I spoke to the man who wrote the book on fermentation – here is his golden rule for gut health
Gut health has become something of a golden goose in health circles recently. With the world turning to prebiotics and probiotics to placate stomach aches (and many other ailments), fermented goods such as kimchi, kefir and kombucha are now the order of the day.
Few know more about this topic than Drain Drain – the man many Michelin-starred restaurants have on speed dial for all their fermentation needs. He has quite literally written the book on the subject Adventures in Fermentation, out on 12 June.
Drain's route to some of the world's top kitchens was far from conventional. It began with a chemistry degree in Bristol, followed by 'a few years in the dark arts [finance]' and then a PhD at the University of Oxford in materials science – a field which, as the name suggests, explores the properties, composition and structure of materials.
It was only when he had completed this that a career in food came into the picture.
'I'd always loved food and cooked a lot, and maybe dreamt of becoming a chef one day,' Drain tells me. 'So I moved back in with my parents in Birmingham and realised there was a place for me as this kind of boffin who understood the science behind the food we eat, why things taste good and how you can make them taste better using science.'
This innovative approach quickly turned heads, and it wasn't long before he was working with a raft of top restaurants including esteemed Copenhagen joint Noma.
'Over the last 10 years I've become this sort of secret weapon of the world's Michelin-starred restaurants ,' Drain says. 'I'm this guy they call up, then I go in and set up research and development labs. I typically work with ingredients from close by the restaurant, teach the chefs how to ferment, and we create these beautiful tasting liquids, pastes and fermented goodies.'
In the process of experimenting according to scientific principles, he as settled on a phrase he believes encapsulates a health-promoting approach to nutrition: eat your CAP. which stands for 'colourful array of plant-based foods', with this method advocating for a diet rich in wholefoods from across the colour spectrum. This formula, he believes, is effective for improving your health, and the many benefits it holds, from increased gut microbiome diversity to improved mental health and immune system function.
What is fermentation and why is it important?
Gut health is trendy in 2025, and fermented foods are rightly seen as a central pillar for achieving it – but fermentation is far from a new phenomenon. According to Drain, the earliest evidence of humans dabbling in fermentation dates back about 13,000 years, and much of our modern diet is dependent on the process.
'The key thing to know about fermentation is that it's done by microbes; bacteria, mould and yeast. These things, and bacteria especially, have been around on our planet for billions of years,' he explains.
'They're really ancient, and they're absolutely everywhere. They're on you, in you and on anything you touch – phones are caked in microbes. We really are living on their planet. So, when you know that, the idea that we use them to make some food shouldn't be particularly remarkable or alarming.'
However, the earliest examples of fermentation among humans would have been an accident, Drain says.
'Our ancestors would have been picking up a peach that's slightly fermented, and it would have become slightly alcoholic, so they'd have been getting drunk off fermented fruits and thinking, 'Oh, that was interesting'.
'But it was about 13,000 years ago that we know for sure humans started directing ferments and working out, 'If we do this process in this particular cave or under these conditions, we'll end up with something that's either kind of tasty or safe, because fermentation sometimes breaks down toxins.
'You could eat a berry that might otherwise be poisonous – it might have killed your granddad and your uncle, but you realise that if you put it in a particular place and leave it for three days, you don't die when you eat it.'
And the benefits don't end there. Fermentation can also make the nutrients in a food more bioavailable.
'For example, the way the nutrients are locked up in a soybean means our guts are not well-designed for getting every single ounce of nutrition out of it. Whereas, if we ferment it first, we can break down some of those macro molecules, as nutritionists would call them, into a form where we can get more nutrients from them.'
Just like the fashion world, the fitness space has trends which tend to dominate conversation for a few years. Currently, gut health is front and centre among them, but why is this worthy topic only now earning some well-deserved recognition.
'I think it's a convergence of trends,' Drain says. 'We're seeing this growing awareness of the gut microbiome because the science in that field is becoming more abundant. Then I think there's this desire to push back against ultra-processed foods (UPFs). People want to know what they're eating, and they are looking to eat more wholefoods.'
He also believes the Covid pandemic played a role in the rising curiosity around gut health, with lockdown sourdough experiments sparking curiosity in the fermentation process.
'I think that opened a lot of gates to this idea of microbes in the kitchen,' Drain says. 'And as soon as you plant that seed, people realise that if you take protein off the plate for a second, and maybe carbs, basically all of the world's favourite flavours come from fermentation.
'Things like vinegar, miso, soy sauce, any alcohol, cheese, butter, tempeh – even chocolate and coffee are fermented. Our favourite flavours come from fermentation, it's just that we don't realise it because we've got divorced from making our own food.'
'[Fermenting is] amazing for your gut microbiome, it helps you eat loads of prebiotic fibres and fibre in general, it's delicious, you can do it at home, it's really cheap and it's kind of fun – it's just a win-win,' Drain adds.
