Kombucha has become part of the wider boom in gut-friendly foods and drinks. But a major heart-health charity is urging consumers to look more carefully at what these products actually contain.
The British Heart Foundation has warned that some popular gut-friendly products can come with drawbacks. Commercial kombucha may be a healthier alternative to some sugary fizzy drinks, but shop-bought versions can contain added sugar. Other fermented foods, such as kimchi and sauerkraut, can be high in salt.
The warning points to a wider problem. Foods and drinks sold with a health halo are not always straightforwardly healthy. The fact that a product contains potentially active compounds does not prove it will produce a meaningful benefit.
Kombucha is often sold as more than a fizzy drink. Because it is fermented, it is commonly linked with gut health, wellbeing and even stress resilience.
But in our controlled human study, the results were more complicated. Drinking kombucha each day changed some metabolic markers in the body, but did not clearly change how healthy adults responded to acute laboratory stress.
That may sound disappointing to consumers, and to researchers hoping to see a clear effect. But it tells us something important: biological activity does not automatically mean a meaningful health benefit.
The metabolic findings from the study have been published in a peer-reviewed journal. The findings on stress responses are currently under review and should be treated as emerging evidence.
Kombucha is made by fermenting tea and sugar with a mixed culture of bacteria and yeasts, often called a scoby.
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The final drink can contain organic acids, tea polyphenols, microbial metabolites and other compounds produced during fermentation. These features make it scientifically interesting and attractive to food and drink companies.
But kombucha is not a single, standardised product. Drinks vary depending on the tea used, the fermentation process, whether they are pasteurised or filtered, and how they are stored. Findings from one kombucha product should not automatically be applied to every bottle or can on the market.
Testing stress in the lab
Because kombucha products vary so widely, our study used a controlled version of the drink and compared it with a placebo. That allowed us to test the effect of this specific kombucha product, rather than making broad claims about every kombucha on the market.
Healthy adults drank either 330ml of kombucha each day or a flavour-matched placebo drink for eight weeks. The kombucha was a prototype product made using organic green and black teas and a controlled four-week fermentation process. The placebo allowed us to ask whether any changes were due to the kombucha itself, rather than simply taking part in a study or expecting a benefit.
Participants completed a laboratory stress test before and after the eight-week intervention. The task, called the Maastricht acute stress test, combines cold-water hand immersion with timed mental arithmetic under observation.
Participants moved between placing a hand in cold water and completing arithmetic under pressure. This allowed researchers to examine physical and psychological stress responses in the same procedure, rather than relying only on how stressed someone said they felt.
They measured salivary cortisol, a hormone involved in the body’s stress response. They also recorded changes in sweating linked to physical arousal, and heart rate variability: small changes in the time between heartbeats that can offer clues about how the body responds to stress. Participants completed questionnaires about stress, mood and emotional state.
What the study found
The stress task worked. Participants showed measurable physical changes during the cold-water and arithmetic tasks, and during recovery. This gave us confidence that the procedure was producing the expected stress response.
But kombucha did not produce a clear change in stress responses compared with the placebo drink. We did not observe clear differences in cortisol recovery, sweating, heart rate variability or self-reported stress that could confidently be attributed to kombucha.
The largest changes appeared to be driven by adaptation to the stress test itself. By their second visit, participants showed some degree of habituation. This is common in repeated laboratory studies: once people know what is coming, their bodies may respond differently.
The data do not support the idea that eight weeks of kombucha clearly improves acute stress responses or recovery in healthy adults.
There is, however, another part of the story.
In related research using the same participant sample, our team reported that daily kombucha consumption influenced patterns of small molecules detected in urine and blood.
Some of these changes were consistent with the idea that fermented tea products can affect measurable aspects of human metabolism. But biochemical changes alone are not evidence of an improvement in mood, stress resilience or mental health.
Together, the findings suggest that kombucha was doing something biologically measurable. Demonstrating a meaningful wellbeing benefit requires further evidence.
For consumers, the findings suggest caution. Kombucha is often discussed in the context of the gut-brain axis, the two-way communication system linking the digestive system and the brain. But showing changes in metabolism is easier than showing meaningful changes in mood or stress resilience.
For companies, health claims need careful evidence. The presence of bioactive compounds, and the fact that a product is fermented, do not establish that it will produce a measurable benefit in people.
For researchers, the findings show where future studies may need to improve. Larger samples and longer interventions could help. Researchers may also need more detailed analysis of the gut microbiome, the community of microorganisms living in the digestive system, and more precise measurements of heart rate variability using an electrocardiogram, or ECG.
It may also be useful to study people with higher baseline stress. Healthy adults may simply have had too little room for improvement.
Kombucha may be biologically interesting, and this study suggests it can alter aspects of metabolism. But the stress-response data do not support selling it as a simple way to become more resilient.
Fermented foods are worth studying properly. Consumers also deserve clear evidence, especially when the marketing is already several steps ahead of the science.
