Key takeaways
- In 2017, an international expert panel defined a prebiotic as something the microbes living in you use selectively in a way that benefits your health, so not every fiber qualifies12.
- Jerusalem artichokes, garlic, leeks, onions and dandelion greens rank among the richest food sources, though published estimates for the same food can differ several-fold56.
- Most proof that prebiotics feed bifidobacteria, a group of helpful gut bacteria, comes from trials of purified powders such as inulin and GOS, not whole foods711.
- Many top prebiotic foods, including garlic, onions, leeks, asparagus and most beans, are also high-FODMAP foods that can bring on bloating in people with IBS10.
- Poppi agreed in 2025 to an $8.9 million settlement over gut-health claims for sodas with 2 grams of fiber a can, while denying any wrongdoing1213.
In 2024, shoppers sued the company behind Poppi, a “prebiotic soda.” Their case turned largely on one small number. Each can held 2 grams of fiber, which the lawsuit called too little to do a gut much good13.
The company denied the claims. In 2025, it agreed to an $8.9 million settlement anyway1213.
Now look at your produce bin. In 2023, a San José State University research team estimated that half of a small onion supplies about 5 grams of prebiotics5. That estimate is preliminary, and other methods give onions far less6. Still, the point holds: some of the richest prebiotic sources are ordinary vegetables.
Prebiotics are compounds that feed particular helpful microbes in your gut. This guide covers what counts as one, which foods carry the most, what the research shows, why the same foods can upset sensitive guts, and a one-week plan to add them without misery.
A prebiotic has to feed the right microbes and do you some good
Two scientists, Glenn Gibson and Marcel Roberfroid, coined the word “prebiotic” in 19951. In 2017, an expert panel convened by the International Scientific Association for Probiotics and Prebiotics (ISAPP) updated what it means12.
The panel’s definition is “a substrate that is selectively used by host microorganisms conferring a health benefit”1. In plain words, a prebiotic must pass three tests:
- It’s food for microbes. Bacteria living in you can break it down and use it.
- It’s selective. Certain microbes use it, not every microbe in your gut2.
- It helps you. Research has to show a health benefit that runs through those microbes2.
That’s why ISAPP warns shoppers not to assume prebiotics and dietary fiber are the same thing4. A fiber earns the label only if the selective feeding and the documented benefit both hold up2.
Three prebiotics are especially well studied: inulin, fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS)37. The 2017 panel also opened the door wider. It said substances other than carbohydrates might qualify, and that prebiotics might act at body sites beyond the gut2.
Probiotics are a different thing: live microbes in some foods and supplements. Our companion article on fermented foods covers them.
Three prebiotic families, and the foods that carry them
Fructans (inulin and FOS). These are chains built mostly from fructose, a sugar. Natural sources include onions, garlic, leeks, bananas, artichokes and asparagus7. Commercial inulin is extracted from chicory root7, though Poppi’s came from agave12.
Americans don’t eat much of it. A 2008 review estimated U.S. intake at about 1 to 4 grams a day, compared with 3 to 11 grams in Europe7.
Galacto-oligosaccharides (GOS). GOS is one of the best-studied prebiotics, and studies often test it as a supplement powder311. Beans, peas and lentils carry their own non-digestible oligosaccharides, which gut bacteria ferment8.
Resistant starch. Some starch escapes digestion in the small intestine and reaches the colon. There, Harvard’s Nutrition Source explains, it acts like fiber and serves as a prebiotic for bifidobacteria8. Beans and lentils are good sources8.
When microbes ferment these compounds, they make butyrate, a short-chain fatty acid that Harvard’s reviewers say may be linked to a lower risk of colorectal cancer8. Our article on fiber’s role in digestion covers that chemistry.
How much is in your food depends on who’s measuring
Researchers have to estimate prebiotic content food by food. The San José State team, led by graduate student Cassandra Boyd, used published data to score 8,690 foods in a U.S. Department of Agriculture database5. About 37% contained some prebiotics5.
Five foods topped the list, at roughly 100 to 240 milligrams per gram: dandelion greens, Jerusalem artichokes, garlic, leeks and onions5. Asparagus and cowpeas, which include black-eyed peas, came in at about 50 to 60 milligrams per gram5. Wheat foods ranked lower. Dairy, eggs, oils and meats had little or none5.
That work was presented at a 2023 American Society for Nutrition meeting. It hadn’t passed the full peer review a journal requires, so treat it as preliminary5.
