Are fermented foods and probiotics good for your gut?
Fermented foods, particularly yoghurt and kefir, show positive indications for gut health; for most other products such as kimchi and kombucha, this has not yet been demonstrated in humans.
In a 17-week randomised trial, fermented foods increased the diversity of gut bacteria and lowered measurable inflammatory markers in the blood. A comparably high-fibre diet did not achieve the latter. These are concrete, positive results in humans, although the study was relatively small (36 participants).
Of all fermented products, kefir has been studied most thoroughly in controlled trials. There is evidence that kefir helps with lactose malabsorption and with the treatment of the stomach bacterium H. pylori. That sounds promising, but the body of evidence is still limited. For kombucha, miso, kimchi and tempeh, no controlled human trials are available. Positive laboratory results do exist, but that is something very different from a proven effect in humans.
Micro-organisms from fermented foods demonstrably reach the gut. Whether they also remain there long-term varies considerably by bacterial strain and by individual. They are present temporarily, but they far from always establish a permanent home.
Fermentation also changes food in other ways: it breaks down substances that can hinder the absorption of nutrients (such as phytic acid and lactose) and produces new bioactive compounds. Whether those compounds actually deliver measurable gut benefits in humans has not yet been demonstrated in large studies for most cases.
Population research links yoghurt and kefir to a lower risk of type 2 diabetes and cardiovascular disease. Those are associations, not proven cause and effect. Findings on a possible influence on mood via the gut-brain axis have so far been mixed and contradictory.
Claims are based on one randomised trial (n=36, Stanford), a number of RCTs for kefir, mechanistic studies, and epidemiological/observational research. No large independent RCTs or meta-analyses are available as a direct source.
Is fermented food good for your health and your microbiome?
Fermented food, and kefir and yoghurt in particular, likely has beneficial effects on gut flora and several health markers, but the evidence in humans is still limited and does not apply equally to every product.
Which fermented products have the greatest effect on gut health?
Yoghurt and kefir have the most human evidence supporting a beneficial effect on gut health; for kimchi, kombucha and tempeh, clinical evidence in humans is still entirely absent.
Do probiotic pills really help your gut?
Probiotics demonstrably work for acute diarrhoea, but for most other gut complaints it depends on which strain you take and what you are taking it for. Always check whether there is evidence for your specific complaint and for the strain listed on the label.
Does fermented food such as yoghurt, kefir and sauerkraut help your gut microbiome?
Fermented foods, and kefir and yoghurt in particular, have the strongest clinical support for a positive effect on the gut microbiome. Feel free to add other fermented products to your diet as well, but be aware that for many of them no good studies in humans yet exist.
Does eating a varied diet help your gut bacteria?
Fermented products have the strongest evidence base for increasing gut bacterial diversity. Eating broadly varied meals also helps, but above all avoid heavily processed products containing emulsifiers such as CMC.
Are plant-based diets better for your gut than mixed diets?
A plant-based diet appears to be beneficial for your gut bacteria, but fermented foods perform at least as well in a direct comparison. Whether healthy people also notice a meaningful long-term benefit has not yet been demonstrated.