Fermented foods vs probiotic supplements: how they compare

Is a glass of kefir doing the same job as a probiotic capsule? It's a fair question. The honest answer is that they sometimes overlap, but they're really built for different jobs.

Here's how fermented foods and probiotic supplements compare on what's inside, how much you get, what the research shows and who each approach suits.

First, the definitions

An international expert panel defines fermented foods as "foods made through desired microbial growth and enzymatic conversions of food components" (Marco et al., 2021). Yogurt, kefir, sauerkraut, kimchi, miso, tempeh, kombucha and sourdough bread all qualify.

A probiotic has a stricter definition: live microorganisms that, given in adequate amounts, provide a health benefit (Hill et al., 2014). The fermented foods panel made a point of separating the two categories (Marco et al., 2021). A fermented food isn't automatically a probiotic, and a probiotic doesn't have to come from a food.

Comparison 1: What's inside

Fermented foods contain whatever microbes did the fermenting, plus the substances created along the way. A review of fermented foods lists several possible ways they could affect health beyond their microbes, including bioactive peptides produced during fermentation, converted plant compounds and a reduction in anti-nutrients (Dimidi et al., 2019). Some fermented foods contain no live microbes by the time you eat them. Sourdough bread, for example, is baked.

Probiotic supplements contain specific strains chosen and named by the manufacturer, ideally with full strain codes so you can look up the research on each one.

Comparison 2: How many live microbes you get

Fermented foods vary widely. A survey of published counts found that many fermented foods contained about 100,000 to 10 million lactic acid bacteria per gram or milliliter. Cultured dairy products consistently had higher levels, up to about a billion. Numbers depended on how products were made and processed, and on how long and under what conditions they were stored (Rezac et al., 2018).

If live microbes are what you're after, a few label clues help:

• For yogurt and kefir, look for wording about live and active cultures.
• For sauerkraut, kimchi and pickles, look in the refrigerated section and check for "raw" or "unpasteurized." Shelf-stable jars are usually heat-treated.
• Be wary of products that were baked, pasteurized or heat-treated after fermentation, since heat kills most microbes.

Probiotic supplements state a count on the label, usually in colony-forming units per serving. That number isn't a guarantee of benefit, but it does give you something concrete to check and compare.

Comparison 3: What the research shows

Fermented foods: The clinical evidence is thinner than their reputation. A 2019 review found randomized controlled trials on digestive effects for only kefir, sauerkraut, natto and sourdough bread, and none for kombucha, miso, kimchi or tempeh. Kefir had the most research, including at least one trial suggesting benefits for people with lactose malabsorption. Overall, the authors described the clinical evidence for most fermented foods as very limited (Dimidi et al., 2019).

One of the more interesting studies looked at whole diets rather than a single food. In a 17-week randomized study, 36 healthy adults were assigned to eat either more fiber or more fermented foods, with 18 people in each group. The fermented-food diet steadily increased the diversity of gut microbes and lowered markers of inflammation. The high-fiber diet didn't change microbial diversity over the study period (Wastyk et al., 2021).

Probiotic supplements: Research is organized by strain, which makes it easier to match a product to a specific goal, such as a symptom a particular strain was tested for. The trade-off is that you have to check that your product contains the same strain, at a similar dose, as the studies.

Comparison 4: Practical pros and cons

Fermented foods

• Pros: They're food, so they bring flavor, nutrients and variety, and they're easy to add to meals you already eat.
• Cons: Microbe counts are unknown and variable, strain-level research is rare, and some are high in salt or sugar. Larger servings of fermented vegetables may cause gas for some people.

Probiotic supplements

• Pros: Named strains, a stated amount and a clearer link to specific studies.
• Cons: Ongoing cost, no nutritional value beyond the microbes, and you're relying on the accuracy of the label.

Which approach makes sense for you?

• If you want a general, food-first habit: Fermented foods are an easy place to start. Begin with small servings. If live microbes are the point, look for refrigerated products that haven't been heat-treated after fermentation.
• If you're targeting a specific symptom: A supplement containing a strain that was studied for that symptom is the more precise tool.
• If you want to try both: Add one change at a time, a couple of weeks apart, so you can tell what's helping and what isn't.

If you're pregnant, immunocompromised or have been told to avoid unpasteurized foods, check with your doctor before adding live fermented foods or supplements.

And if your bloating is new and happening most days, or comes with weight loss, blood in your stool or pelvic pain, get a medical check before experimenting with your diet.

Sources

1. Marco ML, Sanders ME, Gänzle M, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol. 2021;18(3):196-208. PMID 33398112
2. Hill C, Guarner F, Reid G, et al. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506-14. PMID 24912386
3. Dimidi E, Cox SR, Rossi M, Whelan K. Fermented foods: definitions and characteristics, impact on the gut microbiota and effects on gastrointestinal health and disease. Nutrients. 2019;11(8):1806. PMID 31387262
4. Rezac S, Kok CR, Heermann M, Hutkins R. Fermented foods as a dietary source of live organisms. Front Microbiol. 2018;9:1785. PMID 30197628
5. Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153.e14. PMID 34256014

This article is for general information and isn't medical advice. If you're pregnant, immunocompromised or managing a digestive condition, talk to your healthcare provider before adding live fermented foods or supplements.