A 2026 review of inulin research gives different numbers for some foods6. Its figures for Jerusalem artichoke (16 to 20 grams per 100 grams) and garlic (9 to 17 grams) roughly match. For onions, it lists just 2 to 4 grams6.
Why the gap? Boyd’s team counted several kinds of prebiotics in onions, not inulin alone5. The review also notes that values shift with plant variety, processing and lab method6. By our arithmetic, half of a 4-ounce onion, the size Boyd used, holds about 1 to 2 grams by the review’s figure, versus about 5 by Boyd’s.
This is a guide built from both sources. Amounts are per 100 grams of food, about 3½ ounces.
| Food | Main prebiotic | Published estimate per 100 g | FODMAP flag |
|---|---|---|---|
| Jerusalem artichoke (sunchoke) | Inulin | 16 to 20 g6; a top-five food5 | Rich in fructans, a FODMAP group610 |
| Garlic | Inulin-type fructans | 9 to 17 g6; a top-five food5 | High FODMAP10 |
| Leeks | Inulin-type fructans7 | A top-five food, about 10 to 24 g5 | High FODMAP10 |
| Onions | Fructans and other forms5 | 2 to 4 g6 up to about 24 g5 | High FODMAP10 |
| Dandelion greens | Not specified | A top-five food, about 10 to 24 g5 | Not rated in our sources |
| Asparagus | Inulin-type fructans7 | About 5 to 6 g5 | High FODMAP10 |
| Black-eyed peas and other beans | Oligosaccharides, resistant starch8 | Cowpeas about 5 to 6 g5 | Most legumes high FODMAP10 |
| Wheat breads and cereals | Various, smaller amounts | Lower than the foods above5 | Wheat, rye and barley breads high FODMAP10 |
| Bananas | Inulin-type fructans7 | No reliable figure found | Not rated in our sources |
| Meat, eggs, dairy, oils | None to speak of | Little or none5 | Milk and yogurt carry lactose, another FODMAP10 |
What prebiotics can do, and where the proof runs out
The clearest finding is about bacteria. Human trials have consistently found that inulin and oligofructose raise counts of bifidobacteria and lactobacilli, according to a 2008 review by Gibson and Nathalie Delzenne7. Gibson helped coin the word, and the review grew out of a conference organized by Orafti, a company that makes inulin ingredients7. Weigh it as you would any field, or industry, reviewing its own work.
A 2018 pooled analysis of 64 trials in healthy adults, published in the American Journal of Clinical Nutrition, points the same way. Added fiber raised bifidobacteria and lactobacilli, and fructans and GOS drove most of that rise3.
The same review describes several trials with health measures7:
- Young children in day care. After 21 days of oligofructose, bifidobacteria rose and another bacterial group, clostridia, fell. Children taking it had fewer reports of gas, diarrhea, vomiting and fever.
- Teenagers and calcium. A yearlong trial gave adolescents 8 grams a day of inulin enriched with oligofructose. Calcium absorption and bone mineral density improved, and about 31 more milligrams of calcium a day went into bone.
- Adults and appetite. Eating 8 grams twice a day for two weeks cut calorie intake by almost 10% and left people feeling fuller.
Notice what these trials have in common. Nearly all used purified powders in measured doses, not plates of leeks. The San José State team said the effects of cooking and mixed dishes still need study5. So the case for prebiotic foods rests partly on what their isolated compounds do.
| Claim | Evidence level | What it rests on |
|---|---|---|
| Purified prebiotics such as inulin and GOS raise bifidobacteria | Consistent human trials, including a pooled analysis of 643711 | |
| Prebiotics improve calcium absorption | A yearlong trial in teens using a supplement7 | |
| GOS eases gas and bloating in people with gut disorders | A single trial against a low-FODMAP diet11 | |
| Prebiotics help you eat less | A two-week trial of 16 grams a day7 | |
| Prebiotic-rich foods prevent disease in their own right | Few food-based trials; most data come from powders57 | |
| One 2-gram can of prebiotic soda improves gut health | No trial we found; disputed in a lawsuit that settled13 |
The FODMAP catch: the same foods can bloat a sensitive gut
FODMAPs are a group of fermentable carbohydrates that can set off symptoms in people with irritable bowel syndrome (IBS). The letters stand for fermentable oligosaccharides, disaccharides, monosaccharides and polyols10. Researchers at Monash University in Australia developed the low-FODMAP diet10.
Two of the FODMAP groups, fructans and GOS, are the same compounds behind many prebiotics310. Monash’s high-FODMAP list includes garlic, onions, leeks, asparagus, artichokes, most beans and lentils, and wheat, rye and barley breads10. Put that list next to the table above and the overlap is hard to miss.
