Fermented foods “have phenomenal benefits in your overall wellness,” gushes Dr. Mercola. “7 must-eat fermented foods for a healthy gut,” promises EatingWell online.
Humans have enjoyed fermented foods—from wine, beer, and vinegar to pickles, olives, yogurt, and cheese—for millennia. Before refrigeration, people used fermenting to preserve foods. But can fermented foods make you healthier? Here’s a look at the evidence.
Fermentation starts when microbes—bacteria or yeast—gobble up carbs in fruits, vegetables, milk, or grains. Over time, the bacteria turn some of the carbs into acids (in foods like sauerkraut), or the yeast turns some of the carbs into alcohol and carbon dioxide (in beer, wine, and liquor). The acids and alcohol help fend off other microbes that would cause the food to spoil.
Does a fermented food still contain live microbes when you eat it? If it has been filtered, heated, or canned, they’ve probably been removed or inactivated.
But foods like refrigerated yogurt, kefir, kimchi, kombucha, and sauerkraut can contain as many as 1 million to 1 billion cells of live microbes in every gram.1 That’s as many bugs as a typical American gets from a whole day’s worth of food.2
Where’s the evidence?
“Yogurt and fermented dairy is where the lion’s share of the knowledge is on the benefits of fermented foods,” says Maria Marco, associate professor of food science and technology at the University of California, Davis. (Marco’s research has been partially funded by the dairy and olive industries, as well as the National Institutes of Health and others.)
For example, yogurt cultures deliver enzymes to your gut, where they break up the milk sugar (lactose) that some people can’t digest.3 And some studies find a lower risk of type 2 diabetes in yogurt eaters.4
But what excites many enthusiasts is the idea that each swallow of yogurt or sauerkraut or kimchi sends an army of probiotics—do-good bacteria—to take up residence in your gut, warding off disease, obesity, and GI problems. Not so fast.
“Those microorganisms will not colonize and become part of your gut microbiota,” says Robert Hutkins, professor of food science at the University of Nebraska. “But if you consume a diet rich in fermented foods with live microbes every day, it becomes the near equivalent of having those microorganisms living there.”
What good does that do? For most microbes, the jury is still out.1 Here are two examples.
“ReVitalize. From the Inside,” says KeVita’s website. KeVita (now owned by Pepsi) makes Master Brew Kombucha. Kombucha is sweet tea plus yeast that ferments some of the tea’s sugar into alcohol and bacteria that ferment the alcohol into acid.
“Vitality begins in your gut,” says KeVita’s website.
So should skepticism. No published studies have tested the benefits of kombucha in people. And only one small industry-run trial in India has tested one of the two microbes that KeVita adds to Master Brew Kombucha after fermentation.
(KeVita won’t say exactly how much Bacillus coagulans MTCC 5856 it puts in.)
People suffering from diarrhea due to irritable bowel syndrome who were treated with drugs and 2 billion live B. coagulans MTCC 5856 cells reported fewer symptoms after three months than those who got drugs and a placebo.5
That’s promising but nowhere near enough evidence.
“There are lots of suggestions for benefits from kombucha, but little evidence to support those claims,” Hutkins notes. “That’s partly because there is no kombucha or sauerkraut lobby to fund studies.”
Kimchi is spicy fermented napa cabbage. Many Koreans eat it daily. But the evidence for kimchi is skimpy. Some media reports, for example, say that it lowers blood sugar. Yet in one good study, levels were no lower when 21 Koreans with prediabetes ate fermented kimchi than when they ate unfermented kimchi.6
And too much kimchi may have a downside: East Asians who eat the most pickled vegetables like kimchi and other salt-preserved foods have a higher risk of stomach cancer.7 (There’s not enough evidence to know whether that’s true in other countries.)
The bottom line
“We need clinical studies to measure what kind of additional benefits beyond the basic nutrition these fermented foods provide,” says Marco.
A sip of kombucha while you wait?
1 Curr. Opin. Biotechnol. 44: 94, 2017.
2 PeerJ 2: e659, 2014.
3 Am. J. Clin. Nutr. 99: 1251S, 2014.
4 BMC Med. 12: 215, 2014.
5 Nutr. J. 15: 21, 2016.
6 Ann. Nutr. Metab. 63: 111, 2013.