Our bodies naturally contain many types of microorganisms, mainly bacteria and yeast, that live in the gastrointestinal tract. In recent years, pro- and prebiotic products have grown in popularity and are widely available – you may even have a few in your own cabinet! But what exactly are they, and how do they affect the body? Let’s dive into some of the research and debunk some potential myths around their use.

What is a Probiotic?

A probiotic is a live microorganism that, when consumed in adequate amounts, provides a health benefit to the host. Most probiotics are bacteria, though some are yeast. Importantly, research must show that a specific strain produces measurable health benefits in people to qualify it as a probiotic.

While probiotics are being researched for many conditions, they are most often studied in relation to digestive health. Probiotics are available in many forms, including:

Capsules and powders
Milk and soy based drinks or yogurts
Nutrition bars
Some fruit and vegetable juices

Some require refrigeration, while others are shelf-stable. You can identify probiotics in food products by looking for the specific strain on the ingredient list. For example, some fermented foods have probiotics added to enhance their health benefits.

What is a Fermented Food?

Fermented foods are not the same as probiotics, though there can be overlap.

Fermented foods are made by creating conditions for microorganisms naturally present on the food to grow. During the process, these microbes change food components into new substances, which alter flavor and texture. For instance, sauerkraut is made when naturally occurring microorganisms on cabbage, combined with salt and water, convert cabbage sugars into acid and gas, giving it a tart flavor.

Examples of fermented foods include yogurt, miso, tempeh, and kombucha. Some fermented products, like kefir, may also have probiotics added, making them both fermented and probiotic foods.

What is a Prebiotic?

A prebiotic is a substance that feeds the beneficial microorganisms already living in your gut, helping them thrive and function effectively. In other words, prebiotics are “food” for your gut microbes.

When paired together, prebiotics and probiotics are sometimes sold as synbiotics, meaning they are combined for convenience.

Where Do We Find Prebiotics?

Prebiotics occur naturally in many high-fiber foods, such as fruits, vegetables, beans, and grains.

Inulin and pectin: found in asparagus, artichokes, carrots, blackberries, and dates
Galacto-oligosaccharides: found in beans, soybeans, and wheat bran
Resistant starch: found in cooked and cooled potatoes, rice, and pasta

While prebiotics can also be taken as supplements, it’s best to speak with a doctor or dietitian first. Dosage and effectiveness may vary depending on your health history and individual diet. A dietitian can also help you include natural food sources of prebiotics in your meals.

Busting Probiotic Myths

More is not always better. Probiotic doses are measured in colony-forming units (CFU). Most research supports doses between 1–10 billion CFU. Higher numbers don’t necessarily mean better results—what matters is whether the specific dose has been studied in humans.
Not all probiotics have the same effect. Each strain provides different benefits. Choose a probiotic strain that has been researched for your specific health concern, and discuss your options with your doctor or dietitian.
Probiotics don’t permanently change your microbiome. They may provide benefits while you’re taking them, but research shows limited long-term changes in gut diversity once supplementation stops.
Probiotics affect more than just digestion. Emerging research suggests potential benefits for brain health, liver function, the oral cavity, skin, vaginal health, and urinary tract health—but more studies are needed in these areas.

Probiotics and Prebiotics for GI Symptoms

If you deal with symptoms like diarrhea or constipation, you may be considering probiotics or prebiotics. Some evidence shows they can help, depending on the condition, but research is still developing and does not clearly point to which strains are best.

In many cases, dietary changes guided by a dietitian are a good first step. If you try probiotics, it may help to trial them for about 4 weeks, track your symptoms, and discontinue use if you don’t notice improvements. Always check in with your physician or dietitian before starting supplements.

Works Cited:

Sanders M. E. (2016). Probiotics and microbiota composition. BMC medicine, 14(1), 82. https://doi.org/10.1186/s12916-016-0629-z

Schneider, E., O'Riordan, K. J., Clarke, G., & Cryan, J. F. (2024). Feeding gut microbes to nourish the brain: unravelling the diet-microbiota-gut-brain axis. Nature metabolism, 6(8), 1454–1478. https://doi.org/10.1038/s42255-024-01108-6

International Scientific Association for Probiotics and Prebiotics. (2025, March 18). Probiotics: Dispelling Myths. ISAPP Science. https://isappscience.org/wp-content/uploads/2019/04/Dispelling-Probiotic-Myths.pdf

International Scientific Association for Probiotics and Prebiotics. (2025, March 18). Probiotics. ISAPP Science. https://isappscience.org/for-scientists/resources/probiotics/

Probiotics and Prebiotics