Prebiotics

What it is

According to the definition of an international expert panel, a prebiotic is a substrate that is selectively utilized by host microorganisms and confers a health benefit. Both parts are decisive: the substance must preferentially feed certain bacteria, and a benefit for you must follow from this. So not every type of dietary fiber is a prebiotic.

As a supplement, there are mainly three groups. Inulin is a long-chain fructose building block, usually obtained from chicory root. Fructo-oligosaccharides, FOS for short, are shorter chains of the same type. Together they are called inulin-type fructans. Galacto-oligosaccharides, GOS for short, consist of lactose building blocks and are added to infant formula, among other things.

The forms do not behave the same in the gut. In the irritable bowel syndrome meta-analysis by Wilson and colleagues, inulin-type fructans worsened flatulence, while other prebiotics and low amounts of up to 6 g daily slightly improved it. So anyone who reacts sensitively will not fare the same with every prebiotic.

This page is about prebiotics as an isolated supplement. How much dietary fiber from food does for the heart, metabolism and life expectancy is a separate, very well-supported story with different data, which is covered on the page on dietary fiber.

How it is supposed to work

Prebiotics are not digested in the small intestine and arrive unchanged in the colon. There they are fermented by certain bacteria, above all bifidobacteria. This produces short-chain fatty acids, which serve the intestinal cells as energy and slightly acidify the environment.

The first step of this chain is well measured in humans. So and colleagues evaluated 64 studies and found considerably more bifidobacteria and lactobacilli in stool under fiber interventions, especially with fructans and GOS. The overall diversity of the gut flora, by contrast, did not change. Prebiotics thus shift the weights within the community; they do not make it more diverse.

The second step, from the altered microbiome to a noticeable benefit, is supported to different degrees depending on the goal. The route is most direct for bowel movements: more bacterial mass and more bound water make stool softer and more frequent.

Where else prebiotics have been studied

Besides digestion and blood glucose, there are individual findings on minerals. In a randomized study of adolescents in puberty, 8 g of inulin-type fructans daily increased calcium absorption from the gut, with a difference of 8.5 % after 8 weeks, and improved bone mineralization after 1 year.

In premature infants, a meta-analysis of 18 studies with 1,322 children found less sepsis and lower mortality with prebiotics, but no significant difference for the dreaded intestinal inflammation NEC. This is a specialized clinical area and cannot be transferred to adults, but it shows that prebiotics have also been systematically studied in vulnerable groups.

What the studies show

Meta-analysis 2018: bifidobacteria in healthy people

So and colleagues pooled 64 randomized studies with 2,099 healthy adults. Fiber interventions considerably increased bifidobacteria, with a standardized mean difference of 0.64, and also lactobacilli. Fructans and galacto-oligosaccharides had the strongest effect. The diversity of the gut flora remained unchanged.

Meta-analysis 2014: inulin in constipation

Collado Yurrita and colleagues evaluated 5 randomized studies with 252 participants, 144 of them with inulin. Stool frequency, consistency on the Bristol scale, transit time and stool hardness improved significantly. Abdominal pain and bloating did not improve.

Meta-analysis 2019: irritable bowel syndrome, no benefit

Wilson and colleagues pooled 11 randomized studies with 729 patients with irritable bowel syndrome or similar bowel disorders. With prebiotics, 54 % responded, with placebo 63 %; there was no difference. Bifidobacteria rose nonetheless. Inulin-type fructans worsened flatulence; low doses of up to 6 g daily and other prebiotics slightly improved it.

Meta-analysis 2019: blood glucose in diabetes

Wang and colleagues evaluated 33 randomized studies with 1,346 participants using GRADE. Inulin-type fructans lowered fasting blood glucose, HbA1c, fasting insulin and insulin resistance, most clearly in prediabetes and type 2 diabetes. From this the authors derive 10 g daily for at least 6 weeks.

Where the data stop

More bifidobacteria are a measured value, not a health gain in itself. In a study with 20 older and 24 younger people, bifidobacteria rose with GOS in both groups, but immune function and oxidative stress did not change. Which change in the gut flora brings which benefit has not been shown for many promises around the immune system, mood or skin.

In irritable bowel syndrome, prebiotics do not help on average. This is shown by the Wilson meta-analysis and a 2024 network meta-analysis. A single study with 44 patients found an improvement with GOS; that is not enough against the overall picture. Inulin-type fructans and GOS are also among the FODMAPs that many people with irritable bowel syndrome deliberately avoid. On metabolism, the effects are largely limited to people with diabetes: a meta-analysis of 33 studies found no significant effect in healthy people, people with overweight and people with high blood lipids, and body weight remained unchanged. And for constipation, the inulin meta-analysis with 5 studies is narrow.

Status, approval and legal

In Germany, inulin, FOS and GOS are food ingredients and freely available as dietary supplements. The only health claim approved in the EU for a prebiotic concerns chicory inulin: at 12 g daily it contributes to normal bowel function by increasing stool frequency. Other advertising claims, for example about strengthening the immune system, are not approved for prebiotics. There is no official maximum amount from EFSA or the BfR. For competitive athletes, the general advice of the German National Anti Doping Agency applies: dietary supplements can be contaminated; the Cologne List offers guidance without a guarantee.

Safety

Prebiotics are considered safe; the side effects play out in the gut. Because they are fermented in the colon, they can cause a bloated abdomen, flatulence, abdominal pain and diarrhea, strongly dependent on the amount and the person. According to a review, inulin is considered safe and well tolerated in healthy adults up to 40 g daily if the amount is increased slowly. People with irritable bowel syndrome often react more sensitively; in the meta-analysis, inulin-type fructans worsened flatulence there. In a large network meta-analysis on constipation, there were no more adverse events with prebiotics than with placebo.

Sources

• Gibson et al., Nat Rev Gastroenterol Hepatol 2017 – ISAPP consensus on the definition
• So et al., Am J Clin Nutr 2018 – meta-analysis of dietary fiber and gut flora
• Wilson et al., Am J Clin Nutr 2019 – meta-analysis of prebiotics in irritable bowel syndrome
• Collado Yurrita et al., Nutr Hosp 2014 – meta-analysis of inulin in constipation
• Wang et al., J Transl Med 2019 – meta-analysis of inulin-type fructans and blood glucose
• Li et al., Food Sci Nutr 2021 – meta-analysis of inulin-type fructans and metabolism
• Wilms et al., Clin Nutr 2021 – GOS in older adults
• Abrams et al., Am J Clin Nutr 2005 – inulin-type fructans and calcium absorption
• Precup et al., Int J Environ Res Public Health 2022 – prebiotics and the EU health claim
• Canazza et al., Gels 2025 – review of inulin, tolerability