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The increasingly trendy trio of kefir, kimchi and kombucha may not be familiar to you, but experts say fermented foods like these can help the home of most of your immune system – your gut.

How and why some (not all) fermented foods work is an unraveling mystery that goes back to hunter-gatherer humans. Today, nutrition scientists say to look beyond "probiotic" and "prebiotic" labels to select the right fermented foods for you.

Don't fall for the "best superfoods" lists that rank fermented foods highly, warned the International Scientific Association for Probiotics and Prebiotics (ISAPP) in January in a consensus statement published in Nature Reviews Gastroenterology & Hepatology. The science is still mixed on the specific nutritional benefits, and the organization calls for more randomized controlled trials to bear out some of the promising effects researchers have seen in labs.

These tips from experts can help sort what's hype and what's the real thing. First, a primer.

What is a -biotic anyway?

Your gut is home to trillions of microorganisms. While an antibiotic medicine stunts or destroys microorganisms, a prebiotic is non-digestible fiber that feeds good bacteria.

Probiotics-containing fermented foods like yogurt, sauerkraut, kefir, kimchi and kombucha contain live microorganisms known to restore balance to the digestive system. The microbes and their genetic material are known as the gut microbiome, which is a control center for the immune system. When you eat too much processed food, drink excessive alcohol and don't exercise, the microbiome deteriorates.

Dysbiosis, a gut microbial imbalance, can have a role in cardiovascular disease and cardiometabolic disorders like Type 2 diabetes. So having a healthy gut microbiome helps the rest of you stay healthy.

"There are strains of bacteria in your gut that are good for you and others that are bad," said Christopher Gardner, a nutrition scientist at Stanford University in California. "I can't tell you what to eat today for the good, bad or missing bacteria that is in your body. But stay tuned, because we are starting to unlock some of the mysteries."

Be careful in the kitchen

Cooking fermented food – think beer, pickles, sourdough – kills good bacteria. "You want to drink live microbes, not destroy them by cooking them first," Gardner said.

Any health risks for consuming over-fermented good bacteria? Gardner said no – but it might not taste very good. Every seven to nine days for the last two years, he has created his own kombucha from a SCOBY (symbiotic culture of bacteria and yeast). One batch was left alone for four weeks.

"We tasted it, and it was awful," Gardner said. "So, I didn't drink more than a sip."

Researcher Gail Cresci, from Cleveland Clinic Children's Department of Pediatric Gastroenterology, Hepatology and Nutrition, advised closely following recipes for fermented foods. Her husband didn't, and erupted a volcano of beer in their kitchen.

"Fermentation has been around for centuries as a way to preserve food, but you want to be careful," she said. "If you are growing mold and the food is spoiling, you're not doing it right."

Real food over tablets

Supplements labeled as "prebiotic" or "probiotic" don't deliver the same benefits as fermented foods, Cresci said.

"There are challenges to keeping microbes viable in encapsulated tablets," she said. "It's also very, very important to know that each strain of bacteria is not the same as the next. For example, lactobacillus has hundreds of different strains, and each one may behave differently. People like to use supplements because they like to think one size fits all, but it doesn't."

Her advice: "Take in prebiotics and probiotics through food sources. Yogurt with added probiotic bacterial strains is much better to consume than supplements also because as it's been waiting for you to eat it, it's been producing more beneficial metabolites. When you eat it, you get all that."

And for those who have yet to hop on the fermented food wagon, just what is kefir, kimchi and kombucha? Kefir, a fermented dairy beverage, is like a drinkable yogurt. Kimchi, a staple in Korean cuisine, typically is salted and fermented vegetables, such as cabbage and radish. Kombucha is a lightly fizzy drink made with green or black tea, sugar, bacteria and yeast.

In search of the optimal strain

Nutritional labels on fermented foods are not required to name the strain of bacteria present, Cresci said. "Yogurt contains lactobacillus bulgaricus and streptococcus thermophilus, but the store brand is just going to say, 'live active cultures.'"

Specific bacteria may become part of a personalized prescription for gut health. In 2020, the National Institutes of Health unveiled a 10-year research agenda for "precise, targeted approaches and interventions guiding us to a much clearer and precise answer to the question, 'What should we eat?'"

For now, Cresci offered this strategy: "Eat 25 to 30 grams of mixed fiber – a variety of fresh fruits, vegetables and whole grains – every day, and you'll keep your microbiome pretty happy. Once you've cleaned up your diet, add a probiotic-containing food like yogurt or kefir, and just be careful of any added sugar or fat. Fermented sauerkraut and pickles can be good also but can have a lot of sodium."

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