The Science of Fiber

The Science of Fiber

Fiber with the receipts.

Every BUM fiber product is built around a fiber with decades of published research behind it: prebiotic inulin from chicory root in our Gummies, and psyllium husk in our Cleanse. Here's what each one does, the studies behind them, and why those ingredients make our fiber HSA/FSA eligible.

See the research Shop fiber
  • Clinically studied fibers
  • 32 sources, linked below
  • HSA/FSA eligible
Smiling man with curly hair holding a red BUM fiber gummy between his teeth

How fiber works

Fiber has two jobs. We hired a specialist for each.

One kind of fiber feeds the good bacteria already living in your gut. Another holds onto water so stool comes out soft, formed and easy to pass. Instead of asking one fiber to do everything, we built each product around one of the most studied fibers for its job. Take one, or take both.

Feeds your gut

Inulin from chicory root

4gper 2 gummies

In our Prebiotic Fiber Gummies

Inulin is a fiber found naturally in chicory root, onions, garlic and bananas. Your body can't digest it, so it reaches your colon intact. There, your good bacteria ferment it as food. That's what makes it a prebiotic.

  • An established prebiotic in the scientific consensus on prebiotics2
  • Its most consistent effect in human studies: more beneficial Bifidobacteria4,5
  • More frequent, softer stools in controlled trials at 12 to 15g a day*3,6,7
  • Allowed by the FDA to be counted as dietary fiber on labels1
Adds soft bulk

Psyllium husk

4gfiber per scoop

In our Psyllium Fiber Cleanse

Psyllium is the seed husk of the Plantago ovata plant. Mixed with water, it forms a soft gel that holds moisture in stool, so it's bulkier, softer and easier to pass. Your gut bacteria ferment very little of it, so it's generally gentle on gas.

  • Named in the FDA's own definition of dietary fiber11
  • The strongest regularity results in a 2022 review of fiber trials*12,13
  • Poorly fermented, so it generally causes less gas than fast-fermenting fibers12,15
  • Gastroenterology guidelines recommend fiber supplements like psyllium14,16,17

Want both jobs covered? The Fiber Bundle pairs Gummies and Cleanse: two fibers, two jobs, 8g of fiber a day.

Shop the Bundle

The research

Don't take our word for it. Take theirs.

A few of the studies and reviews behind our two fibers. Every source on this page is linked in the references at the bottom.

Inulin · Randomized controlled trial

Chicory inulin, four weeks

Adults with mild constipation took 12g of chicory inulin a day. Bowel movements went from a median of 3 to 4 a week and stools got softer, and a follow-up analysis of the same trial found more Bifidobacteria than on placebo.3,4

Micka et al., 2017; Vandeputte et al., Gut, 2017. Read it

Inulin · Systematic review

What inulin does for your gut bacteria

Across 9 human studies using 5 to 20g of inulin a day, the most consistent change was an increase in Bifidobacteria, a group of beneficial gut bacteria.5

Le Bastard et al., European Journal of Clinical Microbiology & Infectious Diseases, 2020. Read it

Inulin · Regulatory review

European regulators weighed in

Europe's food safety authority found that chicory inulin increases stool frequency at 12g a day, and the EU authorized the claim that it contributes to normal bowel function.8,9

EFSA Journal, 2015; Regulation (EU) 2015/2314. Read it

Psyllium · Meta-analysis

The 2022 fiber review

A review of 16 controlled trials in 1,251 adults found fiber supplements helped chronic constipation. Psyllium had the strongest results, adding about 3 bowel movements a week, with the best effect above 10g a day for at least four weeks.12

van der Schoot et al., American Journal of Clinical Nutrition, 2022. Read it

Psyllium · Randomized controlled trial

Psyllium vs. a stool softener

In 170 adults with chronic constipation, about 10g of psyllium a day increased stool water and bowel movement frequency more than docusate, a common stool softener.13

McRorie et al., Alimentary Pharmacology & Therapeutics, 1998. Read it

Guideline · American College of Gastroenterology

What gastroenterologists are told

The ACG's clinical guideline for IBS recommends soluble fiber for overall symptoms, with the strongest trial evidence for psyllium,14 and suggests against probiotics for the same purpose.16

Lacy et al., American Journal of Gastroenterology, 2021. Read it

Many of these studies use more fiber than one serving of BUM, often 10 to 15g a day. One serving of Gummies or Cleanse gives you 4g, and taking both gives you 8g.

