). These two systems talk to each other constantly, via the vagus nerve, the immune system, and - here’s where it gets interesting - via SCFAs and other microbial metabolites.
Approximately 90% of the body’s serotonin - often called the ‘happiness hormone’ - is produced in the gut, not the brain. The production of serotonin, dopamine, oxytocin, and endorphins is influenced by the tens to hundreds of trillions of bacteria that live in our guts. This is not metaphor. It is biochemistry. A 2019 study in Nature Microbiology found that two genera of gut bacteria, Coprococcus and Dialister, were consistently depleted in people with depression, independent of antidepressant use. Gut microbiome and mental health are not separate conversations. They are the same conversation.
A related and growing body of microbiome research news today concerns the role of intestinal permeability in neuroinflammation - the hypothesis that a damaged gut lining allows bacterial toxins (lipopolysaccharides, or LPS) to enter the bloodstream and trigger a low-grade inflammatory response that reaches the brain, contributing to conditions from anxiety to cognitive decline. Butyrate’s role in maintaining gut wall integrity makes it directly relevant here. How to fix your gut microbiome, in this light, is also how to protect your brain.
Get into tweaking your microbiome
In academic parlance, an out-of-whack microbiome has ‘gut microbial imbalance’ or ‘dysbiosis’. This is caused by your genes, antibiotic use, a crap diet, and/or a lifestyle that causes inflammation. By eating wide and smart we can do so much to improve health. One example is happiness: the production of ‘happy hormones’ like serotonin, dopamine, oxytocin and endorphins are influenced by the tens to hundreds of trillions of bacteria that live in our guts.
Your microbes are fun lovers
While your average health influencer wants you existing on green juices and herb teas, our microbes are up for a way more exciting menu. Spector starts every day with good coffee; in his first book about food, The Diet Myth, he turbocharged his microbiome with a daily diet of stinky, delicious French cheeses. In his recent book, Spoon Fed, he gives a science-led thumbs up for wine, butter, olive oil, decent chocolate, good bread, steak, and the list goes on.
All play a role in gut health if bundled along with a rainbow cornucopia of plant foods - 30 different types in a week he recommends. Basically, what your gut really, really wants is decent food that hasn’t been messed about with, and in as many different colours and varieties as possible.
Obesity is determined by our microbes
A study by Spector of nearly 500 identical twins, published in 2014, found a bacteria species called Christensenellaceae that is more common in slender people. Transplanted into mice, it caused them to lose weight. It’s the supermodel microbe and one day, rather than bariatric surgery or other painful traumatic procedures, a probiotic containing this may be the answer to weight loss.
Microbes need exercise and fibre
Exercise helps produce a microbe byproduct called butyrate, a very simple ‘short-chain’ fatty acid. It’s also produced when certain microbes break down fibre. Butyrate is an incredible substance that regulates everything from colon health to mental health. Exercise and fibre are the least we can do to thank it for all its incredible health-preserving work.
Let your gut have some down time too
Research has found that fasting periods - ranging from several hours to a day - support the health of the gut microbiome. It doesn’t have to be too biblical in scale: a simple overnight fast, and a general reduction in snacking between meals, will give the microbes that clean your gut lining a chance to do their work.
Fermented foods are the new probiotics
Big one, this. Especially of interest to the gen Z. There is research that links a reduction in our ingestion of fermented foods - which are stuffed unusually full of good microbes - to a decline in human health. In the days before artificial preservatives, fermentation was an essential means of keeping foods edible and healthy long term. Nowadays our processed foods and drinks, and especially ultra-processed foods, kill those friendly bugs with their armoury of chemicals that can keep them ‘fresh’ on the shelves for years.
Living a fermented life is actually pretty fun. Wine, after all, is fermented (though obviously to be consumed responsibly). Cheese, kimchi and sourdough - all fermented - are the three ingredients required for an absolutely banging toastie. One study found consumption of fermented foods “is associated with lower severity and prevalence of depression.” Which I think means a grilled cheese is the new Prozac.
Probiotics are not all they’re cracked up to be
Spector’s take on over-the-counter probiotics is that they aren’t helpful, and could even cause disruption to gut health. His advice is to get it all from food, wine and ale.
Gut Microbiome Testing: How to Know Where You Stand
One of the biggest advances in the understanding of this innerverse came when it became possible to read the DNA of your microbes via a small poo sample. This is called ‘taxonomic functional shotgun metagenomic sequencing’ and it’s expensive. Subscribers to ZOE can have this done, or for the rest of us, there’s the blue poo route.
Gut microbiome testing has expanded significantly in the past few years. Where once the best gut microbiome test required a clinical referral and weeks of waiting, home-based options now make it possible to understand your microbial profile - and act on it - relatively quickly. The question of how to test your gut microbiome has become genuinely answerable for the first time.
Beyond direct microbiome sequencing, blood-based biomarkers are increasingly used to infer gut health status. Markers of systemic inflammation, metabolic function, and hormonal balance - the same panel of 17 key biomarkers tracked by clinically validated at-home blood testing programmes - can reveal the downstream effects of gut dysbiosis long before you feel them. Chronic low-grade inflammation, disrupted blood glucose regulation, and altered hormonal signals are all consistent fingerprints of a compromised gut microbiome. Understanding these numbers - and tracking how they change over time - gives you the kind of precision picture that the Blue Poo Challenge, charming as it is, cannot provide on its own.
If you’re serious about improving your gut microbiome, starting with data is the most intelligent approach. Know where you’re beginning from.
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Get acquainted with your microbial visitors
After the Blue Poo result, I started thinking about this crucial inner quantum farm far more. I wanted to tend to it; it was an almost maternal feeling. I started fermenting my own ginger beer (non-alcoholic), I drank ayran (lightly salted drinking yoghurt) instead of fruit juice, I got over my fear of cheese being fattening and instead viewed it as a health food. It’s a long list, and while one person’s experience does not a clinical trial make, my weight remained steady while inflammation hot spots in my body (like an old dodgy ankle that was often a bit puffy) noticeably calmed down.
The whole concept still feels trippy to me. But actively nurturing my microbial guests paid off for me. Today, Spector’s belief that the gut microbiome would prove to be some of the most crucial human science of our times has been proven correct beyond any reasonable doubt. Targeted microbial medicines are the future. Good health is thanks to the subtle interactions of these trillions of organisms with our own cells - so if we trash our microbiome, we are going to get ill. Happy farming.
