The global probiotic supplement market is worth billions, and many people spend considerable sums hoping to improve their gut health. But researchers have raised an awkward question: are these supplements actually doing anything for healthy people?

A systematic review from the University of Copenhagen examined seven studies on probiotics and found something that might disappoint supplement manufacturers. Among participants with gut-related illnesses, some did benefit from probiotics. But for healthy people with normally functioning digestive systems? The researchers found “no convincing evidence” of consistent effects on faecal microbiota composition 1.

This doesn’t mean probiotics are worthless. It does mean that if you’re healthy and hoping to “optimise” your gut bacteria with expensive supplements, you might be wasting your money. There’s another option though: fermented foods. These have been part of human diets for thousands of years, long before anyone worried about their microbiome.

What Makes Fermented Foods Different?

Fermentation is genuinely ancient technology. Humans figured out how to preserve food with bacteria and yeast long before we understood what bacteria even were. The process transforms food chemically, often making nutrients more bioavailable while producing beneficial microorganisms along the way.

What’s interesting about fermented foods is that they come with a complete package: not just live bacteria, but also the metabolites those bacteria produce during fermentation, plus the food matrix itself. Whether this matters for health outcomes compared to isolated probiotic strains is still being studied, but there’s a certain logic to getting your bacteria the way humans have for millennia rather than from a capsule.

Here are five fermented foods worth knowing about. I’ve tried to be honest about what the evidence actually shows rather than just listing claimed benefits.

1. Yogurt

Yogurt forms when specific bacteria (typically Lactobacillus bulgaricus and Streptococcus thermophilus) ferment milk at controlled temperatures. The bacteria convert lactose to lactic acid, which thickens the milk and creates that characteristic tangy flavour.

Beyond probiotics, yogurt provides calcium, magnesium, zinc, and B vitamins. It’s a reasonable protein source. For older adults concerned about bone health or anyone looking for nutrient-dense dairy, yogurt has genuine nutritional value independent of any probiotic benefits 2.

The catch: Not all yogurt contains live cultures by the time you eat it. Some manufacturers heat-treat yogurt after fermentation (which kills the bacteria) to extend shelf life. If you’re eating yogurt specifically for live bacteria, look for labels mentioning “live and active cultures.” Even then, bacterial counts decline with time. A two-week-old yogurt won’t contain the same amount of live bacteria as a fresh one.

There’s also the sugar issue. Many commercial yogurts contain substantial added sugar. If you’re consuming yogurt for health benefits but it’s essentially a dessert, you might be undermining the point. Plain yogurt with your own added fruit is generally the better choice.

2. Kimchi

Kimchi is Korea’s answer to vegetable preservation. At its core, it’s salted and fermented vegetables, most commonly napa cabbage, mixed with seasonings like chilli powder, garlic, ginger, and fish sauce or shrimp paste. The fermentation happens naturally via lactobacillus bacteria.

The research on kimchi is actually fairly interesting. Studies have found associations between regular kimchi consumption and improved markers of metabolic health, including cholesterol levels and blood glucose regulation 3. Whether the benefits come from the live bacteria, the compounds in the vegetables, the spices (particularly capsaicin from the chilli), or some combination isn’t entirely clear.

One study of overweight adults found that eating fermented kimchi led to greater improvements in metabolic parameters compared to fresh (unfermented) kimchi 4. This suggests the fermentation process adds something beyond what you’d get from just eating the vegetables.

My honest take: Kimchi is nutrient-dense, low in calories, and contains fibre. Even if the probiotic angle turns out to be less important than claimed, you’re still eating vegetables. That’s not nothing.

3. Natto

Natto is one of those foods that reveals cultural differences in what people consider edible. This Japanese fermented soybean dish has a stringy, sticky texture and a pungent smell that many Westerners find challenging. But it’s been a dietary staple in parts of Japan for over a thousand years.

The fermentation uses Bacillus subtilis var. natto, which transforms cooked soybeans over about 24 hours. The result is dramatically different from the original soybeans: higher in certain vitamins, particularly vitamin K2, and containing unique enzymes like nattokinase.

Vitamin K2 is where natto genuinely stands out. It contains the highest concentration of vitamin K2 (specifically MK-7) of any commonly eaten food—roughly 100 times more than cheese 5. K2 has been studied for its potential role in bone health and cardiovascular health, though the evidence is still developing 6.

Nattokinase, the enzyme in natto, has fibrinolytic (clot-dissolving) properties that have attracted research interest. Some studies suggest potential cardiovascular benefits, though taking nattokinase supplements isn’t the same as eating natto, and the clinical evidence for cardiovascular disease prevention remains limited 7.

Worth noting: If you’re on blood thinners like warfarin, natto’s high vitamin K content makes it something you’d need to discuss with your doctor. The vitamin K can interfere with how these medications work.

4. Kefir

Kefir originated in the Caucasus mountains and has spread worldwide. It’s made by fermenting milk with kefir “grains”—not actual grains, but gelatinous clumps of bacteria and yeast that look a bit like tiny cauliflower florets.