Real talk

Better digestion starts with a little change.

Fiber is meant to improve how you digest, and improving means changing. For some people that comes with an adjustment period: a little more gas, or a shift in your bathroom routine. That's common sense, not a red flag, and it isn't guaranteed to happen.

Possible, not promised

Your gut may need a minute.

Adding fiber can bring some extra gas or bloating at first, especially if your diet is low in fiber to begin with, which is why doctors suggest easing in.24 At 4g a serving, BUM is a gentle place to start. Plenty of people notice nothing at all: in one study of adults adding fiber-rich beans to their diet, fewer than half reported more gas in the first week.25

It usually settles

Your gut learns the routine.

Your gut bacteria adapt to new fiber. In a small study of a prebiotic fiber, gas production went up at first, then dropped back to normal within two weeks.26 For most people who stick with it, any adjustment passes as their body gets used to the new routine, usually within the first week or two.

Pick your start day

Maybe not date night.

If your diet is light on fiber, don't start your new routine on the day of a big date night. Start on a quiet day, take your serving with a glass of water, and give it a week or two before you decide how it's going.

Not for you? That's what our guarantee is for.

Changing your digestion can take some getting used to. If you give it a fair shot and find fiber just isn't for you, our Money-Back Guarantee covers your first opened product bought on bottomsupmovement.com. Request it within 30 days of delivery and we'll refund the product price, minus a $5 shipping and handling fee.

See the guarantee

About FODMAPs

Yes, inulin is a FODMAP. That's not a bad word.

FODMAPs are fermentable carbs that your gut bacteria break down. Inulin is a fructan, the same kind found in everyday staples like wheat, rye, onions, garlic and beans.27

  • Most people do fine with them

    Fermentation happens in everyone, and most people eat high-FODMAP foods every day without issue.27 Some FODMAPs, like inulin, are prebiotics: food for your good bacteria.2,29

  • Sensitivity is the difference

    In a 2017 MRI study, inulin produced similar amounts of colon gas in people with and without IBS. The people with IBS noticed it more because their gut was more sensitive to stretching, not because they made more gas.28

  • Low-FODMAP is a short-term tool

    The low-FODMAP diet was designed for IBS: a few weeks of cutting back with a dietitian, then reintroducing foods to find your personal triggers.16,29 It isn't meant for life, and cutting FODMAPs long term can lower beneficial gut bacteria like Bifidobacteria.30,31

  • Sensitive gut? You have options.

    If you follow a low-FODMAP plan, check with your dietitian before adding any fiber. Psyllium appears to be well tolerated by many people with IBS, which can make our Cleanse an easier place to start.32

What about probiotics?

Probiotics have great marketing. The guidelines are less convinced.

Probiotics are everywhere, but gastroenterologists have been careful about them. When the American Gastroenterological Association reviewed the evidence in 2020, it recommended probiotics for most digestive conditions only as part of a clinical trial.18

  • Not the go-to for IBS

    The American College of Gastroenterology suggests against probiotics for overall IBS symptoms, while recommending soluble fiber.16

  • They don't always move in

    In a 2018 study that sampled the gut lining directly, some people's guts resisted colonization by the probiotic strains they took.19

  • More isn't always better

    In a small 2018 study, people who took an 11-strain probiotic after antibiotics took longer to get their normal gut bacteria back than people who didn't.20

So we feed the good bacteria you already have.

That's the job of a prebiotic fiber like inulin.* Probiotic research is strain-specific, and some strains do show promise for specific uses.21 If you're curious, that's a great conversation to have with your doctor.

Two smiling men in a kitchen, one holding a jar of BUM Psyllium Fiber Cleanse toward the camera

HSA/FSA eligible

Eligible because of what's inside.