That’s the tension. The fermentation that feeds helpful microbes also makes gas, which can cause bloating and cramps in some people8.
The low-FODMAP diet isn’t meant to last forever. Monash describes three steps: two to six weeks of low-FODMAP eating, eight to 12 weeks of testing each FODMAP group, then a personal long-term diet that limits only what you can’t tolerate10.
A 2018 trial in the journal Gastroenterology suggests why the last step matters. People with functional gut disorders and troublesome gas got either 2.8 grams a day of a commercial GOS product or a low-FODMAP diet for four weeks11. Both groups felt better.
Two weeks after stopping, relief lasted in the GOS group but faded in the diet group11. Bifidobacteria rose with GOS and fell on the low-FODMAP diet11.
If you have IBS, work with a clinician or a FODMAP-trained dietitian before you add or cut these foods. Our coming article on low-FODMAP eating covers the method in detail.
When symptoms need a clinician. Some extra gas is expected when you add these foods811. See a clinician if you have rectal bleeding, weight loss you can’t explain, low iron, new gut symptoms later in life, or a family history of colon cancer, the American College of Gastroenterology says14. If you’re due for colorectal cancer screening, our article on no-cost preventive care explains what’s covered.
Prebiotic sodas: what 2 grams of fiber buys
Three lawsuits filed against Poppi’s maker in 2024 were combined that September in federal court in California12. The suits targeted slogans such as “for a healthy gut”12.
The plaintiffs argued that the drink’s main prebiotic, agave inulin, came in amounts too small to matter, and that its sugars could work against good bacteria12. Those were allegations, and no court ruled on the science.
In 2025, the company agreed to an $8.9 million settlement while stating that it admitted no fault, liability or wrongdoing12. Buyers from 2020 to mid-2025 could claim up to $16 a household without receipts, by a September 2025 deadline13. A final approval hearing was set for November 202513; we could not confirm its outcome. At the time of the deal, PepsiCo was completing its purchase of the brand12.
The FDA lets inulin count as dietary fiber on a Nutrition Facts label9. So a soda can post a fiber number with no fruit, vegetable or grain inside.
Is 2 grams worthless? Not quite. It’s real inulin, and ISAPP’s suggested range starts at 3 grams a day4.
But half a small onion beats it by Boyd’s estimate and roughly matches it by the review’s figures56. The onion also brings vitamins, minerals and other plant compounds, without the soda’s sugar12. Prebiotic supplements are the subject of a coming article.
A one-week plan to add prebiotic foods slowly
This plan is our own design. It adds one food at a time, so you can tell what sits well. Gas often rises when microbes meet a new food8. In a pilot study of GOS in healthy volunteers, gas climbed at first, then fell back to normal within three weeks11.
| Days | What to add | Why |
|---|---|---|
| 1 and 2 | A few tablespoons of cooked onion or leek at one meal, such as in soup or eggs | A small, familiar start |
| 3 and 4 | Keep that, and add ¼ cup of black-eyed peas, lentils or other beans at another meal | Brings oligosaccharides and resistant starch8 |
| 5 | Add a clove of garlic to a sauce or dressing | One of the richest sources by weight56 |
| 6 | Add a few asparagus spears or a small handful of dandelion greens | Broadens your prebiotic mix5 |
| 7 | Try 2 or 3 thin slices of roasted Jerusalem artichoke | The richest food, so test it last56 |
A few rules make it work:
- Write down how you feel each evening: gas, bloating, bowel habits, comfort.
- Hold steady if you’re uncomfortable. Repeat the same day’s amounts for a few more days before you add more.
- Rotate. After the first week, mix several foods through the week rather than piling on one.
- Drink fluids and keep meals regular while your gut adjusts.
- Stop and call a clinician if you notice any of the warning signs above.
Food first is a natural-health instinct, and here it holds up
Onions, garlic, leeks and beans are old friends in kitchens around the world. Long before anyone measured bifidobacteria, cooks were building soups, stews and sauces on these foods.
Federal advice points the same way, without the buzzword. The 2025–2030 Dietary Guidelines for Americans, released in January 2026, credit vegetables, fruits and high-fiber foods with supporting a diverse gut microbiome, and warn that highly processed foods can upset that balance15. The word “prebiotic” never appears in them.
The same skepticism you’d bring to a drug ad belongs here, too. “Gut health” sells sodas, bars and powders, and a small dose of added inulin can carry a big claim. Whole prebiotic foods ask less of your trust and offer more on the plate.
