It was while munching through a slice of Marmite on toast and doom-scrolling Instagram that I first stumbled across Tim Spector’s latest revelation: this divisive spread, beloved and loathed in equal measure, might actually be good for me. Not just for its salty umami hit, but for my gut. Hurrah – a health win I didn’t even need to try for.
Because Marmite, Spector explains, is a ferment. Not in the bubbling-kimchi-on-your-countertop sense, but because it’s made from yeast leftover from brewing beer. Technically, it’s dead by the time it hits the jar, but it still contains proteins and B vitamins, including high levels of B12, which a recent study showed could help ease anxiety and depression. “A lot of people have Marmite regularly,” Spector says, “and in people who have it for years on end, it could well be beneficial.”
Before you rush out to stockpile Marmite, the point is less about toast-as-therapy and more about the hidden ferments we overlook every day. According to Spector, the roll call of everyday ferments is longer and stranger than you might think.
“Most people are surprised at soy sauce and Tabasco,” he says. Even ketchup, before Heinz industrialised it, was a fermented food. “Then you’ve got the dead fermented foods, like chocolate and coffee. Beers are also a good example. Some highly filtered beers have no dead microbes at all, while in other ones you’ll be getting quite a lot of dead yeast in there.”
This is where things get interesting. For years, the orthodoxy was that if microbes were dead, they were useless. Spector himself once thought the same. “Initially, I was very down on anything that was killed or pasteurised. I thought they were conning people,” he says, “but it turns out they may still have some benefit.”
Now he calls them “zombie microbes” – organisms that, although lifeless, still interact with our bodies. Admittedly, it could do with a marketing overhaul. “It’s not yet approved by food authorities to call something a post-biotic,” he says, “but the public will soon know about it.”
For Spector, fermentation is less kitchen craft and more molecular sorcery. In layman’s terms, it’s “food that has been transformed by microbes into something that tastes different and has different health properties, and usually it allows it to be preserved longer,” he explains.
If cupboard staples like Marmite are at the lower end of the spectrum with zombie microbes, at the top end are the “Premier League” ferments: kefir, kimchi, sauerkraut, kombucha. These are the stars of his new book Ferment: The Life-Changing Power of Microbes, which mixes research, recipes and a call to arms for a more microbe-friendly way of eating.
“I’ll always try and eat three ferments a day, sometimes more,” he says. “For me, it’s a Greek yoghurt and milk kefir for breakfast, then at lunch, maybe some sauerkraut and a salad. And in the evening, I’d probably have a glass of kombucha and another kefir. If I have kimchi, it makes five, but I think three is achievable.”
Spector isn’t just practising what he preaches – he’s tested it. For Ferment, he set up a citizen science project through Zoe, the personalised nutrition company he co-founded. Ten thousand people were asked to do exactly what he does: eat three fermented foods a day, every day, for two weeks.
The results, he says, were striking: 56 per cent reported an improvement in mood. Many also said they had more energy, felt less hungry and noticed reduced bloating. For Spector, it was proof of concept – that fermented foods can affect not just our guts but our minds.
The obvious question is: how does eating sauerkraut or sipping kombucha end up making us feel calmer, happier, healthier? “We don’t know exactly. We’re filling in some gaps,” he admits.
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‘Ferment: The Life-Changing Power of Microbes’ – part recipe book, part manifesto (Jonathan Cape)
His best guess? When you drink kefir, for example, the microbes interact with the immune cells in your intestines and tell them to “calm down”. That same message then travels through the vagus nerve (connecting your brain, heart and digestive system) to “send that same signal: ‘We don’t have inflammation, all is well, you don’t have to go into defence mode! If you’re depressed, don’t be depressed!’ That’s roughly what we think is happening,” he explains.
“You’re sort of setting the thermostat lower while it’s receiving these signals from your kefir drink,” Spector says. “And if you stop, it will revert. You can’t just have a kefir binge once a month.”
The concept of microbes whispering to immune cells and signals travelling up nerves like coded telegrams sounds a little abstract, wishy-washy even, but Spector points to studies that suggest the loop is real. A detailed Stanford trial, for instance, compared people eating fibre supplements to those eating five daily ferments and found the latter showed profound shifts in immune markers.
Researchers had expected fibre to win; instead, ferments came out on top. “That was the first moment I thought, ‘wow, this isn’t just speculation’,” Spector tells me. The team took blood daily and tracked dozens of markers and the changes were striking. What was meant to be a control arm – fermented foods – ended up stealing the show.