Every BUM fiber product is on the official list of HSA/FSA eligible products, reviewed against IRS standards for eligible medical care by the national industry organization that maintains the list. It comes down to what's inside: the list's criteria name fiber products like inulin and psyllium as eligible when they're marketed as fiber.22 No doctor's note, survey or diagnosis needed.

BUM fiber: inulin and psyllium

Eligible as fiber, so you typically won't need extra paperwork. Buy here and get reimbursed, or pay with your HSA/FSA card on eligible Amazon listings.

Probiotic supplements

The same criteria class probiotics as dual-purpose, so they aren't on the list and need a Letter of Medical Necessity to be reimbursed. Fiber supplements that add ingredients marketed for general health benefits beyond fiber don't qualify either.22 Under IRS rules, supplements taken to maintain your general health aren't considered medical care.23

How to use your HSA/FSA

Eligibility and reimbursement rules vary by plan. Check with your plan administrator for your plan's specific rules. Our site checkout doesn't take HSA/FSA cards yet.

Do the math

More gummies. More servings. Less per day.

Fiber works best when you take it every day, so your bottle should go the distance. Here's a bottle of BUM next to a popular peach-flavored fiber and probiotic gummy.†

BUM Prebiotic Fiber Gummies

60 gummies · 30 servings

30 days at 2 gummies a day.

Peach probiotic gummies

48 gummies · 24 servings

24 days at 2 gummies a day. That's 12 fewer gummies.

BUM Gummies Peach probiotic gummies†
Gummies per container 60 48
Servings (2 gummies) 30 24
Fiber per serving 4g inulin from chicory root 4g resistant maltodextrin (from corn)
One-time price $35.00 $39.99
Cost per day, one-time $1.17 $1.67
Cost per day, subscription $1.05 $1.25
HSA + FSA eligibleNo doctor's note needed Yes No‡

† Based on a popular peach-flavored fiber and probiotic gummy, using the pack size and single-bag one-time and subscription prices listed on the brand's website on October 22, 2026, before limited-time sales and multi-bag offers. BUM subscription pricing reflects our 10% subscribe and save discount. Prices can change.

‡ Under the HSA/FSA eligible product list's criteria, probiotic supplements and fiber supplements with added ingredients marketed for general health benefits are dual-purpose: they aren't on the list and need a Letter of Medical Necessity to be reimbursed.22 The brand's website doesn't list HSA/FSA eligibility (as of October 22, 2026).

Want both fibers? The Fiber Bundle gets you Gummies and Cleanse for $53, about $1.77 a day for 8g of fiber.

FAQ

Questions about the science.

What's the difference between a prebiotic and a probiotic?

A probiotic is live bacteria you swallow. A prebiotic is food for the good bacteria already living in your gut. Inulin, the fiber in our Gummies, is a prebiotic: it reaches your colon undigested, and your own bacteria ferment it.2

Will inulin make me gassy?

It might, especially at first, and it might not. Gas is a byproduct of your gut bacteria fermenting fiber, which is how a prebiotic feeds them. In a tolerance study, single doses of up to 10g of chicory inulin were well tolerated, with mostly mild gas and bloating,10 and a serving of our Gummies has 4g. If you do notice an adjustment, it usually settles as your gut gets used to the routine.26 More on the adjustment period.

If you're sensitive to gas, psyllium is poorly fermented and generally gentler, which makes our Cleanse a good place to start.15

Is inulin a FODMAP?

Yes. Inulin is a fructan, one of the FODMAPs, also found in foods like wheat, onions and garlic. Most people eat FODMAPs every day without issue; they mainly bother people with sensitive guts, like many people with IBS.27,28 If you follow a low-FODMAP plan, check with your dietitian. More on FODMAPs.

What if fiber doesn't agree with me?

Give it a week or two, since an adjustment period often settles as your gut adapts.26 If you find fiber just isn't for you, our Money-Back Guarantee covers your first opened product bought on bottomsupmovement.com. Request it within 30 days of delivery, and we'll refund the product price minus a $5 shipping and handling fee.

Should I take the Gummies, the Cleanse or both?

