Are fermented foods and probiotics good for your gut?

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.