Still, the science of the gut-brain axis is in its infancy, and the evidence isn’t without cracks. Ten thousand people signed up to the Zoe trial that inspired the book – but nearly 30 per cent dropped out, many because of bloating or because they struggled to keep up with three ferments a day. Of those who remained, 56 per cent said their mood improved – about 4,000 people out of the 10,000. Not nothing, but not the sweeping majority the topline suggests.
Even on Zoe’s own website, the numbers aren’t overwhelming: just 51 per cent of Daily30+, their wholefood gut supplement, users say they experienced improved energy, and 65 per cent felt more satisfied. Curiously, the total number of participants isn’t shared.
And what about the fact that it’s all based on self-reported mood, which can be swayed by placebo or expectation? He bristles at the question. “That’s the whole of psychiatry, it’s based on self-reported symptoms. So it’s like saying psychiatry doesn’t exist. It does, and that’s how they’ve tested antidepressant drugs and things, because there are no other measures.”
This is the future. All the trials about these ferments are showing that for many patients, particularly those with milder mental health issues, it works as well as drugs for anxiety and depression, obviously with far fewer side effects
Tim Spector
He acknowledges, though, that this is a new and imperfect field. One reason, he says, is that medicine is siloed. “Science generally is not very good at linking up different bits of the body, because you have specialists in all those areas that don’t talk to each other. You’ve got the microbiome, which is a new area, so very few specialists in it, and you’re linking gastro, neuro, psychiatry and psychology. So that’s one of the reasons it’s been slow to be taken up.”
Fermented foods themselves have been around for millennia, even marketed as health tonics, but without serious research. “Yakult’s been around since the 1920s,” he points out. “People took it as a general health tonic. No one ever said, ‘Well, Yakult’s going to help your depression’. They probably could have done the studies in the last 90 years, but they didn’t – probably in case it was negative, and they had to withdraw their product.” Insert side-eye emoji here.
So where does this leave us – and should diet now be part of how we think about treating mental health? At the moment, Spector says, it isn’t. “Psychiatrists are even less exposed to diet advice than other specialities, and they generally dismiss patients, saying it’s a waste of time.”
But he believes this will change. “This is the future. All the trials about these ferments are showing that for many patients, particularly those with milder mental health issues, it works as well as drugs for anxiety and depression, obviously with far fewer side effects. I’m not saying you should treat all mental illness with diet – there is definitely a place for the drugs – but no one’s sitting down and saying, this is because of your diet.”
Of course, not everyone has the time, money or inclination to fill their fridge with bubbling jars of kimchi.
One of Spector’s goals is to make fermentation less intimidating. For those less inclined to DIY, supermarket options are fine, he insists, so long as they’re alive. “If you want to get maximum microbes from your food, go for more traditional foods with the least processing and look at sell-by dates. Generally, shorter ones have more microbes,” he says. “If it’s got vinegar in it, they’ve definitely killed it. If it’s got a bit of fizz or something that suggests there’s something alive in it, then they’re good.”
How to ease into the flavours? He suggests “mixing kimchi with cream cheese and spreading it on toast or mixing it in with avocado and cream cheese or scrambled egg. It’s gorgeous with scrambled egg. Put it into soups and stews at the end. Some people put it in mashed potatoes, which sounds a bit weird to me.”
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From kombucha to kimchi: Spector’s kitchen experiments have even left stains on his ceiling (Handout)
The only real rule to stick to? “Putting the right amount of salt in and giving it time to work, which is normally two or three days. If you drop below 2 per cent salt, you can be in trouble.”
His own family, though, aren’t quite as evangelical. “I’ve got stains on my kitchen ceiling from my experiments,” he laughs. “Occasionally, my wife, when I’m not around, just puts some stuff in the garden and thinks I won’t notice.” He notices. Still, they’ve changed their breakfasts to include kefir and yoghurt, and they’ll happily eat sauerkraut or sip kombucha – just perhaps not with quite the same zeal.
Not everyone, in other words, is convinced. Which brings us to the bigger question.
Spector, like Marmite, divides opinion as sharply as the spread itself. Is he a pioneering scientist quietly building a new field, ready to say “I told you so” two decades from now, or a wellness entrepreneur selling hype to the worried well?
Zoe, the company he co-founded, is now valued at over a million dollars. Its nutrition programme costs nearly £300 upfront for at-home tests and up to £60 a month for membership. With Ferment, the science is still young, the studies imperfect… but the possibilities are tantalising.