Either works on its own, and they're designed to work together. The Gummies feed your good bacteria. The Cleanse adds soft bulk. If regularity is your main goal, psyllium has the deepest research behind it.12

Taking both gives you 8g of fiber a day from two fibers doing two different jobs, and the Fiber Bundle gets you both for less than buying them separately.

Why don't BUM Gummies have a probiotic?

Because the research favors feeding the bacteria you already have. Major gastroenterology groups don't recommend probiotics for most everyday digestive concerns.16,18 And under the HSA/FSA eligible list's criteria, fiber supplements that add ingredients marketed for general health benefits don't qualify.22 We'd rather put that money into fiber with the research behind it.

Do the studies use the same amount of fiber as BUM?

Not always. Many studies use 10 to 15g a day, which is more than a single serving of our Gummies or Cleanse. One serving of either gives you 4g, and taking both gives you 8g. The research summaries on this page describe the ingredients studied, not BUM products specifically.

Is BUM fiber HSA/FSA eligible?

Yes. Our Prebiotic Fiber Gummies, Psyllium Fiber Cleanse and Fiber Bundle are on the SIGIS Eligible Product List, so you don't need a Letter of Medical Necessity. Our site checkout doesn't take HSA/FSA cards yet: buy with any card and submit your receipt, or use your card on eligible Amazon listings. See how it works.

I have a health condition. Is fiber right for me?

Talk to your doctor before starting any supplement, especially if you have a digestive condition, take medication or have trouble swallowing. Always take psyllium with plenty of water.

References

The receipts.

Peer-reviewed studies, clinical guidelines and official sources behind this page.

Inulin (our Gummies)

  1. 1U.S. Food and Drug Administration. Guidance for Industry: The Declaration of Certain Isolated or Synthetic Non-Digestible Carbohydrates as Dietary Fiber on Nutrition and Supplement Facts Labels. 2018. FDA
  2. 2Gibson GR, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491-502. PubMed
  3. 3Micka A, et al. Effect of consumption of chicory inulin on bowel function in healthy subjects with constipation: a randomized, double-blind, placebo-controlled trial. Int J Food Sci Nutr. 2017;68(1):82-89. PubMed
  4. 4Vandeputte D, et al. Prebiotic inulin-type fructans induce specific changes in the human gut microbiota. Gut. 2017;66(11):1968-1974. PubMed
  5. 5Le Bastard Q, et al. The effects of inulin on gut microbial composition: a systematic review of evidence from human studies. Eur J Clin Microbiol Infect Dis. 2020;39(3):403-413. PubMed
  6. 6Marteau P, et al. Effects of chicory inulin in constipated elderly people: a double-blind controlled trial. Int J Food Sci Nutr. 2011;62(2):164-170. PubMed
  7. 7Collado Yurrita L, et al. Effectiveness of inulin intake on indicators of chronic constipation; a meta-analysis of controlled randomized clinical trials. Nutr Hosp. 2014;30(2):244-252. PubMed
  8. 8EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the substantiation of a health claim related to "native chicory inulin" and maintenance of normal defecation by increasing stool frequency. EFSA Journal. 2015;13(1):3951. EFSA Journal
  9. 9Commission Regulation (EU) 2015/2314 of 7 December 2015 authorising a health claim made on foods, other than those referring to the reduction of disease risk and to children's development and health (chicory inulin and normal bowel function). EUR-Lex
  10. 10Bonnema AL, et al. Gastrointestinal tolerance of chicory inulin products. J Am Diet Assoc. 2010;110(6):865-868. PubMed

Psyllium (our Cleanse)

  1. 11Code of Federal Regulations. 21 CFR 101.9(c)(6)(i): definition of dietary fiber, which names psyllium husk. eCFR
  2. 12van der Schoot A, et al. The effect of fiber supplementation on chronic constipation in adults: an updated systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2022;116(4):953-969. PubMed
  3. 13McRorie JW, et al. Psyllium is superior to docusate sodium for treatment of chronic constipation. Aliment Pharmacol Ther. 1998;12(5):491-497. PubMed
  4. 14Moayyedi P, et al. The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis. Am J Gastroenterol. 2014;109(9):1367-1374. PubMed
  5. 15McRorie JW Jr, et al. Psyllium: the gel-forming nonfermented isolated fiber that delivers multiple fiber-related health benefits. Nutr Today. 2021;56(4):169-182. Nutrition Today

