We’re back again with another edition of Gut Check, our series on the relationship between food and the gut microbiome. So far, we’ve taken a closer look at how whole grains and fiber affect our gut microbes. Today we’re exploring a popular topic: fermented foods.

Just a few years ago, a search for foods like miso, kimchi and kefir would have led us to a specialty store or a crunchy granola food co-op. These days, they’re on shelves in most grocery stores and are commonplace in farmers markets across the country. From the acidic hit of sauerkraut to the cooling tang of yogurt and the fizzy, sweet-and-sour flavor of kombucha, fermented foods offer a wide range of taste and health attributes that have been sought after for thousands of years.

Not yet equipped with microscopes and modern technology, our ancestors may not have known that microbes like bacteria and yeasts were responsible for making these foods and beverages. However, based on what we now know about their microbial content, fermentation has been caught up in a probiotics-fueled health food frenzy. But just because a food is fermented, does that automatically make it a probiotic? Exactly what are the health benefits of fermented foods? Let’s take a closer look.

How are fermented foods made?

Fermentation is typically an anaerobic process (meaning that it happens without oxygen) in which bacteria or yeasts convert sugars in food to other compounds like alcohol or organic acids, while also producing energy for themselves. Usually, the end product is either alcohol or lactic acid. Microbes that convert sugars to alcohol are responsible for beer and wine; those that make lactic acid give foods like yogurt, sour cream, miso, kimchi, sauerkraut and some pickles the tangy flavors we know and love. Different foods and drinks are made by different species of bacteria and yeasts, which give each food distinct flavor characteristics. Traditionally, many foods underwent fermentation naturally, but today the process is usually automated, making the end product more consistent in taste and texture.

How do fermented foods affect gut health?

Some precursors to fermented foods like cabbage and other vegetables are rich in fiber and act as prebiotics. Fiber serves as food for the beneficial bacteria in our gut and results in production of short-chain fatty acids that fuel the cells lining the intestines and maintain gut health. Some fermented foods — especially yogurt and kefir — have demonstrated probiotic capabilities and associations with reduced inflammation and improved gut health. However, outside of the realm of yogurt research, studies are limited on the impact of fermented foods on gut health. Randomized, controlled trials are needed, but these studies are challenging to design because blinding participants to fermented foods versus a control food would be next to impossible given their noticeable flavor profiles.

Fermented ≠ Probiotic

Because many fermented foods are made with the help of bacteria and are associated with healthfulness, it’s tempting to think that they’re one and the same with probiotics, which are defined as live bacteria that confer health benefits when consumed in adequate numbers.

In reality, not all fermented foods contain live organisms. Microbes that are essential for making beer and wine are filtered out in the finished product. Sourdough breads and canned sauerkraut, other fermented vegetables, and many chutneys are heat-treated, which inactivates the microorganisms. So while these foods and drinks might still be flavorful and nutritious, they do not have probiotic activity. Read on for more Debbie Downer probiotics real-talk:

• While many fermented foods may still contain live and active cultures when they make it to store shelves, many of these microbes do not have known health benefits—one of the key requirements of being considered a probiotic.
• I hate to break it to you, but a cup of yogurt once in a while or a few spoonfuls of kimchi with your takeout probably aren’t going to make much of a sustained impact on gut health. Studies suggest that probiotic bacteria have to be eaten regularly to maintain a presence among the microbes that have already staked their claim to valuable real estate in the gut.
• Because there is no recommended daily allowance for probiotics, it’s not clear how much we need to eat to optimize gut health.
• It’s also unclear if probiotics are particularly beneficial for people who are already in good health. However, there is evidence that fermented dairy products may have therapeutic properties in some people with severe Clostridium difficile infection, by repopulating the gut with healthy bacteria and reducing risk of regrowth of harmful microbes.

All of that being said, many fermented foods do contain live, probiotic organisms. Dairy products like kefir and yogurt are among the best vehicles for delivering beneficial bacteria to our gastrointestinal tract, and most cheeses, non-heated kimchi and sauerkraut, kombucha and miso contain large numbers of viable bacteria that can provide a probiotic boost.

While we can’t always rely on a boost of beneficial bacteria from them, there are still tons of reasons to seek out fermented foods and drinks. They offer a wealth of benefits outside of their probiotic possibilities, including being rich in many vitamins, minerals and other nutrients; contributing unique flavors; and giving us a reason to meet up at that new taproom serving beer and kombucha alongside a house-made pickle plate. (Sounds fun—can I invite myself?) There may still be many unanswered questions about fermentation and gut health, but thousands of years of eating and drinking experience indicates that they’re here to stay.

For more info, check out our pre- and probiotics fact sheet and infographic, as well as our microbiome podcast.