Gut health has gone mainstream, and two words show up everywhere: probiotics and prebiotics. They sound almost identical, and they are often sold together, but they are very different things. Understanding the difference makes it much easier to decide what you actually need.
Here is a clear guide to what each one is, how they work together, where to find them in food, and what the research shows.
Your gut microbiome in brief
Your large intestine is home to trillions of microbes, mostly bacteria, along with yeasts and other organisms. Together they are called the gut microbiome. These microbes help break down fibers your body cannot digest on its own, produce compounds such as short-chain fatty acids, and interact with your immune system and the lining of your gut. Researchers are also studying how the microbiome communicates with the brain, a field we explore in our article on the gut-brain connection and focus.
A diverse, well-fed microbiome is generally considered a sign of good gut health. Both probiotics and prebiotics aim to support it, but in different ways.
What probiotics are
Probiotics are live microorganisms that, when consumed in adequate amounts, provide a health benefit to the person taking them. That is the formal definition agreed upon by an international panel of scientists (Hill et al., 2014). In practice, most probiotic supplements contain strains of Lactobacillus (some now reclassified under new genus names) and Bifidobacterium, along with some other species.
An important detail is that benefits are strain specific. Different strains have been studied for different purposes, and a result found with one strain does not automatically apply to another. Probiotics also tend not to settle permanently in the gut; they generally have their effects while you take them.
What prebiotics are
Prebiotics are the food for beneficial microbes. The formal definition describes a prebiotic as a substrate that is selectively used by host microorganisms to confer a health benefit (Gibson et al., 2017). In plain terms, prebiotics are mostly specific types of fiber that your own body cannot digest but that certain gut bacteria can.
Common prebiotics include inulin and fructooligosaccharides (FOS), found in chicory root, onions, garlic, leeks, asparagus and bananas; galactooligosaccharides (GOS), found in legumes and dairy; and resistant starch, found in cooked and cooled potatoes and rice, green bananas and oats. When gut bacteria ferment these fibers, they produce short-chain fatty acids such as butyrate, which help nourish the cells lining the colon.
The simple way to remember it
Think of your gut like a garden. Probiotics are seeds or new plants you add. Prebiotics are the fertilizer that helps the beneficial plants already there, and the new ones, thrive. You can have a healthy garden without adding new plants, but you cannot keep one going without feeding it.
When they are combined: synbiotics
Products that combine probiotics and prebiotics are sometimes called synbiotics. A 2020 international consensus statement defined synbiotics as mixtures of live microorganisms and substrates selectively used by host microorganisms that confer a health benefit (Swanson et al., 2020). The idea is that the prebiotic fiber may help support the probiotic microbes and the beneficial bacteria already living in your gut.
Most Americans fall short on fiber
Here is a big reason prebiotics matter. Most adults in the US eat far less fiber than recommended. Guidelines suggest about 14 grams of fiber per 1,000 calories, roughly 25 grams a day for women and 38 grams for men, yet average intake is around 15 grams (Quagliani & Felt-Gunderson, 2016). That gap means many people are not feeding their gut microbes as well as they could.
What a food-based study showed
A notable 2021 study from Stanford randomly assigned healthy adults to eat either a high-fermented-food diet or a high-fiber diet for 10 weeks (Wastyk et al., 2021). The fermented food group, eating foods such as yogurt, kefir, kimchi and kombucha, showed increased microbiome diversity and decreases in several markers of inflammation. The high-fiber group did not see the same rise in diversity over that time, though responses varied depending on each person's starting microbiome.
The takeaway is not that fiber does not matter, since fiber has many well-established benefits. It is that both fermented foods, a source of live microbes, and fiber, a source of prebiotics, each play distinct roles.
Food sources of each
For probiotics and live microbes, look to yogurt and kefir with live and active cultures, sauerkraut and kimchi that are refrigerated and unpasteurized, miso, tempeh and kombucha. For prebiotics, focus on onions, garlic, leeks, asparagus, bananas, oats, barley, beans, lentils, chickpeas, chicory root, and cooked and cooled potatoes or rice.
A practical goal is to include at least one fermented food and several fiber-rich plant foods every day.
Do you need a supplement?
Not everyone does. If you regularly eat fermented foods and plenty of fiber, you may already be supporting your gut well. A supplement can be a convenient option if you rarely eat fermented foods, if your fiber intake is low, or if you want a consistent daily dose of studied strains. A combined probiotic and prebiotic product covers both sides in one step.
How to take them
Follow the label for timing; many probiotics can be taken with or without food, while others are best taken with a meal. Consistency matters more than timing. If you are new to prebiotic fibers, increase them gradually, since a sudden jump can cause gas and bloating while your microbes adjust. Drink plenty of water as you add fiber, which our water intake calculator can help with.
Give any new gut routine at least four weeks before judging how it feels. Our guide to the best probiotics for every need covers strain selection in more detail.
Safety notes
Probiotics and prebiotics are generally safe for healthy people. Mild gas or bloating is common at first, especially with prebiotic fibers. People who are seriously ill, have a weakened immune system, or have had recent surgery should talk with their healthcare provider before taking probiotics. If you have ongoing digestive symptoms, see your healthcare provider.
The evidence at a glance
Probiotics are live microbes that provide a benefit in adequate amounts (Hill et al., 2014), prebiotics are substrates selectively used by beneficial microbes (Gibson et al., 2017), and synbiotics combine the two (Swanson et al., 2020). Most Americans fall well short on fiber (Quagliani & Felt-Gunderson, 2016), and a fermented food diet increased microbiome diversity in a Stanford trial (Wastyk et al., 2021).
The bottom line
Probiotics add beneficial microbes; prebiotics feed them. Most people benefit from more of both, starting with fermented foods and fiber-rich plants. For a simple daily step that covers both, our Probiotic 40 Billion with Prebiotics combines four probiotic strains with prebiotic fiber (fructooligosaccharides) in one daily serving.
References
- Hill et al. (2014). The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology. doi.org/10.1038/nrgastro.2014.66
- Gibson et al. (2017). Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology. doi.org/10.1038/nrgastro.2017.75
- Swanson et al. (2020). The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics. Nature Reviews Gastroenterology & Hepatology. doi.org/10.1038/s41575-020-0344-2
- Quagliani et al. (2016). Closing America’s Fiber Intake Gap. American Journal of Lifestyle Medicine. doi.org/10.1177/1559827615588079
- Wastyk et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell. doi.org/10.1016/j.cell.2021.06.019
Health & Cognition is not affiliated with the researchers, institutions or publishers mentioned in this article.
