What Are Prebiotics?

Health benefits, side effects, and what to look for

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Prebiotics are non-digestible food ingredients that are often added to "functional foods." These ingredients are believed to promote the growth of helpful gut bacteria—thereby increasing gastrointestinal health and potentially providing other health benefits.

Science is increasingly recognizing the importance of prebiotics for gut health, but the jury is still out on whether or not functional foods with prebiotics are necessary for optimal health.

Health Benefits

Prebiotics are components of foods that are not able to be digested and through their interaction with gut bacteria are thought to be health-promoting. Prebiotics are usually ingredients in functional foods, or certain conventional or modified foods that provide a benefit that goes beyond basic nutrition.

Prebiotics are not digested in the small intestine because we lack the enzymes necessary to break them down into components where they can be absorbed into our bloodstreams. This lack of breakdown brings them into contact with gut bacteria, where they play a role in stimulating the growth and activity of select bacteria that are good for our health. Much of this beneficial interaction with gut bacteria is due to fermentation.

Prebiotics are most likely to increase the number of bifidobacteria (a friendly type of bacteria often targeted by probiotic supplements), but also appear to increase the amount of various other host-friendly bacteria.

Prebiotics for General Health

Ongoing research has shown that prebiotics may provide health benefits to the general population. These benefits include improved calcium absorption, decreases in allergy risk, improved immune system defense, and other positive effects on metabolism.

Research is ongoing to understand the full effects of these foods on gut health, metabolism, and certain diseases. But not all nutrition experts are able to confirm that consuming functional foods or prebiotics will necessarily boost specific health outcomes.

Prebiotics for IBS

Prebiotics may play a role in the treatment of irritable bowel syndrome. Some studies have been conducted to see if increasing prebiotic intake can help reduce IBS symptoms. Results have been mixed.

In some studies, it does appear that higher amounts of prebiotics resulted in worsening symptoms for study participants—not surprising given what we know about FODMAPs effect on IBS symptoms (more fermentation leads to increased gas which results in gassiness, bloating and abdominal pain).

However, in one preliminary study on the effectiveness of a prebiotic supplement for IBS, researchers found that prebiotics may provide a therapeutic benefit. However, the number of study participants was quite small so we cannot draw any firm conclusions from this trial.

Possible Side Effects

According to the Mayo Clinic, most prebiotics and probiotics can be safely consumed without side effects by most healthy adults. In some cases, abdominal discomfort, bloating and gas may occur while your digestive system adjusts.

However, if you have IBS or another gastrointestinal disorder, you should speak with your healthcare provider to get a personalized recommendation for including prebiotics in your diet.

Dosage and Preparation

Most people can get prebiotics by setting a goal to reach the recommended intake of fiber. The recommended fiber intake for adults is 25 grams to 38 grams per day. Consuming whole grains and plenty of fruits and vegetables is often the best way to reach that goal.

Many prebiotic supplements provide a dose of about four to five grams per day. If you take a prebiotic supplement, start slowly (once a day) until you see how your body reacts to the supplement. If gas or bloating occurs, then cut your dose in half.

Many people combine prebiotics with probiotics for an increased benefit. According to Memorial Sloan Kettering Cancer Center, because probiotics are short-lived, prebiotics are sometimes added to probiotics to maintain their levels in the gut. This combination of pro- and prebiotics is called “synbiotic therapy" or "synbiotics."

What to Look For

Prebiotics can be consumed in foods or in supplement form. Since prebiotics are non-digestible fibers (carbohydrates) they are found in many plant-foods that provide good nutrition. So when you increase your intake of prebiotic foods you gain health benefits from the other nutrition that they provide.

These prebiotic foods include many items that you'd find in your local market.

  • Asparagus
  • Chicory root
  • Fennel
  • Garlic
  • Jerusalem artichoke
  • Legumes (beans, chickpeas, lentils, soybeans)
  • Nuts such as cashews and pistachios
  • Onions, leeks, shallots, scallions
  • Wheat products, such as cereal

If you look for prebiotic supplements, you may see certain terms on the label identifying the prebiotics offered by the product. Commonly consumed prebiotics include:

  • Fructans (inulin and fructooligosaccharides)
  • Galacto-oligosaccharides (GOS)
  • Oligofructose (fructose)
  • Resistant starch

Oligosaccharides are the best-known prebiotics.

If you choose to buy a supplement, the National Institutes of Health (NIH) recommends that you look for a Supplement Facts label on the product that you buy. This label will contain vital information including the amount of fiber per serving, and other added ingredients like fillers, binders, and flavorings.

Lastly, the organization suggests that you look for a product that contains a seal of approval from a third party organization that provides quality testing. These organizations include U.S. Pharmacopeia, ConsumerLab.com, and NSF International. A seal of approval from one of these organizations does not guarantee the product's safety or effectiveness but it does provide assurance that the product was properly manufactured, contains the ingredients listed on the label, and does not contain harmful levels of contaminants.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Silk, D., et.al. "Clinical trial: the effects of a trans-galactooligosaccharide prebiotic on faecal microbiota and symptoms in irritable bowel syndrome" Alimentary Pharmacology & Therapeutics 2009 29:508–518. DOI:


  2. Spiller, R. "Review article: probiotics and prebiotics in irritable bowel syndrome" Alimentary Pharmacology & Therapeutics 2008 28:385–396. DOI:


  3. Whelan, K. "Mechanisms and effectiveness of prebiotics in modifying the gastrointestinal microbiota for the management of digestive disorders" Proceedings of the Nutrition Society 2013 72:288-298. DOI: 10.1017/S0029665113001262

  4. Markowiak, P., & Śliżewska, K. (2017). Effects of Probiotics, Prebiotics, and Synbiotics on Human Health. Nutrients9(9), 1021. doi:10.3390/nu9091021

Additional Reading

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.