Probiotics are live microorganisms that benefit your health when you consume enough of them. Prebiotics are types of fiber that feed the beneficial bacteria already living in your gut. Together, they support a thriving community of trillions of microbes in your digestive tract, influencing everything from immune function to how well you absorb nutrients.
How Probiotics Work in Your Gut
Your intestines house a vast ecosystem of bacteria, fungi, and other microbes. Probiotics tip the balance of that ecosystem in your favor through several mechanisms. They compete with harmful bacteria for space along your intestinal lining, essentially crowding out organisms that could make you sick. They also produce short-chain fatty acids and other compounds that directly inhibit the growth of pathogens.
Beyond fighting off harmful microbes, probiotics strengthen your gut’s physical barrier. They stimulate the production of the mucus layer that lines your intestines and help tighten the junctions between intestinal cells, making it harder for toxins and unwanted bacteria to slip into your bloodstream. Probiotics also boost production of a key immune antibody called secretory IgA, which acts as a first line of defense in your gut.
How Prebiotics Fuel Beneficial Bacteria
Prebiotics are not bacteria themselves. They’re specific types of dietary fiber and carbohydrates that your body can’t digest but your gut bacteria can. When beneficial microbes ferment these fibers, they produce three main short-chain fatty acids: acetate, propionate, and butyrate. Each one plays a distinct role in your health.
Butyrate is the star of the three. It’s the primary fuel source for the cells lining your colon, and it helps reduce intestinal inflammation by calming down key inflammatory pathways. All three fatty acids also activate receptors that influence metabolic hormones involved in appetite regulation and blood sugar control. Animal studies have shown that supplementation with these fatty acids can protect against weight gain, improve insulin sensitivity, and reduce inflammation associated with obesity.
Health Benefits Beyond Digestion
Probiotics have shown the most promise for digestive conditions. They can help treat or prevent diarrhea, including the kind triggered by antibiotics. In children, one well-studied strain reduced antibiotic-associated diarrhea risk by 71% at adequate doses. Probiotics have also been studied for irritable bowel syndrome, ulcerative colitis, and Crohn’s disease, though results vary depending on the strain used.
Prebiotics deliver benefits that extend well past the gut. Higher prebiotic intake is linked to improved calcium absorption from food, better blood sugar control, stronger immune function, and a reduced risk of colorectal cancer. One intriguing study found that people who ate 30 grams daily of foods rich in the prebiotic fiber inulin for two weeks became more likely to choose lower-calorie foods. Brain scans showed less activation in their reward centers when they looked at high-calorie foods, suggesting that prebiotic-rich diets may subtly shift food preferences.
Where to Find Them in Food
Probiotic foods are fermented. Yogurt, kefir, sauerkraut, kimchi, miso, tempeh, and kombucha all contain live cultures, though the specific strains and amounts vary widely by brand and preparation. Heat-treated versions (like pasteurized sauerkraut) no longer contain live organisms, so check labels if the probiotics are the point.
Prebiotics are naturally present in many plant-based foods. Good sources include bananas, almonds, garlic, onions, leeks, asparagus, whole grain wheat products (bread, pasta, cereals), and corn (including popcorn). Beans and other legumes are also rich in prebiotic fibers. For children, nutrition guidelines recommend at least 5 grams per day of prebiotic fiber specifically, with a total fiber goal of the child’s age plus 5 to 10 grams. Adults benefit from simply eating a wide variety of these plant foods regularly.
If you check ingredient lists on packaged foods, prebiotic fibers often appear as inulin, wheat dextrin, acacia gum, psyllium, or abbreviated terms like FOS (fructooligosaccharides) and GOS (galactooligosaccharides).
Choosing a Probiotic Supplement
Probiotic supplements are measured in colony-forming units, or CFUs, which indicate the number of live organisms per dose. Most products contain 1 to 10 billion CFU, though some pack in 50 billion or more. A higher CFU count does not necessarily mean a more effective product. What matters more is the specific strain and whether it’s been studied for the condition you’re targeting.
This is a key distinction: probiotic benefits are strain-specific. A strain studied for antibiotic-associated diarrhea won’t necessarily help with bloating or constipation. Supplements are not tested by the FDA the way medications are, and the strains in a given product may not match the condition you’re hoping to address. Look for products that name the specific strain (not just the genus and species) and ideally reference clinical evidence for that strain.
Storage matters too. Some strains are heat-sensitive and need refrigeration to stay alive. Others, particularly those in the Bacillus genus, are more resilient and remain stable at room temperature. Many shelf-stable products are freeze-dried and sealed in moisture-resistant packaging. If the label doesn’t mention refrigeration, the product is likely shelf-stable. If you buy online and it arrives with an ice pack, store it in the fridge.
Synbiotics and Postbiotics
You may see these newer terms on supplement labels. Synbiotics combine probiotics and prebiotics in a single product. They come in two forms: complementary synbiotics, where a probiotic and a prebiotic each work independently toward their own benefits, and synergistic synbiotics, where the prebiotic is specifically chosen to feed the accompanying probiotic strain, and the two are designed to work as a team.
Postbiotics are different entirely. They contain inactivated (non-living) microorganisms or their components and byproducts. There are no viable cells in the final product. The idea is that some of the health benefits from bacteria come not from the living organisms themselves but from the substances they produce or the structural components of their cells. Postbiotics don’t require the same careful storage as live probiotics, which makes them more practical for some uses.
Safety and Side Effects
For most people, probiotics and prebiotics are safe. The most common side effects are temporary gas and bloating, especially when you first increase your intake of prebiotic fiber or start a new probiotic. Starting with smaller amounts and increasing gradually helps your gut adjust.
There is one important exception. People with severely weakened immune systems, those who are critically ill, or those with central venous catheters face real risks from certain probiotic organisms. Cases of bloodstream infection from the yeast-based probiotic Saccharomyces boulardii have been documented in critically ill patients. Notably, nearly half of the patients who developed these infections were taking the probiotic directly, and another 8% were simply near patients receiving it, meaning the organism posed an environmental risk on hospital wards. For anyone with a compromised immune system, the decision to use probiotics should involve a healthcare provider who knows the specific clinical situation.