Short answer: they aren’t interchangeable, and neither is simply “better”. Probiotic supplements have trial evidence only for specific strains in specific situations – most clearly, preventing diarrhoea caused by antibiotics – and gastroenterologists don’t recommend them for most digestive conditions. Fermented foods are everyday foods with a promising but still small body of evidence for gut health. If you have a medical reason to consider a probiotic, the strain matters more than the brand, and it’s worth asking your doctor.
What Each One Is
A probiotic, in the definition used by the International Scientific Association for Probiotics and Prebiotics (ISAPP), is live microorganisms that, when given in adequate amounts, confer a health benefit; the term applies to defined strains with evidence behind them (Hill et al., 2014). A fermented food is one made through desired microbial growth and enzymatic conversion of its ingredients (Marco et al., 2021). Many fermented foods – yogurt, kefir, raw sauerkraut, kimchi – contain live microbes when eaten, but others are baked, cooked, pasteurised or filtered (sourdough bread, most beer and wine, shelf-stable pickles), and even live ones usually haven’t been tested for a specific health effect. So a fermented food isn’t automatically a probiotic, and a probiotic capsule isn’t a substitute for food.
The Evidence for Fermented Foods
The best-known trial is from Stanford. Healthy adults were randomly assigned to a high-fiber or a high-fermented-food diet in a 17-week study with 18 people per group. The fermented-food diet steadily increased the diversity of their gut microbiota and lowered markers of inflammation, while the high-fiber diet didn’t change diversity. The study’s main pre-planned measure (an immune “cytokine response score”) didn’t change in either group (Wastyk et al., 2021). It’s an encouraging result from a small study, not proof of health benefits, and much of the remaining evidence for fermented foods is observational. See how much fermented food per day for the amounts people in such studies ate. For the expert definitions of probiotics, prebiotics and synbiotics, see probiotics vs prebiotics.
The Evidence for Probiotic Supplements
Probiotic research is strain- and condition-specific, and the results vary a lot:
- Antibiotic-associated diarrhoea in children. A Cochrane review of 33 trials (6,352 children) found diarrhoea in 8% of children given probiotics with their antibiotics, against 19% of controls – about one case prevented for every nine children treated. Higher doses (at least 5 billion CFU a day) worked better (Guo et al., 2019).
- C. difficile diarrhoea. In a Cochrane review of 31 trials (8,672 people) taking antibiotics, C. difficile-associated diarrhoea occurred in 1.5% of people on probiotics and 4.0% of controls. The benefit was seen in trials where the baseline risk was above 5%, not in lower-risk groups (Goldenberg et al., 2017; the review was updated in 2025).
- Guidelines. The American Gastroenterological Association’s 2020 guideline conditionally suggests certain specific strains or strain combinations (for example Saccharomyces boulardii) to prevent C. difficile infection in people on antibiotics, on low-quality evidence. For C. difficile treatment, Crohn’s disease, ulcerative colitis and irritable bowel syndrome, it recommends probiotics only within clinical trials (Su et al., 2020).
Two points follow. First, a benefit shown for one strain doesn’t carry over to a different strain of the same species, let alone to a yogurt. Second, for most everyday gut complaints, the clinical evidence for any probiotic product is weak.
Safety
For healthy people, probiotics in trials have mostly caused only mild effects such as gas and bloating. The exceptions matter. In 2023 the US Food and Drug Administration warned that probiotic products given to hospitalised preterm infants had been linked to invasive disease, including an infant’s death (FDA, 2023), and Cochrane reviewers note serious infections reported in severely ill or immunocompromised children, for example those with central venous catheters (Guo et al., 2019). If you’re immunocompromised, critically ill, or considering probiotics for a premature baby, talk to the medical team first; the same caution applies to unpasteurised home ferments.
If You’re Choosing a Probiotic
- Match the strain to the evidence. Look for the full name – genus, species and strain code (for example Lacticaseibacillus rhamnosus GG) – and check that this strain was tested for your purpose.
- Match the dose. Use the amount used in the trials, with the count guaranteed until the end of shelf life, not just “at manufacture”.
- Follow storage instructions, since live microbes die with heat and time (see what kills probiotics).
- Ask a doctor or pharmacist if you’re taking it for a medical reason, including alongside antibiotics (our guide to gut recovery after antibiotics covers food).
And eat fermented foods because you enjoy them: a spoon of kimchi or a glass of kefir is food, not medicine, and that’s fine.