What distinguishes kefir from yogurt is the complexity of its microbial community. While yogurt typically contains two or three bacterial strains, kefir grains host around 30 different species of bacteria and yeast 8. Whether this diversity translates to better health outcomes is an open question, but it’s an interesting difference.

Research on kefir has shown various potential benefits: antibacterial properties, improved lactose digestion in people with lactose intolerance, and possible effects on bone health and cholesterol. A systematic review found some evidence supporting kefir’s benefits for glycaemic control and cholesterol management, though the authors noted that more rigorous trials are needed 9.

My take: Kefir is drinkable, which makes it easier to consume than some fermented foods. If you tolerate dairy and want something with probiotic potential, it’s a reasonable choice. Water kefir and coconut milk kefir exist for those avoiding dairy, though they’re fermented with different organisms.

5. Miso

Miso is a Japanese fermented paste made from soybeans, salt, and koji (a mould called Aspergillus oryzae). Sometimes rice or barley is added. The fermentation can take anywhere from weeks to years, depending on the variety.

Most people encounter miso in soup, but it’s also used as a marinade, spread, or flavouring in various dishes. The fermentation process breaks down proteins into amino acids (including glutamate, which is why miso adds that savoury, umami quality to food) and produces various bioactive compounds.

The research on miso is mixed. Epidemiological studies from Japan have found associations between miso soup consumption and various health markers, including lower blood pressure in some populations 10. However, miso is quite high in sodium, which typically raises blood pressure, so the relationship is complicated.

One intriguing area involves miso and radiation protection. After the Nagasaki bombing, a doctor reported that hospital staff who regularly consumed miso soup seemed to experience fewer radiation effects. Animal studies have since provided some support for this idea 11, though the mechanisms and applicability to humans remain unclear.

Reality check: Most miso available in Western supermarkets has been pasteurised, which kills the live organisms. If you want miso with active cultures, you’ll need to look for unpasteurised varieties, usually at health food shops or Asian markets. And cooking miso soup kills the bacteria anyway—adding miso after the soup cools slightly preserves more of the live organisms.

Do Fermented Foods Actually Work Better Than Supplements?

Here’s the honest answer: we don’t know for certain. The question of whether fermented foods provide meaningful probiotic benefits comparable to or better than supplements hasn’t been definitively answered.

What we do know is that fermented foods have been part of healthy traditional diets for thousands of years. They provide nutrients beyond just bacteria. They’re generally affordable. And they don’t require you to trust a supplement manufacturer’s claims about how many live organisms are actually in each capsule (a common issue, as quality varies significantly among probiotic supplements).

The Copenhagen researchers who questioned probiotic benefits in healthy people weren’t suggesting people shouldn’t eat fermented foods. They were pointing out that healthy people might not need to spend money on supplements specifically marketed for gut health.

Practical Considerations

If you’re interested in adding fermented foods to your diet, here are some realistic points to consider:

Start gradually. Introducing lots of fermented foods at once can cause digestive discomfort, particularly gas and bloating. Your gut needs time to adjust.

Look for live cultures. Not all fermented products contain live bacteria. Pasteurisation, heating during cooking, and extended storage all reduce or eliminate live microorganisms. “Contains live cultures” or similar labels indicate the product should have active bacteria at the time of packaging.

Watch the sodium. Fermented foods like kimchi, miso, and some pickles are quite high in salt. If you’re managing blood pressure or watching sodium intake, be mindful of portion sizes.

Don’t expect miracles. Fermented foods are food. They’re part of a healthy diet, not a treatment for serious health conditions. If you have significant digestive issues, see a doctor rather than trying to fix them with yogurt.

References

Kristensen NB, et al. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Med. 2016;8(1):52.
Rizzoli R. Dairy products, yogurts, and bone health. Am J Clin Nutr. 2014;99(5 Suppl):1256S-62S.
Kim HJ, et al. Fermented kimchi reduces body weight and improves metabolic parameters in overweight and obese patients. Nutr Res. 2011;31(6):436-43.
An SY, et al. Beneficial effects of fresh and fermented kimchi in prediabetic individuals. Ann Nutr Metab. 2013;63(1-2):111-9.
Kaneki M, et al. Japanese fermented soybean food as the major determinant of the large geographic difference in circulating levels of vitamin K2. Nutrition. 2001;17(4):315-21.
Akbari S, Rasouli-Ghahroudi AA. Vitamin K and Bone Metabolism: A Review of the Latest Evidence in Preclinical Studies. Biomed Res Int. 2018;2018:4629383.
Chen H, et al. Nattokinase: A Promising Alternative in Prevention and Treatment of Cardiovascular Diseases. Biomark Insights. 2018;13:1177271918785130.
Bourrie BC, et al. The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir. Front Microbiol. 2016;7:647.
Rosa DD, et al. Milk kefir: nutritional, microbiological and health benefits. Nutr Res Rev. 2017;30(1):82-96.
Watanabe H. Beneficial biological effects of miso with reference to radiation injury, cancer and hypertension. J Toxicol Pathol. 2013;26(2):91-103.
Yoshino K, et al. Radioprotective effect of miso (Japanese fermented soybean paste) on intestinal crypt cells irradiated with gamma-rays in rats. Nippon Rinsho. 1996;54(7):1829-33.