What’s the Difference Between Prebiotics and Probiotics?

Our food choices depend on so many things — culture, preferences, momentary cravings — but perhaps we should consider what our microbial partners like to eat. 

Gut microbes help us metabolize food, protect against pathogens, and bolster our immune system. Although the science linking gut microbes to health and disease still has a way to go, researchers are learning that beneficial gut bacteria have nutritional preferences of their own. Appealing to our gut microbes by including foods they prefer in our diets can expand communities of beneficial gut microbes. 

We hear so much about probiotics, but not as much about prebiotics, so don’t be surprised if you’re confused. Dietary scientists have kept the definition broad because researchers are still adding to the list prebiotic foods, but basically prebiotics are substances selectively used by certain groups of microbes that benefit the host. 

Here’s an easy way to tell the difference: Probiotics are live bacteria, like what you find in yogurt. Prebiotics are dead material, most commonly dietary fibers, which enrich organisms already in your gut. Gut bacteria can ferment some, but not all, fibers. Fermentable fibers are considered prebiotics only if microbes produce byproducts beneficial to health.  

“Dietary fibers are more complicated than you may think, because there are so many subtle differences in their chemical makeup,” says Julie Stefanski, registered dietitian, nutritionist, and spokesperson for the Academy of Nutrition & Dietetics. 

Adding certain fibers to your diet, like those found in a variety of fruits and vegetables, is a worthwhile strategy, because our own good health depends on a vibrant community of beneficial gut microbes. We need their help because the human genome doesn’t come equipped with the genetic utensils to break down indigestible fibers. Gut microbes do the job for us. 

Researchers are studying prebiotics as a possible aid to many health conditions, including allergies, osteoporosis, osteoarthritis, skin care, and also how they could bolster the health of lungs and the reproductive tract. Prebiotics are even being studied as an intervention for COVID-19 with the idea that a healthy immune system — bolstered by diet and nutrition — can help your body fight off viral infections. If some of the research pans out, prebiotics could fuel a new generation of food products. 

Most prebiotics are low-digestible, complex carbohydrates. Some examples of prebiotics are whole grains, bananas, onions, legumes like chickpeas and lentils, and greens. If you choose fermented foods like sauerkraut and kimchi, you’ll get the double benefit of both a prebiotic and a probiotic, says Stefanski, because these foods contain both live bacteria and fiber.  

Gut microbes break down chains of fiber and produce short-chain fatty acids, which the gut uses as fuel, and which reduce inflammation and strengthen the immune system. While there are no dietary guidelines for the amount of prebiotics one should consume, Stefanski notes that we would need at least 3 grams orally, per day with a goal of reaching 5 grams.  

Prebiotics are very healthy foods, but if you don’t have the right mix of bacteria to digest them, you can have a lot of gastric distress. “You may not feel good on these foods, especially if you have gastric issues, such as irritable bowel syndrome,” says Stefanski. You can get help with a nutrition expert like Stefanski, who can check your symptoms and figure out which foods are causing the problems. If you’re having problems, she often recommends starting out with a high-quality probiotic and slowly adding in prebiotics.  

Prebiotic supplements may offer convenience, but you’ll lose any benefit from eating whole food. “When you consume these foods, you also get vitamins, minerals, phytochemicals, which give other benefits, and may work together in synergy,” says Stefanski. 

Also, watch out for ingredients in fiber bars because they often contain inulin, a commonly studied prebiotic. Like other fibers, inulin can help keep blood sugar steady and make you feel full, but at high levels the side effects may be unpleasant. “I work with a lot of teen athletes and heard funny stories about using these products right before a game or a match, it’s not a good idea,” says Stefanski. She also cautions people on the popular low-carbohydrate, ketogenic diets — and fiber is in the category of carbohydrates — to add vegetables to get fermentable fibers. 

Jeffrey I. Gordon, a microbiologist at Washington University School of Medicine in St. Louis, is working to deliberately fashion food to recruit communities of microbes that can efficiently process that specific food. 

“Prebiotics are not as simple as adding asparagus to the diet,” says Robert Chen, a graduate student in Gordon’s lab. “When you think about something like food, we are very cautious when we use this kind of language. We think there has to be a really deep commitment to understanding the components of food that affect functions of the microbiota that in turn affects humans.” 

Here's the idea: If foods were designed in ways to specifically recruit communities of organisms that may improve their nutritional value, says Gordon. “Microbes are master chemists able to sense what kind of molecules are in their environment and different microbes have different capacities to sense, acquire and metabolize nutrients” he says. 

In a recent series of papers in Cell, Elife, and Cell Host & Microbe the team studied how prebiotic fiber is selectively used by gut microbes by designing a biosensor — a series of artificial food particles attached to microscopic glass beads. A color label helps track the beads. The team sent the nutrient-decorated beads into the intestinal tracts of specialized mice with specific human gut communities.  

The biosensors can detect how well a microbial community extracts and uses nutrients, so they can also assess the microbiota’s state of health. They can help identify food formulations that might be particularly nutritious.  

With information like this, researchers can test what kinds of therapeutic foods might be most optimal for different populations around the world, and how effective they are, says Gordon. His team has been working with the International Center for Diarrheal Disease in Bangladesh to help design optimal food for undernourished children, on what they call microbiota-directed complementary foods. The goal is to create formulations of foods with similar biological activities appealing to microbial communities, and that would also be culturally acceptable to people in different parts of the world.  

“Whether that’s called a prebiotic, or whether that’s something with more detail and resolution,  there’s still a lot to understand about the interaction between what we call food, specifically the chemicals in food, and how that impacts us at a molecular level and a microbial level,” says Chen.