How Much Fermented Food Do You Actually Need?

If you type "how much fermented food" into Google, it finishes the question for you: "should you eat a day," "is too much," "for gut health." The number people usually land on is six servings a day, and it goes back to one study. In 2021, a team at Stanford published a trial in the journal Cell where a diet full of yogurt, kefir, kimchi, and kombucha made people's gut bacteria more diverse and lowered signs of inflammation in their blood. Kombucha brands still cite it on their websites.

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The paper does report both of those results. It also comes from 18 adults in their fifties on average, eating far more fermented food than most people ever will, and the trials that have tried something similar since (one of them in college students) mostly haven't gotten the same result.

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What did the study test?

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Researchers randomly split 36 healthy adults into two groups for 10 weeks. One group added fiber (beans, whole grains, nuts, vegetables, and fruit). The other added fermented foods: yogurt, kefir, fermented cottage cheese, kombucha, kimchi and other fermented vegetables, and vegetable brine drinks (the salty, sour liquid that pickled vegetables sit in, sold in small bottles as a drink).

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The fermented group went from about 0.4 servings a day to an average of 6.3. The study counted a serving as 6 ounces of yogurt, kefir, or kombucha, a quarter cup of kimchi or sauerkraut, or 2 ounces of brine. Six of those is a yogurt at breakfast, a kombucha at lunch, kimchi with dinner, and three more on top of that, every day for weeks. Before the study, the researchers had turned away anyone already eating more than two servings a day, so everyone started low.

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The fiber group's gut bacteria didn't get more diverse, which is covered in Is Fiber-Maxxing Actually Good for You?.

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What changed in the fermented-food group?

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Microbiome diversity (how many different kinds of bacteria live in your gut) went up slowly over the 10 weeks. It kept rising after the strict diet ended, during a four-week stretch when people could eat as much or as little fermented food as they wanted, and it peaked there.

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In people's blood, 19 of the 93 inflammation-related proteins the team measured went down. One of them, IL-6, is a protein that runs high in conditions like type 2 diabetes and rheumatoid arthritis. Justin Sonnenburg, one of the lead scientists, called the results "a stunning finding" in Stanford's press release.

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Blood sugar, cholesterol, blood pressure, and waist size didn't change in either group, and neither did people's own ratings of stress, fatigue, or well-being. The fermented group did report more bloating while they were ramping up their servings, and by the end of the full-intake weeks the difference was gone.

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Where did the new bacteria come from?

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If you've wondered whether stomach acid kills the bacteria in your kimchi before they get anywhere, the study has part of an answer. The researchers compared the new bacteria that turned up in people's guts with the bacteria in the foods themselves, and found that the new ones "were largely not from the fermented foods themselves." Whatever the diet did, it mostly changed which bacteria thrived in the gut (or let new ones in from somewhere else) rather than moving the yogurt's bacteria into your colon.

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Health-Ade, a kombucha company, says on its blog that the study showed "that eating fermented foods like yogurt, kefir, and kombucha led to a significant INCREASE in overall microbial diversity." Everyone in the fermented group ate a mix of foods, so the study can't say what kombucha does on its own, and its own data point away from the bacteria in the bottle doing the work.

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How strong is the evidence?

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With 18 people per group, the authors write, the study "limits statistical power and presents challenges with regard to generalizability." There was no control group eating their normal diet (both groups changed what they ate), and nobody was blinded, because you can't hide from someone that they're eating kimchi.

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Before it started, the team registered one main thing to measure, an immune test called the cytokine response score. That score didn't change in either group. The diversity and inflammation results were secondary findings, which studies report all the time, but a trial is built and sized to test its main question, and this one's answer was no.

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ISAPP (the International Scientific Association for Probiotics and Prebiotics, a group of scientists who research this area) describes the study as a trial "in a small cohort of healthy people." Its 2021 consensus paper on fermented foods says that "with the exception of yoghurt and other cultured dairy products, few well-designed, randomized controlled trials" on their health effects have been published.

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Has anyone gotten the same result since?

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Mostly not. The trials that came after tested one fermented food at a time, usually in smaller amounts, and found little change.

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A University of North Florida team had 87 adults at higher risk of heart disease eat 100 grams of fermented vegetables at least five days a week for eight weeks. Their inflammation markers and gut diversity didn't change, and the authors wrote that "higher doses of FVs," meaning fermented vegetables, may be needed.

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At UC San Diego, 16 adults drank a 16-ounce bottle of kombucha every day for four weeks while 8 others didn't. Their median age was about 25, the closest to a student of any trial here. Inflammation markers didn't move, and one measure of diversity went slightly down. GT's Living Foods, which makes kombucha, paid for the study.

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A trial in South Korea had 28 college students ages 19 to 25 drink 150 milliliters (a little over half a cup) of kefir, milk, or yogurt every day for two weeks. Kefir raised a few specific kinds of bacteria, but the researchers found "no statistically significant differences in alpha diversity" (the variety of species) in any group.

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In Germany, 87 healthy adults ate 100 grams of sauerkraut a day for four weeks, both fresh and pasteurized versions. Blood pressure dropped slightly with both, and the authors concluded that four weeks "was not linked to an appreciable systemic health benefit in healthy individuals."

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None of these copied the Stanford diet. They used less food, shorter timelines, or a single food, so they don't prove Stanford wrong. They do show that the six-servings result hasn't been reproduced, and that a daily kombucha or a forkful of kimchi is a different thing from what Stanford tested.

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So how much should you eat?

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Nobody has an official number. The US Dietary Guidelines say that fermented foods like "sauerkraut, kimchi, kefir, miso" support "a diverse microbiome, which may be beneficial for health." On the same page they give daily servings for vegetables, fruit, dairy, and whole grains, and none for fermented food. Stanford's own nutrition research group, the one that ran the trial, suggests starting "with one per day" and building slowly "to have at least 2 servings per day and more as tolerated."

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If you want live bacteria, check how the food was made. Bread, chocolate, and beer are fermented and then "baked, roasted, and/or filtered," according to ISAPP, so nothing is left alive in them, and its consensus paper puts pasteurized fermented vegetables in the same category. Yogurt that's been heat-treated after it's made has to say "does not contain live and active cultures" on the label, under FDA rules. For kimchi and sauerkraut, that means the refrigerated jars rather than the shelf-stable ones (more on those in The 10 Best Foods for Your Gut Health). If you're weighing a probiotic supplement instead, Do Probiotics Actually Work? covers what the gut doctors' guidelines say.

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Fermented foods are ordinary, cheap food, and fermented dairy like yogurt and kefir has the most trials behind it. If you add a lot at once, you might be bloated for the first few weeks, the way the Stanford group was while they ramped up.

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