Fermented Foods and Your Gut: What’s Worth It and What’s Hype

Summary

Fermented foods have become a staple of gut health advice. But many of the products on supermarket shelves have been pasteurised, which kills the live bacteria most people are buying them for. This post looks at what the evidence actually shows, why fermented foods matter more as you get older, and how to make your own at home for a fraction of the price.

Key Takeaways

• Fermented foods can increase the diversity of your gut bacteria and reduce markers of inflammation.
• Most supermarket sauerkraut and kimchi has been pasteurised, which destroys the live cultures.
• Even pasteurised fermented foods still offer some benefits through compounds called postbiotics.
• For the best effect, choose unpasteurised products from the chilled aisle, or make your own.
• Homemade sauerkraut and kimchi are simple, inexpensive, and quick to prepare.

Introduction

Fermented foods have been part of traditional diets for centuries. Sauerkraut in Germany, kimchi in Korea, yoghurt across the Middle East, kefir in the Caucasus. We have our own examples here too. Traditional British cheeses, including Cheddar, Cheshire and Caerphilly, are fermented foods, even though they’re not always a source of live microbes by the time we eat them. Scotland also had sowans, a sour oat food made by soaking oat husks in water until they fermented. Long before anyone understood gut bacteria, people preserved food this way because it worked, and because they noticed it helped.

Now the science is catching up. There’s good evidence that fermented foods can do something fibre alone doesn’t always achieve. They can increase the range of bacteria in your gut and lower inflammation at the same time.

But there’s a gap between the marketing and the reality. Not all fermented foods are equal. Many of the jars sitting on the ambient shelf in the supermarket have been heat-treated, which kills the very bacteria you’re hoping to eat. And if you’ve ever looked at the price of a small jar of unpasteurised sauerkraut, you’ll know it’s not cheap.

The good news is that making your own is one of the easiest and most rewarding things you can do in the kitchen. You don’t need special equipment. You just need cabbage, salt, and a bit of patience.

What Fermented Foods Do in Your Gut

When a food is fermented, bacteria break down its natural sugars and starches. This produces lactic acid, which preserves the food and gives it that familiar tangy taste. It also creates a range of useful compounds along the way.

Live bacteria

The most talked-about benefit is the live bacteria themselves. These are mainly lactic acid bacteria, including Lactobacillus and Bifidobacterium species. When you eat unpasteurised fermented food, these bacteria travel through your digestive system. Some of them may settle temporarily in your gut. Others pass through but still interact with your immune system on the way.

Postbiotics

Fermentation also produces compounds that are useful even without live bacteria. These include short-chain fatty acids, peptides, and organic acids. Researchers call these postbiotics. They’re created during the fermentation process and remain in the food even if it’s been pasteurised or cooked. Heat-treated fermented foods are therefore not without value. They’ve lost the live cultures, but the postbiotic compounds are still there.

Prebiotic fibre

Fermented vegetables like sauerkraut and kimchi also contain prebiotic fibre from the vegetables themselves. This fibre feeds the bacteria already living in your gut. So even without the live cultures, the fibre content still offers support. I cover the wider range of foods that support your gut microbiome in a separate post.

What the Research Shows

The most widely cited trial on fermented foods and gut health was published in 2021. Researchers compared a high fibre diet with a high fermented food diet in 36 healthy adults over ten weeks.

The group eating fermented foods, including yoghurt, kefir, kimchi, sauerkraut and kombucha, showed an increase in overall gut microbial diversity. The group eating more fibre, who reached around 45 g a day, did not. This is one small trial, so it does not unseat fibre. It does suggest that fermented foods act on the gut in a different way. The same trial comes up in my post on the gut microbiome and ageing.

Inflammation dropped too

In the fermented food group, 19 inflammatory proteins decreased. These included interleukin-6 and interleukin-12, both of which are linked to chronic, low-grade inflammation. The effect was consistent across the group, and it became stronger with higher servings.

Chronic low-grade inflammation is now recognised as a driver behind many long-term health conditions. It’s also something that tends to increase with age, particularly after menopause, when falling oestrogen can tip the balance further.

Diversity increased with dose

The trial found that the more fermented food people ate, the greater the increase in microbial diversity. Participants worked up to about six servings a day. That sounds like a lot, but a serving could be a small pot of yoghurt, a few tablespoons of sauerkraut, or a glass of kefir. Once you start including these regularly, it adds up quickly.

