Yogurt, kimchi and natto are fermented foods. Are they good for your stomach? Scientists at Stanford University studied 36 healthy adults. One group ate more fermented foods, and the other group ate more fiber.

After some weeks, the fermented food group had more kinds of gut bacteria. Some signs of inflammation in their blood went down. But this was a small study, so we cannot say these foods prevent illness.

Fermented foods such as miso, natto and yogurt are often said to be good for the gut, but how strong is the evidence? In 2021, researchers at Stanford University randomly divided 36 healthy adults into two groups for 17 weeks. One group gradually increased fermented foods, eating about six servings a day at the peak, while the other ate more fiber, such as beans and whole grains.

In the fermented food group, the variety of gut bacteria slowly increased, and many markers of inflammation in the blood fell. However, the study was small, and it measured markers rather than actual illness. It is too early to say that fermented foods prevent disease.

Fermented foods are a staple of the Japanese diet, and many people assume they benefit the gut. A 2021 randomized trial at Stanford University put this idea to the test. Thirty-six healthy adults were assigned to one of two diets for 17 weeks: one group steadily increased fermented foods such as yogurt, kefir, kimchi and kombucha, reaching about six servings a day, while the other increased fiber from beans, whole grains and vegetables.

In the fermented food group, the diversity of gut bacteria rose gradually and many inflammatory markers in the blood declined. The fiber group showed no major change in diversity, and its immune responses varied depending on each person's original gut bacteria. These findings attracted attention because chronic inflammation is linked to many lifestyle diseases. There are caveats, though.

Each group had only 18 people, the study measured biological markers rather than disease outcomes, and the primary measure chosen in advance showed no clear difference. In addition, many fermented foods, such as bread and soy sauce, contain few live microbes after processing. An international expert panel concluded in 2021 that rigorous human trials are still lacking.