On Nutrition

Given the long history of fermented foods — fermentation was a biological method of food preservation long before we had refrigeration or preservatives — it can seem a bit surprising that fermented foods are currently so trendy. Indeed, interest in fermented foods including kefir, kimchi, kombucha and sauerkraut is higher than ever, driven by the growing number of small artisan producers of these foods and beverages, as well as by the health properties fermented foods may offer.

Even though industrialization of the food supply reduced the use of fermented foods in Western nations, they remained part of many traditional cuisines. Today, fermented foods and beverages account for about one-third of the global human diet and, as interest in supporting a healthy gut microbiota (the community of bacteria and other microorganisms that live in our large intestine) increases, so has interest in fermented foods. Some nutrition and health experts even recommend that fermented foods be included in national dietary recommendations.

The mechanics of fermentation

Fermentation is the slow, controlled decomposition of organic substances by microorganisms or enzymes. This process may occur naturally and spontaneously, or through careful addition of a starter culture. The most frequently used microorganisms in fermentation are bacteria and yeast, and there are thousands of food-microorganism combinations, which means there are thousands of different fermented foods and beverages, each with unique flavors and textures.

With some fermented foods, microorganisms are still alive when we consume them. These include fresh kimchi, sauerkraut and sour dill pickles as well as yogurt, kefir (a fermented dairy beverage), kombucha (fermented tea), miso, some cheeses, water- or brine-cured olives, traditional salami and European-style dry fermented sausages. Microbrewed beer that hasn’t been filtered or heated still contains live yeasts and bacteria.

With other foods, those microorganisms are destroyed by further processing, such as baking, pasteurization or filtering. These include tempeh, most soy sauce, most beer and wine, sourdough bread, chocolate and self-stable sauerkraut and kimchi. Fresh cheeses, including cottage cheese, are heated. Aged cheese may not contain many live bacteria, since most die during storage. However, in many of these cases, the microorganisms altered the foods in ways that benefit us, such as making cabbage or wheat more digestible.

Fermented foods and gut health

One common myth about fermented foods is that they are the same thing as “probiotics.” The World Health Organization defines probiotics as live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. Many fermented foods contain millions or trillions of viable microorganisms, and many of those do survive passage through our digestive tracts. While these microorganisms might not take up permanent residence in our gut microbiota, they still have demonstrated health benefits, including benefits for immune function. But is it because they’re probiotic? Maybe yes, maybe no. Even though many fermented foods offer “adequate amounts” of live microorganisms, not all of those foods specifically contain microbes that have been shown to confer a health benefit on us, their hosts.

To be fair, demonstrating that a microorganism actually confers a health benefit requires substantial clinical research using specific and well-defined microorganism strains. However, live microorganisms in fermented foods are rarely characterized or defined, let alone tested for to determine if they offer probiotic health benefits. One reason is logistical — the specific organisms in foods like kimchi, sauerkraut and miso vary by batch, manufacturer and location. The one exception is yogurt. European health authorities have determined that even the generic yogurt bacteria can help improve digestion of lactose among individuals who are lactose intolerant.

A quick look at the science

There’s little downside to consuming fermented foods as part of a healthy diet, and in fact, there may be benefits. But it’s important not to let the hype get ahead of the science. Fortunately, research on functional health benefits of fermented foods is evolving. A 2020 review of results from 19 human intervention studies found that eating fermented foods may change the amounts and types of good bacteria that live in the human gut, and that these changes may be linked to other health benefits. However, the authors noted that there’s not enough data to show whether one particular fermented food can modify the gut microbiota in a specific pattern.

A 2020 study analyzed stool samples from 6,811 individuals enrolled in the American Gut Project, including 115 people who were recruited specifically because they frequently eat fermented foods. The researchers found that higher consumption of fermented foods — especially fermented plants — was associated with subtle differences in the gut microbiome of healthy individuals.

A study from Stanford University published last month randomized 36 people to eat either a diet that included fermented foods or one that included high-fiber foods for 10 weeks. The fermented food diet resulted in a more diverse gut microbiota and a decrease in multiple markers of inflammation. (Systemic inflammation in the body can contribute to a number of health conditions.) While the high-fiber diet resulted in few changes, the authors noted that the study may have been too short to allow fiber to have noticeable effects.

Ready to slip more fermented foods into your meals? Here are a few of my favorite ideas:

  • Add sauerkraut or kimchi to grain bowls or on top of scrambled eggs.
  • Use sauerkraut instead of pickles in tuna or potato salad, on burgers and sandwiches, and in place of cabbage on fish tacos. Making coleslaw? Mix in some sauerkraut, starting with 5 parts cabbage to 1 part kraut.
  • Pair cereal or granola with kefir or plain yogurt instead of milk, and add one of these forms of fermented dairy to smoothies.
  • Enjoy kombucha as a low/no-alcohol cocktail swap — “nonalcoholic” kombucha has less than 0.5% alcohol by volume, while “hard” kombucha has about 3% ABV, which is less than a light beer.

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