Why This Matters More as You Get Older

After menopause, gut microbial diversity tends to decline. That’s partly because falling oestrogen affects the gut lining and the bacteria that live there. But it also works the other way around.

A group of gut bacteria known collectively as the estrobolome plays a role in how your body handles oestrogen. These bacteria produce an enzyme called beta-glucuronidase, which helps to recirculate oestrogen that would otherwise be excreted. When your gut bacteria are diverse and balanced, this system works well. When microbial diversity is low, oestrogen metabolism can become less efficient.

So there’s a two-way relationship. Falling oestrogen reduces microbial diversity, and reduced microbial diversity makes oestrogen metabolism less effective. That’s why supporting your gut bacteria through diet, including fermented foods, becomes more relevant as you move through perimenopause and beyond.

This doesn’t mean fermented foods replace hormonal support. It means they’re one piece of a wider picture, and a practical one at that.

The Pasteurisation Question

Most advice on fermented foods stops one layer too early.

The standard line is: buy unpasteurised fermented foods so you get the live bacteria. That’s true, as far as it goes. Heat-treating a product kills the beneficial bacteria along with everything else.

But a 2025 crossover trial involving 87 healthy participants compared four weeks of daily fresh sauerkraut with four weeks of pasteurised sauerkraut. Both versions led to small reductions in blood pressure. Pasteurised sauerkraut raised serum levels of short-chain fatty acids, including butyrate, more than the fresh version did.

What does that mean in practice?

It means pasteurised fermented foods aren’t useless. The postbiotic compounds created during fermentation, the organic acids, the peptides, the short-chain fatty acids, survive the heat. And those compounds can still support gut health, even without live cultures.

But if you want the full range of benefits, including the live bacteria that can temporarily join and influence your gut community, you need unpasteurised. The simplest way to tell: if it’s on the ambient shelf, it’s been heat-treated. If it’s in the chilled section and says “raw”, “unpasteurised”, or “live cultures”, it hasn’t.

What to Look for When You’re Buying

Not every product labelled “fermented” is what it seems.

Some supermarket sauerkraut is made with vinegar rather than natural fermentation. It tastes sour, but it was never fermented and contains no beneficial bacteria or postbiotics. Check the ingredients. Genuine sauerkraut should list cabbage and salt, possibly water. If it lists vinegar, it’s a pickle, not a fermented food.

Yoghurt is a more reliable option. It’s made from milk that’s been pasteurised first, and then the live cultures are added to ferment it. So the finished yoghurt still contains live bacteria, even though the milk was heat-treated. The exception is yoghurt that’s been heat-treated after fermentation to extend its shelf life, which kills the cultures. Look for “live cultures” on the label and choose plain yoghurt without added sugar.

Kefir is widely available in supermarkets now and usually contains live cultures. It has a wider range of bacterial strains than yoghurt, often including yeasts alongside lactic acid bacteria.

Miso, tempeh, and kombucha are also fermented, though their bacterial profiles differ. Miso and tempeh are often cooked before eating, so the live cultures don’t survive. Kombucha varies widely in quality and sugar content.

Traditional British cheeses belong on the list too. Cheddar, Cheshire, Caerphilly and Wensleydale all rely on fermentation during making and maturing. But cheese isn’t always a reliable source of live cultures by the time it reaches your plate, particularly if it’s been pasteurised or aged. It’s still a fermented food. I just wouldn’t put it in the same category as live yoghurt, kefir or unpasteurised sauerkraut.

How to Make Your Own

Making your own is the practical part, and the cheap one. Homemade sauerkraut costs almost nothing. A whole cabbage, a tablespoon of salt, and a clean jar. That’s it.

Basic Sauerkraut

You’ll need:

• 1 medium white cabbage (roughly 1 kg)
• 1 tablespoon fine sea salt (about 2% of the cabbage weight)
• A clean glass jar, about 1 litre

What to do:

1. Remove the outer leaves from the cabbage and set one aside. Quarter the cabbage, remove the core, and shred it finely.
2. Place the shredded cabbage in a large bowl. Sprinkle the salt over it.
3. Massage the salt into the cabbage with your hands for about five to ten minutes. It will soften and release liquid. This liquid is your brine.
4. Pack the cabbage tightly into the clean jar, pressing it down firmly so the brine rises above the cabbage. There should be no cabbage sitting above the liquid.
5. Tuck the reserved outer leaf over the top of the shredded cabbage to hold it under the brine. You can also use a small weight, a clean stone or a smaller jar filled with water.
6. Cover the jar loosely with a cloth or a lid left slightly open. Don’t seal it tightly, because the fermentation produces carbon dioxide and needs to escape.
7. Leave it at room temperature, out of direct sunlight, for three to seven days. Taste it after three days. When it’s tangy enough for your liking, put the lid on and move it to the fridge.