What is the 'eat your CAP' approach to gut health and why does it work?
If you want to eat a nutritious meal, you could do a lot worse than a colourful plate crammed with wholefood ingredients. But it's not just the usual customers of leafy greens and vibrant yellows that play a key role, Drain says.
'Basically, eat anything that's within the rainbow, then also add in things that are brown and black and white,' he explains. 'Obviously green is important, things like leaves, and red things like red cabbage and berries too. But it also extends all the way down to things like coffee and cocoa, which has an incredible amount of prebiotic ingredients.
'That's the stuff the gut microbes need to be fed in order to do their thing [such as aiding digestion and boosting the immune system].'
However, Drain says UPF-heavy modern diets are often lacking in certain nutrients such as fibre.
'Even if you start with a diverse gut microbiome, if you don't feed the microbes the stuff they need, they're going to wither and die. So that's one of my takeaways from the book: eat your CAP. [You should] eat a colourful array of plant-based foods, which essentially means eating lots of different veggies, nuts, legumes and cereals is the way to keep your gut microbes happy.'
For further gut health benefits, he also recommends adding fermented foods into your diet on a daily basis. But this doesn't mean you need to start necking kefir like there's no tomorrow, especially if you aren't convinced by the flavour.
'Start small and aim for consistency,' advises Drain. 'You could eat a spoonful of sauerkraut a day, or eat some sauerkraut one day, a little glass of miso broth the next day, a glass of kefir the next day, and some kombucha after that.
'Cycling through these things means it doesn't have to be this huge undertaking, because I know some of the flavour profiles can be a little bit strong for some people.'
What are the benefits of using the 'eat your CAP' approach
For most people, eating their CAP would result in a more diverse gut microbiome, and the myriad plus-points that come with it. 'There are associations between a more diverse gut microbiome and improved mood and cognition, improved mental health, improved immune function and improved digestion,' Drain says.
This is an appealing list. To understand how improved gut microbiome diversity can offer these benefits, it is helpful to know the processes behind it.
'Humans are essentially a tube,' Drain continues. 'You put something in at the top, and other stuff comes out of the bottom. The game is getting as many nutrients from it as possible in the process.'
For this reason, humans evolved to make use of microbes and their pre-existing expertise in breaking down proteins into amino acids, breaking down carbohydrates into simple sugars, and other functions.
'Your gut microbiome is this ecosystem of microbes that live in your gut, and everyone's is different,' says Drain. 'They get to live in a nice place that works for them, in the middle of your gut, and we get the benefits of them breaking things down and detoxifying things.'
Research, such as this 2023 study from the University of Oxford, seems to agree that improved gut microbiome diversity is associated with better health outcomes, and vice versa.
'People that have some diseases, such as metabolic diseases, might have fewer diverse microbiomes,' explains Drain.
'Eating a multitude of fermented foods – things like kefir, live yoghurt, sauerkraut, kimchi and miso – that contain live microorganisms and might make it all the way down your digestive system into your gut, can help lead to that diversity. So that's one reason to eat lots of live fermented foods.'
This is where the word probiotic comes in. You will no doubt have seen this term slapped on the packaging of supermarket stock claiming to be good for your gut, but Drain says it can be 'somewhat abused and misused'.
'It has a very specific scientific meaning, but fundamentally it's talking about live microorganisms that, in a known quantity, can have a positive health impact on the person that consumes them.'
But probiotics are not the be-all and end-all for health benefits. Drain also wants to draw people's attention to postbiotics.
'Even if you eat fermented food that's been pasteurised, so there are no live microorganisms or probiotics in it, the remains of those organisms can also have positive health benefits,' he says.
'Sometimes, when you pasteurise something, it might break open the cell and the insides of the microbe can come out. Those molecules can also do really powerful things like trigger immune responses inside a human.
'There's been this obsession with probiotics [in recent years], but actually if you eat pasteurised fermented foods, the molecules that are left over from fermentation can also have amazing health benefits. We use the word postbiotics to describe that. So where, for the last two to four years, it's all been about probiotics, we're now talking a lot more about postbiotics.'
Examples of postbiotics include supermarket-bought sauerkraut and kimchi, and there's a reason why their source is important.
'If you make it at home, it will be live and have lots of live microorganisms,' Drain says. 'But if you buy a sauerkraut or kimchi in a supermarket, it might have been pasteurised before it got shipped to extend its shelf life. Similarly, with some kombuchas, but not all, some kefirs, but not all, etc.
'People are passionate about live microorganisms, and they would say, 'Oh, you can't eat pasteurised sauerkraut, it won't do anything'. But actually the new science is saying you can, and these postbiotic elements can have a very profound and positive set of health benefits as well.'
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