Fermented Foods and Your Gut Microbiome: What a Stanford Diet Study Found

By Dr. Ryan Kneessi, ND · Move Forward Natural Health, Odenton, MD · Last reviewed: October 23, 2026

Fermented foods have been part of traditional diets for thousands of years, and they have become popular again as interest in the gut microbiome has grown. For a long time, though, most of the enthusiasm rested on laboratory and animal research, with surprisingly few human trials. That started to change with a carefully designed diet study from Stanford University published in the journal Cell in 2021. Because it is one of the more encouraging nutrition studies of the past several years, and because it is free to read in full, I want to walk through what the researchers actually did and what they found.

How the Stanford fermented food study was designed

The researchers, led by Hannah Wastyk and Justin Sonnenburg, randomly assigned healthy adults to one of two diets for a 17-week study. One group was asked to gradually increase their intake of fermented foods, and the other was asked to increase plant fiber from fruits, vegetables, legumes, whole grains, nuts, and seeds. The full analysis included 36 participants with an average age of 52, and everyone started out eating relatively little of either kind of food. In fact, people who already ate more than 20 grams of fiber a day or more than two servings of fermented foods were not eligible to join.

After a few weeks of baseline measurements, participants spent four weeks ramping up their assigned foods and six more weeks maintaining the higher intake. In the fermented food group, average intake climbed from less than half a serving a day to a little over six servings a day. The foods they chose included yogurt, kefir, fermented cottage cheese, fermented vegetables such as kimchi and sauerkraut, vegetable brine drinks, and kombucha, and each person was free to pick the ones they enjoyed. Throughout the study, the team collected stool and blood samples and ran detailed analyses of gut bacteria and immune activity. You can read the free full text of the study on PubMed Central.

What happened to gut diversity and inflammation markers

The most striking finding was in the fermented food group. The diversity of their gut bacteria increased steadily over the course of the diet. Eating more servings was linked with a larger increase, although time spent on the diet was an even stronger factor. Using a panel of 93 cytokines, chemokines, and other inflammation-related proteins, the researchers found that 19 of them declined during the fermented food phase. Among those was interleukin-6, often called IL-6, a signaling protein that the authors describe as a key mediator of chronic inflammation and a commonly used measure of it. They also saw lower activity in signaling pathways across four major types of circulating immune cells.

One detail I find especially interesting is where the new gut bacteria came from. The team sequenced the commonly eaten fermented foods and compared them with the microbes that newly appeared in participants’ stool. Only a small share of the new bacteria matched the ones in the foods, with a peak overlap of about 5 percent early in the study. In other words, eating more fermented foods seemed to change the wider gut community, not just add the bacteria found in the jar.

It is worth being honest about what the study did not show. The main outcome the researchers chose in advance, a measure of how immune cells respond to stimulation, did not change significantly in either group. The authors also list clear limitations: the study was small, there was no separate control group that kept eating normally, the intervention lasted only ten weeks, and everyone was healthy at the start. So this trial tells us about changes in the microbiome and in blood markers over a short period, not about long-term health outcomes.

What about the high-fiber group?

The fiber group roughly doubled their intake, from about 21 grams a day to about 45 grams a day. Their gut bacteria became better equipped to break down complex plant carbohydrates, but overall diversity stayed stable during the ten weeks, and their immune responses varied from person to person. The researchers found that these different responses tracked with how diverse each person’s microbiome was at the start. I do not read this as a reason to skip fiber, which has a long and well-supported track record. It suggests instead that a gut ecosystem may take longer than ten weeks to adapt to a big jump in fiber, and that the two approaches may work in different ways. The authors themselves called for future research on combining fiber and fermented foods. I cover practical fiber strategies in my guide on how to take a fiber supplement.

A larger look at people who eat fermented foods

A second study helps put the Stanford trial in context. In 2020, Bryan Taylor and colleagues published an analysis in the journal mSystems of stool samples from 6,811 people in the American Gut Project, including 115 people recruited specifically for a four-week follow-up based on how often they ate fermented foods. They found subtle but statistically significant differences in gut bacteria between people who regularly ate fermented foods and people who did not. The stool of fermented food eaters was also richer in conjugated linoleic acid, a fatty acid the authors described as possibly health-promoting, and their analysis suggested that bacteria associated with fermented food consumption may help produce it. Because this was an observational study, it shows an association rather than cause and effect, and the authors called the signal modest. The free full text is available on PubMed Central.

Not all fermented foods are the same

Fermented foods are a broad category, and they differ a great deal in which microbes they contain and how much research supports them. A 2019 review in Nutrients by Eirini Dimidi and colleagues looked at common fermented foods, including kefir, kombucha, sauerkraut, tempeh, natto, miso, kimchi, and sourdough bread. The authors explained that fermented foods may influence health in several ways, including through their live microbes, through compounds created during fermentation, and through changes to the food itself. Kefir was the most studied, with at least one randomized trial suggesting benefits for people with lactose malabsorption. At the same time, the review concluded that clinical evidence for most individual fermented foods is still limited and that more high-quality human trials are needed. You can read the full review on PubMed Central.

It also helps to know that many shelf-stable products labeled as fermented, such as most canned sauerkraut, pasteurized pickles, and many breads, have been heated or processed in ways that remove live microbes. Some fermented vegetables are also high in salt, so portion size matters for anyone watching their sodium.

How I think about fermented foods in a functional medicine plan

In functional medicine, I look at the gut as one of the places where diet, immune activity, and whole-body inflammation meet. The Stanford results fit well with that view, because they connected a simple food change to measurable shifts in both gut bacteria and blood markers of inflammation. My article on microbiome diversity explains how diversity is measured and why it matters, and my guide to the gut barrier and microbiome covers how the lining of the gut and its bacteria work together. If you are curious about the bigger picture of low-grade inflammation and aging, you may also like my articles on inflammaging and the hs-CRP test, which is one of the inflammation markers I check most often.

Fermented foods are not right for everyone in the same amounts. People with histamine sensitivity sometimes notice symptoms after aged or fermented foods, people with significant bloating may do better starting slowly, and anyone who is immunocompromised, pregnant, or managing a digestive condition should talk with their clinician before making large changes. Starting with a small serving and building up gradually, much as the Stanford participants did over four weeks, is usually the most comfortable approach.

Practitioner-grade options in my Fullscript catalog: if you and your clinician decide a supplement makes sense, my supplement guide by health goal links to the practitioner-grade products I trust.

Frequently asked questions

Sources

• Wastyk et al. Gut-Microbiota-Targeted Diets Modulate Human Immune Status. Cell. 2021. Free full text on PubMed Central.
• Taylor et al. Consumption of Fermented Foods Is Associated with Systematic Differences in the Gut Microbiome and Metabolome. mSystems. 2020. Free full text on PubMed Central.
• Dimidi et al. Fermented Foods: Definitions and Characteristics, Impact on the Gut Microbiota and Effects on Gastrointestinal Health and Disease. Nutrients. 2019. Free full text on PubMed Central.

This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before making changes to your diet, exercise, supplements, or medications, especially if you are pregnant, nursing, or managing a health condition.