It will keep in the fridge for several months. Start with a tablespoon a day alongside a meal.

Simple Kimchi

You’ll need:

• 1 Chinese cabbage (napa cabbage), roughly 800 g
• 2 tablespoons fine sea salt
• 1 tablespoon gochugaru (Korean chilli flakes), or adjust to taste
• 3 garlic cloves, finely grated
• 1 teaspoon fresh ginger, grated
• 1 tablespoon fish sauce (or soy sauce if vegetarian)
• 3 spring onions, sliced

What to do:

1. Cut the cabbage lengthways into quarters, then chop it into roughly 5 cm pieces.
2. Toss the cabbage with the salt in a large bowl. Leave it for one to two hours, turning it occasionally, until it softens and releases liquid.
3. Rinse the cabbage briefly under cold water and drain well.
4. Mix the gochugaru, garlic, ginger, and fish sauce into a paste. Add the spring onions.
5. Toss the drained cabbage with the paste until everything is well coated. Use gloves if you prefer, as chilli flakes can irritate skin.
6. Pack the mixture tightly into a clean jar, pressing down so the liquid rises above the vegetables.
7. Leave at room temperature for two to five days, pressing the vegetables down once a day. Taste after two days. When it’s lively and sour enough, refrigerate.

Like sauerkraut, it keeps for months in the fridge and develops more flavour over time.

A Note on Yoghurt and Kefir

Yoghurt and kefir are worth including alongside fermented vegetables, because they offer a different range of bacteria.

Shop-bought natural yoghurt with live cultures is a good everyday choice. It contains Lactobacillus and Streptococcus thermophilus as standard. You can also make your own by warming milk to about 43°C, stirring in a spoonful of live yoghurt as a starter, and keeping it warm, in a thermos or a low oven, for six to twelve hours. It’s simpler than it sounds.

Kefir is made using kefir grains, which are small clusters of bacteria and yeast. You add them to milk, leave it at room temperature for 24 hours, strain out the grains, and drink the liquid. The grains are reusable and last indefinitely. They’re available online and from health food shops. Kefir contains a broader range of organisms than yoghurt, including several yeast species, which is why it has a slightly fizzy, tangy quality.

Both are easy to include daily. A small glass of kefir, a few spoons of yoghurt with breakfast, or yoghurt used as a base for dressings and dips.

How to Start

If fermented foods are new to your diet, introduce them gradually. A tablespoon of sauerkraut or a small amount of kefir is a sensible starting point. Too much too quickly can cause wind and bloating as your gut bacteria adjust.

Build up over a couple of weeks. There’s no fixed target, but the trial found benefits increased with higher servings, up to about six servings a day. A serving is roughly a small pot of yoghurt, a few tablespoons of sauerkraut or kimchi, or a glass of kefir. Most people won’t eat six servings, and that’s fine. Even one or two servings a day is a step in the right direction. They work best alongside plant diversity to feed your microbiome, rather than instead of it.

If you notice a strong reaction to fermented foods, particularly bloating that doesn’t settle after a week or so, it’s worth mentioning to a practitioner. In some cases it can be a sign of an existing imbalance, such as bacterial overgrowth, that may need attention first.

People with weakened immune systems, or those on immunosuppressive medication, should check with their doctor before eating unpasteurised fermented foods.

Bringing It Together

Fermented foods aren’t a magic fix. But they’re one of the most practical and affordable things you can add to your diet to support your gut. The trial evidence for increased microbial diversity and lower inflammation is early, and it points in the same direction.

The key is to choose the right ones. Unpasteurised, naturally fermented products from the chilled aisle will give you live bacteria. Making your own is even better, both cheaper and richer in live cultures than almost anything you can buy. If you’re rethinking your meals more widely, my post on practical nutrition after menopause is a good next read.

A medical herbalist can help you understand how fermented foods fit alongside other gut support, whether that’s herbal medicine, dietary patterns, or addressing an underlying issue that needs attention first.