Clinical guidelines and probiotics

  1. 16Lacy BE, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021;116(1):17-44. PubMed
  2. 17Chang L, et al. American Gastroenterological Association-American College of Gastroenterology Clinical Practice Guideline: Pharmacological Management of Chronic Idiopathic Constipation. Gastroenterology. 2023;164(7):1086-1106. PubMed
  3. 18Su GL, et al. AGA Clinical Practice Guidelines on the Role of Probiotics in the Management of Gastrointestinal Disorders. Gastroenterology. 2020;159(2):697-705. PubMed
  4. 19Zmora N, et al. Personalized gut mucosal colonization resistance to empiric probiotics is associated with unique host and microbiome features. Cell. 2018;174(6):1388-1405. PubMed
  5. 20Suez J, et al. Post-antibiotic gut mucosal microbiome reconstitution is impaired by probiotics and improved by autologous FMT. Cell. 2018;174(6):1406-1423. PubMed
  6. 21National Center for Complementary and Integrative Health (NIH). Probiotics: What You Need To Know. NIH

HSA/FSA eligibility

  1. 22SIGIS (Special Interest Group for IIAS Standards). Eligible Product List Criteria. 2025. SIGIS
  2. 23Internal Revenue Service. Publication 502, Medical and Dental Expenses: Nutritional Supplements. IRS

Adjusting to fiber and FODMAPs

  1. 24Mayo Clinic News Network. Mayo Clinic Q and A: Increasing fiber intake for constipation relief. 2021. Mayo Clinic
  2. 25Winham DM, Hutchins AM. Perceptions of flatulence from bean consumption among adults in 3 feeding studies. Nutr J. 2011;10:128. Nutrition Journal
  3. 26Mego M, et al. Colonic gas homeostasis: mechanisms of adaptation following HOST-G904 galactooligosaccharide use in humans. Neurogastroenterol Motil. 2017;29(9):e13080. Neurogastroenterology & Motility
  4. 27Monash University. About FODMAPs and IBS. Monash University
  5. 28Major G, et al. Colon hypersensitivity to distension, rather than excessive gas production, produces carbohydrate-related symptoms in individuals with irritable bowel syndrome. Gastroenterology. 2017;152(1):124-133. Gastroenterology
  6. 29Whelan K, et al. The low FODMAP diet in the management of irritable bowel syndrome: an evidence-based review of FODMAP restriction, reintroduction and personalisation in clinical practice. J Hum Nutr Diet. 2018;31(2):239-255. J Hum Nutr Diet
  7. 30Staudacher HM, et al. Fermentable carbohydrate restriction reduces luminal bifidobacteria and gastrointestinal symptoms in patients with irritable bowel syndrome. J Nutr. 2012. Journal of Nutrition
  8. 31Halmos EP, et al. Diets that differ in their FODMAP content alter the colonic luminal microenvironment. Gut. 2015;64(1):93-100. PubMed
  9. 32Monash University FODMAP. Fibre supplements and IBS. 2017. Monash University

Less prep. More play.*

Two clinically studied fibers, in a gummy you'll look forward to and a mixed berry drink. Take one or take both, every day.

Shop Gummies Shop Cleanse Shop the Bundle

Backed by our Money-Back Guarantee. A portion of every sale goes to nonprofits that give direct grants to LGBTQ+ individuals and couples who want to become parents.

Two men relaxing on a couch, one holding a bottle of BUM fiber gummies

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Individual results may vary. Research summaries describe the ingredients studied, often at different doses than a serving of BUM fiber, and are shared for education. Guideline summaries describe recommendations made to clinicians, not claims about BUM products. If you have a medical condition or take medication, talk to your doctor before starting any supplement.

HSA/FSA eligibility and reimbursement rules vary by plan. Check with your plan administrator for your plan's specific rules. BUM does not provide tax advice.