A probiotic is a live microorganism that, when consumed in large enough amounts, provides a measurable health benefit. That’s the formal definition established by the World Health Organization and the Food and Agriculture Organization of the United Nations. In practical terms, probiotics are the “good” bacteria and yeasts found in fermented foods like yogurt and kimchi, or sold as dietary supplements in capsule and powder form.
How Probiotics Work in Your Gut
Your digestive tract is home to trillions of microorganisms collectively called the gut microbiome. Probiotics support this ecosystem in a few key ways. They compete with harmful bacteria for space and nutrients along the intestinal lining, physically blocking dangerous microbes from gaining a foothold. They also help break down fibers and other compounds your body can’t process on its own, producing useful byproducts like vitamins, amino acids, and short-chain fatty acids in the process.
One of those short-chain fatty acids, called butyrate, is especially valuable. It fuels the cells lining your colon, helps reduce inflammation, and plays a role in metabolic health. These byproducts of bacterial activity are sometimes called “postbiotics,” and they’re a big part of why probiotics matter. The bacteria themselves do important work, but the compounds they produce as they digest fiber extend those benefits throughout the body.
Common Types of Probiotics
Most probiotic products rely on bacteria from two main groups: Lactobacillus and Bifidobacterium. Within those groups, specific strains like L. acidophilus, L. rhamnosus, and Bifidobacterium longum are among the most widely studied. A third type, Saccharomyces boulardii, is actually a yeast rather than a bacterium. Each strain can have different effects, which is why two probiotic supplements with different strains aren’t interchangeable, even if they contain the same total number of organisms.
Probiotic potency is measured in colony-forming units, or CFUs, which represent the number of living organisms in a dose. Most supplements contain 1 to 10 billion CFU per dose, though some products go as high as 50 billion or more. A higher CFU count doesn’t automatically mean a product is better. What matters is whether that specific strain, at that specific dose, has evidence behind it for the benefit you’re looking for.
Foods That Contain Probiotics
Fermented foods are the oldest and most accessible source of probiotics. Yogurt is the most familiar, typically containing both Lactobacillus and Bifidobacterium species. Kefir, a tangy fermented milk drink, is another dairy-based option with a diverse microbial profile.
Beyond dairy, the list is long: kimchi, sauerkraut, tempeh, miso, natto, and naturally fermented pickles all contain live cultures. Even some cheeses (cheddar, gouda, mozzarella, cottage cheese) and sourdough bread carry probiotic organisms. One important detail with foods like sauerkraut and pickles: look for unpasteurized versions, because the pasteurization process kills the live bacteria that make them probiotic in the first place.
Prebiotics: Fuel for Probiotics
Prebiotics are the fibrous foods that feed the beneficial bacteria already living in your gut. Think of probiotics as planting new seeds and prebiotics as fertilizing the soil. Foods high in prebiotic fiber include garlic, onions, bananas, asparagus, and whole grains. When gut bacteria break down this fiber, they produce the short-chain fatty acids and other postbiotic compounds that support your health. Getting both probiotics and prebiotics in your diet gives the good bacteria the best chance of thriving.
Health Benefits With Strong Evidence
Probiotics have the most convincing research behind them for digestive conditions. A 2017 review of 17 studies found that taking probiotics alongside antibiotics cut the risk of antibiotic-associated diarrhea roughly in half. A separate analysis of over 8,600 patients found moderate certainty that probiotics reduce the risk of C. difficile infection, a serious bacterial complication that can follow antibiotic use.
For irritable bowel syndrome, a large review of 53 studies covering more than 5,500 people concluded that probiotics may improve overall symptoms and abdominal pain, though pinpointing the best strains remains difficult. People with constipation also see modest benefits, particularly with Bifidobacterium strains. And for ulcerative colitis, a review of 21 studies suggested that adding probiotics to standard treatment could help maintain remission.
The benefits extend to specific populations as well. In breastfed infants with colic, the strain L. reuteri was associated with cutting daily crying time by more than half across multiple studies. For premature, very-low-birth-weight infants, a review of over 7,300 babies found that probiotics helped prevent necrotizing enterocolitis, a dangerous intestinal condition. And a review of more than 9,400 premature infants found probiotics reduced the risk of sepsis.
Traveler’s diarrhea is another area where probiotics show promise, with a review of 11 studies and over 5,000 participants finding evidence they may help with prevention.
Side Effects and Safety Concerns
For most healthy adults, probiotics cause nothing more than temporary gas, bloating, or mild digestive discomfort in the first few days of use. These effects typically fade as your gut adjusts.
The real safety concerns apply to people with weakened immune systems. In individuals with certain conditions, including cancer, diabetes, organ transplants, or a compromised intestinal lining, some probiotic strains can act as opportunistic pathogens. In rare cases, this has led to serious infections including pneumonia, endocarditis, and sepsis. Newborns with health complications also fall into this higher-risk category.
How Probiotics Are Regulated
In the United States, most probiotics are sold as dietary supplements, not drugs. This distinction matters. Dietary supplements do not need FDA approval before they hit store shelves. Manufacturers are responsible for ensuring their products are safe and that their claims aren’t misleading, but they don’t have to submit evidence of safety or effectiveness to the FDA before or after marketing. If a probiotic were marketed as a treatment for a specific disease, it would need to go through the same rigorous approval process as any pharmaceutical drug. In practice, almost none do, which is why supplement labels use vague language like “supports digestive health” rather than claiming to treat a condition.
This regulatory gap means quality varies widely between products. Some supplements have been found to contain fewer live organisms than their labels claim, or different strains entirely. Choosing products from manufacturers that use third-party testing can help, as can looking for specific strain names (not just genus names) on the label, since the research supporting probiotics is strain-specific.