Fermentation is one of the oldest ways of preserving food: microbes break down sugars and starches, producing acids, flavors — and, in many cases, live bacteria. Fermented foods have been part of almost every traditional cuisine for thousands of years.
Common fermented foods
- Yogurt and kefir — fermented milk; kefir contains a wider range of microbes
- Sauerkraut and kimchi — fermented cabbage and vegetables
- Miso, tempeh and natto — fermented soy
- Kombucha — fermented tea
- Some cheeses — aged varieties like Gouda and cheddar
Not all fermented foods contain live microbes at the point you eat them. Sourdough bread is baked, killing the culture; many shelf-stable sauerkrauts are pasteurized. Look for “live and active cultures” or refrigerated, unpasteurized products.
What the research shows
In a 2021 Stanford study published in Cell, adults were assigned to a diet high in fermented foods or a diet high in fiber for ten weeks. The fermented-food group increased the diversity of their gut microbiome and showed lower levels of several inflammatory markers. The high-fiber group, surprisingly, didn’t show the same diversity increase over that period.
It was a small study, and diversity is a marker rather than an outcome, so it isn’t proof of long-term health benefits. But it’s one of the clearer pieces of human evidence that what you eat can shift your microbiome measurably within weeks.
How to add them
Start small — a few spoonfuls of sauerkraut, a cup of kefir — especially if you’re not used to them, since they can cause gas at first. In the Stanford study, participants built up to several servings a day.
A few cautions
Kimchi, sauerkraut and miso can be high in salt. Kombucha may contain sugar and small amounts of alcohol. People who are immunocompromised should check with a clinician before eating unpasteurized products.