They're All The Rage RN - So, How Beneficial Are Fermented Foods Really? An Expert Weighs In

Two years ago, most of us wouldn’t have known our kombucha from our kefir. Now, major supermarkets have started selling 'gut shots', and you can pick up a can of tropical kombucha as part of a meal deal without a second thought. But unlike many fleeting food trends, the fermented food craze isn’t just aesthetic - it’s rooted in some genuinely intriguing science.

The global wellness conversation has shifted dramatically in recent years, moving away (thankfully) from calorie counting and detox teas towards a more nuanced fascination with gut health, how our digestive system interacts with trillions of microbes inside us and influences everything from metabolism to immunity and even our mood.

As nutritional therapist Eve Kalinik previously told Marie Claire UK, “The interest in gut health has markedly increased over the past couple of decades." Thanks to increasing scientific research, "we're beginning to understand just how much of an influence it has on our wellbeing," she added.

Fermented foods have become shorthand for “good for the gut,” and there’s growing evidence to suggest they can play a role. These foods - think yoghurt, kefir, sauerkraut, kimchi, miso and kombucha - are produced through microbial fermentation, a process that can introduce live bacteria into the diet. A study found that consuming fermented foods was associated with improved bowel movement frequency, better stool consistency, and reduced digestive discomfort. Meanwhile, other studies suggest they may help increase microbial diversity, a key marker of gut health.

That said, experts are quick to caution against viewing fermented foods as a shortcut to digestive nirvana. As Vassiliki Sinopoulou, registered dietitian and lecturer at the University of Lancashire, explains, “Whether foods are fermented or not is not the main aspect that determines how they interact with the gut and the gut microbiome. The nutrient profile and the quantity of the food are what matter most, and whether the fermentation microbes are still alive or not.”

In other words: guzzling kombucha and drowning your breakfast in kefir isn’t a guaranteed route to perfect gut health. Some fermented foods contain live microbes that may be beneficial; others don’t. Some are nutritious staples; others are high in sugar, alcohol or fat. With that in mind, we asked Sinopoulou to help separate science from social-media spin - and explain how fermented foods can fit into a genuinely healthy diet.

Whilst you're here, if expert-led nutrition content is your thing, don’t miss our guides to fibremaxxing, mindful eating, intuitive eating and the inflammation diet. Plus, we’ve got healthy breakfast ideas, healthy snack ideas, healthy smoothie recipes, and what to eat after a workout - so you can nourish yourself from the inside out.

Fermented foods are trending, we asked an expert to explain how they actually support gut health.

So, what is gut health?

Gut health refers simply to the balance and function of the trillions of microorganisms - bacteria, fungi, viruses and more - that live primarily in your digestive tract. Collectively known as the gut microbiome, this ecosystem helps break down food, synthesise vitamins, regulate immune responses and communicate with the brain via the gut–brain axis.

A healthy microbiome is generally diverse and resilient, able to adapt to dietary changes and environmental stressors. When this balance is disrupted (a state known as dysbiosis), people may experience digestive symptoms such as bloating, constipation or diarrhoea, and potentially wider effects on inflammation and immunity.

Diet is one of the most powerful influences on the microbiome, which is why foods that appear to “feed” or support beneficial bacteria - including fibre-rich plants and some fermented products - have become such a focus in public health and wellness research.

What defines a fermented food, and why are they good for us?

Fermentation is a natural process in which microorganisms break down sugars and starches into acids, gases or alcohol. This transformation alters flavour and texture, extends shelf life, and can change a food’s nutritional profile.

However, not all fermented foods are created equal when it comes to gut health. “In bread, for example, the microbes die during baking, so they can’t interact,” says Sinopoulou. “In yoghurt and most cheeses, they’re alive, so it depends on the quantity consumed and which microbes they are.”

She adds that while some vinegar, beer and wine may still contain live bacteria, “the detrimental effects of alcohol outdo any possible positive effects,” and chocolate-based fermented products are often high in fats and sugar.

In practical terms, fermented foods most likely to offer gut benefits are those that:

• Contain live cultures
• Are minimally processed
• Are low in added sugar
• Are eaten in meaningful portions

Examples include plain yoghurt, kefir, unpasteurised sauerkraut, kimchi and some miso varieties.

What does the science say on fermented foods and gut health?

The research into fermented foods is promising - but far from definitive. Studies and clinical trials suggest that some fermented products can improve stool quality and reduce digestive discomfort, particularly in people with mild gut symptoms.

But Sinopoulou urges caution when interpreting these findings. “This varies heavily from food to food, microbe to microbe, and health outcome to health outcome,” she explains. “If we look at specific probiotic microbes such as Lactobacillus and Bifidobacterium, there is some positive evidence - but overall the certainty is very low.”

Part of the challenge is that probiotic studies often use different strains, doses and populations, making comparisons difficult. Results can also be conflicting, and benefits seen in one group may not translate to another.

“There are specific conditions where probiotics might be beneficial, such as IBS,” she adds, “but it depends on the strain, and the evidence is still not super clear yet. This is an area with a lot of research interest and new studies coming out all the time.”

In short: fermented foods may support gut health, but they’re best viewed as one piece of a much larger dietary puzzle.

What are the risks of eating fermented foods?

For most people, fermented foods are considered safe. “Probiotic side effects are rarely reported,” says Sinopoulou, “and people generally do not risk much by trying them and seeing how they work for them.”

That said, introducing them too quickly can cause temporary bloating or gas, particularly in people with sensitive digestion. Some fermented foods are also high in histamines, which may trigger symptoms in susceptible individuals.

Store-bought products can be misleading, too: many are pasteurised (killing live microbes) or loaded with added sugars. And for those with specific medical conditions - such as SIBO or compromised immune systems - professional guidance may be advisable.

3 simple ways to add fermented foods into your diet, according to an expert

1. Start with low-sugar dairy options

Combining fermented foods with fibre helps support beneficial bacteria. “That’s the symbiotic theory - probiotics plus fibre,” she explains. “Fibre can be metabolised by microbes and produce beneficial byproducts.” While evidence is still emerging, adequate fibre intake is independently linked to better gut health.

This growing interest in fibre’s wider impact is reflected in recent industry-backed research, including studies that show how fibre intake may influence gut–brain signalling, sleep and stress. It’s also why retailers such as Holland & Barrett have expanded their high-fibre ranges - responding to rising consumer demand for foods that support both digestive and overall wellbeing.

2. Pair them with fibre-rich foods

“I often recommend yoghurts and yoghurt drinks without added sugars,” says Sinopoulou. “They are fermented with promising microbes and are high in protein and low in fat.”

3. Treat it as a flavour adventure

"One of the good things of our time is that we have easy access to fermented foods from around the world,” says Sinopoulou. “Many are acquired tastes, but once you like them, you really like them.” Exploring foods like kimchi, kefir, miso and fermented vegetables can help expand dietary variety and support healthier long-term habits.

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