What’s clear is that the microbes – alive, dead or somewhere in between – aren’t going away. Spector has once again forced us to look at what’s hiding in our cupboards and wonder: what else have we been missing?
Simple kimchi
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Spicy, tangy and fizzing with life – kimchi is one of Spector’s ‘Premier League’ ferments (Issy Croker)
This is a shortcut for those who want a faster recipe, cutting out the traditional two-stage salting process. Try both methods and see if you can tell the difference. You can use any whole cabbage, Chinese/napa cabbage and/or daikon (Japanese radish) for this recipe.
Makes: 1 jar
Ingredients:
1 cabbage or Chinese/napa cabbage
1 daikon
Sea salt
4 garlic cloves
1 bunch of spring onions, trimmed and sliced
1-2 tbsp Korean chilli flakes (gochugaru)
1 tbsp soy sauce
1 tbsp fish sauce (or fermented miso paste for vegans)
Method:
1. Trim the cabbage and cut it into 4-5mm thick slices. Trim and peel the daikon and cut into thin half-moons.
2. Weigh the combined vegetables, tip into a bowl and add 2 per cent salt of the total weight. Using your hands, massage the salt into the vegetables until they are starting to soften, then cover the bowl and set aside at room temperature for about 4 hours.
3. Combine the garlic, spring onions, chilli flakes, soy sauce and fish sauce in a food processor and pulse to combine. Add to the vegetables and mix well to coat them thoroughly. Pack tightly into a clean jar, ensuring that the vegetables are submerged beneath the brine. If necessary, add a little 2 per cent brine (ie 2g salt for every 100ml water) to cover. Press weights on top and loosely cover with a lid, then leave in a cool, dark place for 5-10 days until fizzy. Transfer to the fridge and eat within 1-2 months.
Method for leftover kimchi:
1. Chop any spare vegetables, including peppers, brassicas, carrots and onions (but avoid anything too leafy and too soft) into 4-5mm slices. Finely chop a few garlic cloves, a thumb of ginger and 2-3 chillies and add to the mix.
2. Weigh the prepared vegetables and add 2 per cent sea salt and a good amount of chilli flakes (Korean or ordinary) and mix to combine. Using your hands, massage the vegetables for a couple of minutes to soften and then leave them in a covered bowl for an hour. Then, tightly pack/squash forcibly into clean jars, making sure that the vegetables are submerged below the resulting brine. If necessary, you can top up the liquid with 2 per cent brine.
3. Cover loosely with a lid, sit the jar on a plate and store somewhere dark and cool for a week, burping the resulting gas every day or so. Store in the fridge and eat within 2-4 weeks.
Ways to enjoy krauts and kimchi:
- Add to salads or stir into rice or grain dishes – kimchi is particularly delicious in egg fried rice or served with noodles.
- Add to cream cheese as a spread – a great idea for kids.
- Add sauerkraut to a Reuben sandwich with pastrami, salt beef or mushroom pate on rye bread.
- With cheese and bread or in toasted cheese sandwiches.
- Add to dumplings or macaroni cheese or serve alongside scrambled or fried eggs.
Kombucha
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Tea turned tonic: kombucha is one of the simplest (and most explosive) ferments to brew at home (Issy Croker)
You will need a SCOBY (symbiotic culture of bacteria and yeast) to make kombucha. These are available online, can be found at the bottom of a good bottle of shop-bought raw unflavoured kombucha or you can grow your own. Failing that, find a friend who is already brewing kombucha and have one of their babies (a SCOBY baby).
To grow your own SCOBY, you will first need a small bottle (330ml or thereabouts) of raw, unflavoured kombucha. Brew up 1.5 litres of black tea (avoid anything aromatic such as Earl Grey) using 3-4 tea bags, add 100g sugar, stir to dissolve and leave to cool to room temperature. Add the kombucha, including any yeasty residue and strands in the bottom of the bottle, and pour into a spotlessly clean 2-litre Kilner or glass jar. Cover the jar with a double or triple thickness layer of clean cheesecloth secured with a rubber band or string and leave the jar out of direct sunlight and at room temperature for about 4 weeks, or until a 5mm thick SCOBY has formed. Now you can start to brew kombucha.
Makes: 1.5 litres
Ingredients:
3-4 tea bags
500ml freshly boiled water
125-150g granulated or caster sugar
1 litre filtered water SCOBY (see above)
Method:
1. Use the tea bags and boiled water to make up a brew of tea. Leave to steep for 10-15 minutes, then remove the tea bags (do not squeeze them as this will make the tea too bitter) and add the sugar and stir to dissolve. Add the filtered water.