Questions readers ask
Are prebiotic supplements better than food?
Supplements deliver a known dose, and most trials used them7. Food gives you prebiotics plus vitamins, minerals and other plant compounds, but in amounts that vary. A coming article in this series weighs prebiotic and probiotic supplements.
Does cooking destroy prebiotics?
No one knows well enough to say. The San José State team flagged cooking as an open question5. In the meantime, eat these foods both raw and cooked, whichever way you enjoy them.
How do I know prebiotics are working?
There’s no simple home test. Home microbiome kits give inconsistent results, as our article on them explains. Steady bowel habits and comfort after meals are reasonable signs, and a clinician can help if something feels wrong.
I have IBS. Should I avoid prebiotic foods?
Not forever, and not on your own. Many prebiotic foods are high in FODMAPs, and a short low-FODMAP phase can ease symptoms10. The goal of Monash’s method is to reintroduce as many foods as you tolerate, ideally with a trained dietitian10.
The bottom line
Prebiotics are compounds that particular gut microbes use in ways that help you, and they’re concentrated in everyday foods such as garlic, onions, leeks, beans and Jerusalem artichokes. Trials of purified prebiotics show they feed bifidobacteria, with early signs of benefits for calcium absorption, appetite and gut comfort. Whether whole foods deliver the same effects has been studied far less.
Start small, rotate foods and expect some gas at first. If you have IBS, the same foods may need a careful, dietitian-guided approach. And when a soda promises gut health, check the grams and compare them with what an onion offers.
Related reading
About this article
This article was researched using expert consensus statements, peer-reviewed research, FDA guidance, the 2025–2030 Dietary Guidelines, university and medical society resources, and news coverage of a documented lawsuit, all listed below. It was reviewed in October 2026. It is general education, not medical, legal or financial advice.
Sources
- International Scientific Association for Probiotics and Prebiotics. “Prebiotic Definition Updated by ISAPP.” ISAPP, June 16, 2017 (web post updated 2025). isappscience.org
- Gibson GR, Hutkins R, Sanders ME, et al., Reid G. “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, 2017;14:491–502 (University College Cork repository record). cora.ucc.ie
- So D, Whelan K, Rossi M, et al. “Dietary Fiber Intervention on Gut Microbiota Composition in Healthy Adults: A Systematic Review and Meta-Analysis.” American Journal of Clinical Nutrition, 2018 (Bond University record). research.bond.edu.au
- International Scientific Association for Probiotics and Prebiotics. “ISAPP Releases New Video Providing Clarity on Prebiotics and Their Health Benefits.” ISAPP, undated.
- American Society for Nutrition. “Eating More of These Foods Could Benefit Your Gut Microbiome” (on Boyd C, Gieng J, NUTRITION 2023 abstract). ASN, July 22, 2023. nutrition.org
- Salvi SS, Dabade A, Pai N, Sonawane SK. “Inulin: Functional Ingredients in Food Industry, Extraction, and Health Applications, a Comprehensive Review.” Exploration of Foods and Foodomics, 2026;4:1010188. explorationpub.com
- Gibson GR, Delzenne N. “Inulin and Oligofructose: New Scientific Developments.” Nutrition Today, 2008;43(2):54–59 (UCLouvain repository). farm.ucl.ac.be
- Harvard T.H. Chan School of Public Health, The Nutrition Source. “Legumes and Pulses.” Updated November 15, 2024. nutritionsource.hsph.harvard.edu
- U.S. Food and Drug Administration. “Questions and Answers on Dietary Fiber.” Content current as of July 25, 2024. fda.gov
- Monash University Department of Gastroenterology. “3-Step FODMAP Diet” (information sheet). Monash University, 2026. monashfodmap.com
- Guarner F. “Prebiotics Do Better Than Low FODMAPs Diet” (on Huaman JW, et al., Gastroenterology, 2018). ISAPP, August 9, 2019, updated June 21, 2025. isappscience.org
- BevNET. “Poppi Reaches $8.9M Settlement Over Gut Health Claims.” BevNET, March 2025. bevnet.com
- TODAY. “Poppi Prebiotic Soda Might Owe You Money as Part of Settlement: How to File a Claim.” NBC News, 2025. today.com
- American College of Gastroenterology. “Irritable Bowel Syndrome: Overview.” ACG, updated February 2026. gi.org
- U.S. Department of Health and Human Services and U.S. Department of Agriculture. “Dietary Guidelines for Americans, 2025–2030.” January 2026. cdn.realfood.gov