According to some experts, the answer isn't as clear-cut as you might think. "Science has shown that fermented foods do contain various strains of probiotics," says Natasha Bhuyan, MD, a doctor at OneMedical. Yet many of the benefits of fermented food for gut health are extrapolated based on existing knowledge of probiotics, and not confirmed based on a large body of dedicated research to fermented foods themselves.
"We know that probiotics are helpful for gut health, which is why many assume that fermented foods would be beneficial also. However, there isn't enough research to actually make this claim," says Dr. Bhuyan.
That's not to say that there's no research linking fermented foods with gut health. As BZ Nutrition owner Brigitte Zeitlin, RD, points out, some studies do suggest that probiotic-rich foods like yogurt may help improve intestinal health, treat diarrhea and respiratory infections, and strengthen the body's immune and anti-inflammatory responses. There's also some research that suggests fermented foods can be helpful for immunity. "This is mostly due to the fact that [around] 70 percent of our immune system is located within our digestive tract," says Zeitlin. One recent study found that human cells have a receptor that interacts with metabolites found in fermented foods, triggering the immune system into action.
Dr. Bhuyan adds that yet another study has singled out kimchi, in particular, as being beneficial for constipation. But it's hard to say why that is, exactly. "This could be due to both its probiotic properties and its fiber content," Dr. Bhuyan says, both of which are beneficial for digestive health.
"We don’t understand exactly how eating fermented foods affects the gut microbiome." —Abigail Johnson, PhD, RDN
The same question marks apply for all probiotic foods, says Abigail Johnson, PhD, RDN, a post-doctoral associate at University of Minnesota. It's unclear whether it's the probiotics in fermented foods, or something else entirely—fiber, for instance—that causes people to experience positive health outcomes when eating them. "We don’t understand exactly how eating fermented foods affects the gut microbiome," says Dr. Johnson. "Some data from the American Gut Project has shown that eating a large number of different, non-fermented vegetables is associated with microbial diversity in the gut. However, there is little research exploring the impact of fermented vegetables versus fermented dairy or other non-fermented foods on gut health." Scientists are studying this topic now, so Dr. Johnson says we can expect to get a few answers within the next year or two.
There are a few other widely-made claims around fermented foods that experts view with skepticism. "The most common unproven 'fact' about fermented foods is that they contribute to [weight management] due to the probiotics decreasing belly fat. There is very little research to back this up," Zeitlin says. Those who eat yogurt, for example, have been found to have better metabolic parameters than those who don't—however, Dr. Johnson points out that yogurt consumption is also typically linked with higher dietary quality as a whole. That means it's hard to say how big of a role its probiotic content plays in the grand scheme of a person's diet.
The theory that probiotics—and fermented foods, by extension—can decrease anxiety and improve mood also isn't totally proven by existing science. "More research needs to be done," says Zeitlin. "Serotonin, a feel-good hormone, is made both in the gut and in the brain, so the connection has been suggested that a healthier gut will help trigger a healthier mind and mood. But this is still very unclear." Existing research shows an association between regulated gut micobiota and improved anxiety symptoms, for example, but that doesn't show a cause-and-effect relationship. Some sources also claim that fermented foods are good for heart health, although Zeitlin says that their potential cholesterol-lowering properties likely have less to do with probiotics and more to do with their fiber content.
It's also important to consider that not all fermented foods are created equal. "Fermented foods are not just one type of food, but rather, a process that a diverse collection of foods may undergo," says Dr. Bhuyan. "People often perceive fermented foods as 'all-natural' and don't realize they can be highly processed with various additives, including sugars—which are, coincidentally, [detrimental] for gut health." This is the case for many store-bought fermented foods, she adds, such as kombucha. "Plain Greek yogurt is going to be a healthier choice than a flavored Greek yogurt because the flavored one will have more sugar in it, even though the probiotic amount will be the same for each," adds Zeitlin. "The more sugar and salt an item has, the less beneficial it is for your health, so you want to read those nutrition labels."
Kimchi fans, take heart—if you love the taste of fermented foods and they're helping you add diversity to your diet, Dr. Johnson says there's no reason not to keep eating them. Just don't think you need to incorporate them solely for their potential health benefits. "We might eventually find out that fermented foods have benefits over their unfermented counterparts, but at this stage we mostly have data supporting the inclusion of diverse fruits and vegetables as beneficial," she says. "If you don’t enjoy fermented foods, it’s probably just as beneficial to add non-fermented fruits and vegetables to your diet."
For those who are firmly in the "pro" camp, Zeitlin recommends looking for foods that are labeled as "naturally fermented," rather than ones that were fermented using vinegar (which does not always produce the good gut probiotics you want). She suggests adding them to one or two meals a day and switching up which ferments you eat, as each one will have its own set of good bacterial strains. "You can start your day with some berries with kefir or plain Greek yogurt, add olives to your lunchtime salad, have cheese with some carrots or whole grain crackers for an afternoon snack, and top off your dinnertime stir-fry with some kimchi or miso," she says.
But if you've tried fermented foods and their tangy taste just isn't your thing, Dr. Bhuyan doesn't want you to stress. "Honestly, they are not necessary [for health] if someone prefers to avoid them," she says. "We can still get the nutrition we need from other sources. For gut health, I generally just advise my patients to try to follow a healthy diet that is mostly plant-based and has limited to no processed foods." And if that happens to include healthy fermented foods, then by all means, go for it.
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