2. Pour into a clean 2-litre jar and leave to cool to room temperature.
3. Add the SCOBY (make sure your hands are spotlessly clean when handling your live culture) and about 100ml of the original starter brew. Cover the jar with a double or triple thickness of cheesecloth and secure with a rubber band or string; this will let air in but keep insects out. Leave the jar at room temperature and out of direct sunlight and wait for 1-2 weeks as the colour lightens, and the SCOBY rises and grows to fit the container.
4. After 5-7 days, pour a little kombucha into a glass and taste it – it should be fruity and no longer taste of tea. Continue fermenting until it has reached a level that you are happy with – anywhere from 5 days to 2 weeks is about normal.
5. You can now draw off the kombucha and bottle it so it is ready to drink or go for a second fermentation when you will add any flavourings and extra fizz.
6. Leave a cupful of kombucha in the fermenting jar and either refrigerate this until you are ready to brew again or make up more tea and immediately start again. Once you have established a brewing rhythm, you can scale up the quantities you make with each brew depending on how much kombucha you drink.
7. To add flavours and a second fermentation, pour your brewed kombucha into 500ml flip-top bottles and ¼–½ teaspoon of sugar and a choice of fruit and herbs. The bottle should be filled to within 5cm from the top to allow space for the carbon dioxide to expand and escape without liquid overflowing. Peaches, mint, lemon verbena, elderflower, pineapple, hibiscus, grenadine syrup, ginger, turmeric and berries all work well. Seal the bottles and leave at room temperature but out of direct sunlight for 24-48 hours, keeping a close eye on the bottles to avoid too much gas build-up and exploding bottles. Chill thoroughly before serving.
Variation: coffee kombucha
1. Brew a batch of kombucha and have it ready for the second fermentation. Make up a cup of fresh coffee using ground coffee – either using a cafetière, filter, cold press or however you make your coffee – and leave to cool to room temperature.
2. Pour about 150 ml of coffee into a clean flip-top bottle, add 1 teaspoon of sugar and shake to dissolve the sugar. Top up with kombucha, cover and leave at room temperature for 24 hours for second fermentation to occur. Chill before serving.
Simple sauerkraut
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Humble cabbage transformed – sauerkraut shows why fermentation is often called kitchen alchemy (Issy Croker)
Traditionally, sauerkraut is fermented slowly at lowish temperatures – two to four weeks at around 15-20C – to give enough time for the sourness to properly develop. With a pH of 3.5, sauerkraut is around seven times more sour than kimchi, its spicier cousin. Red cabbage takes longer to ferment than white cabbage, and both work faster if you add another vegetable (like a small carrot or two) that contains a range of accessible sugars for the microbes.
Makes: 1 large jar
Ingredients:
1 white or red cabbage
1-2 carrots (optional)
1 tbsp caraway seeds (or juniper berries)
Sea salt
Method:
1. Trim the base of the cabbage and peel away the outer leaves; set these aside for use later. Cut the cabbage into quarters and use a mandoline, food processor, coarse grater or sharp knife to finely shred the cabbage and carrot (if using).
2. Tip the cabbage into a large bowl, add the caraway seeds and make a note of the total weight. Add 2 per cent salt to this total weight, so if the total weight is 600g, you will need 12g salt.
3. Using your hands, massage the salt really well into the shredded cabbage for a couple of minutes until it starts to soften. Cover the bowl with a clean cloth or plate and set aside for at least 30 minutes and up to 3 hours until the cabbage is very soft and has released water.
4. Scoop the cabbage and any resulting liquid into a clean 1–2 litre jar and really pack it down hard so that the shredded cabbage is submerged under the briny liquid and you have a clear 5cm gap between the cabbage and the top of the jar. Cover the top of the cabbage with the reserved outer leaves and place a weight on top. Close the lid but leave it slightly loose – if the jar is sealed tight, it runs the risk of exploding!
5. Place the jar on a plate in a dark, cool cupboard out of direct sunlight and leave for 2-3 days for fermentation to start. Burp the jar daily to release any collected gas. After 7 days, the sauerkraut should be fermenting nicely, so it can now be stored in the fridge. Use within 1-2 months, depending on how soft you like it.
Recipes from ‘Ferment’ by Tim Spector (Jonathan Cape, £25), published 11 September 2025