Inulin Supplementation and the Microbiome: Its Effect on Bacterial Growth

Prebiotics are a type of dietary fibre present in various fruits and vegetables that cannot be digested in the small intestine but are instead fermented by gut bacteria in the large intestine. (1) Main sources of prebiotics include foods such as garlic, onions, leeks, asparagus, and bananas, and due to their potential beneficial effects, they are often extracted and commercially produced into supplements or added to functional food products. (1)

One particular prebiotic is inulin, which is frequently added to prebiotic supplements or highlighted in products marketed as “high-fibre” or “gut-friendly.” Inulin is often associated with an increase in certain beneficial bacteria, particularly Bifidobacteria, and may contribute to overall gut health. (2) It can also lead to the production of short-chain fatty acids, which are by-products of its fermentation and may play a role in metabolic and immune processes. (1) However, how consistent and meaningful these effects are in practice remains an important question. This article explores whether these mechanisms reliably translate into measurable improvements in gut health.

What does the evidence say?

• A 2025 double-blind randomised controlled trial investigated the effect of Inulin and fructooligosaccharides (FOS), another prebiotic, during a 4-week intervention on normal-weight and overweight subjects. The inulin group showed an increased abundance of beneficial bacteria such as Lactobacillus and members of the Bacteroidetes group in overweight participants. It also decreased the abundance of a bacterial genus called Ruminococcus, which has been associated with impaired blood glucose regulation. However, these findings were not observed in the normal-weight group. (3)

• Another double-blind randomised crossover study from 2017 reported modest changes in the abundance of bacteria such as Anaerostipes, Bilophila and Bifidobacterium. Inulin did not have an effect on microbial evenness and diversity, which suggests that its effects are relatively small and targeted rather than reshaping the overall microbiome structure. (4)

• These findings are consistent with another randomised crossover trial from 2025, which did not observe significant changes in overall microbial composition. However, the main outcome of the study focused on bowel habits in participants with diagnosed constipation and showed an increase in stool frequency. The authors also noted that a carry-over effect may have influenced the results. (5)

• Lastly, a systematic review from 2022 summarised the prebiotic effects of inulin across 78 studies, of which 45 reported on microbial changes. While a consistent increase in some bacterial strains was observed, the baseline gut microbiota appeared to strongly influence the magnitude and direction of these changes, highlighting individual variability in response to inulin. (6)

What does this mean in practice?

• Inulin supplementation may increase certain targeted bacterial strains, particularly within the Bifidobacterium genus.

• However, current evidence suggests that these changes are selective rather than global, with no consistent effects on overall microbiome diversity or structure.

• The potential benefits of inulin appear to depend strongly on an individual’s baseline gut microbiota, meaning that responses can vary considerably between individuals. Those with different dietary patterns or microbiome compositions may experience different outcomes

• As with other types of dietary fibre, inulin intake should be increased gradually to minimise potential digestive side effects such as bloating, gas, or abdominal discomfort.

  
  

Key takeaway

Inulin is a well-studied prebiotic fibre that can selectively stimulate the growth of certain beneficial gut bacteria, particularly Bifidobacterium. However, its effects on the overall microbiome appear modest and are not consistent across all individuals. While it may offer benefits in specific contexts, such as low fibre intake or certain digestive conditions, inulin should be viewed as a supportive dietary component rather than a standalone solution for gut health.

References:

1. Davani-Davari D, Negahdaripour M, Karimzadeh I, Seifan M, Mohkam M, Masoumi SJ, et al. Prebiotics: definition, types, sources, mechanisms, and clinical applications. Foods. 2019;8(3):92. doi:10.3390/foods8030092

2. Alonso-Allende J, Milagro FI, Aranaz P. Health effects and mechanisms of inulin action in human metabolism. Nutrients. 2024;16(17):2935. doi:10.3390/nu16172935

3. Li J, Liu F, Luo Y, Wijeyesekera A, Wang S, Chen X, et al. Differential effects of inulin and fructooligosaccharides on gut microbiota composition and glycemic metabolism in overweight/obese and healthy individuals: a randomized, double-blind clinical trial. BMC Med. 2025;23(1):372. doi:10.1186/s12916-025-04189-6

4. Vandeputte D, Falony G, Vieira-Silva S, Wang J, Sailer M, Theis S, et al. Prebiotic inulin-type fructans induce specific changes in the human gut microbiota. Gut. 2017;66(11):1968–74. doi:10.1136/gutjnl-2016-313271

5. Puhlmann ML, Wegh CAM, van der Zalm SCC, Dam V, Doolan A, Meyer D, et al. Inulin-induced improvements on bowel habit and gut microbiota in adults with functional constipation: findings of a randomized, double-blind, placebo-controlled study. BMC Gastroenterol. 2025;25(1):806. doi:10.1186/s12876-025-04409-6

6. Hughes RL, Alvarado DA, Swanson KS, Holscher HD. The prebiotic potential of inulin-type fructans: a systematic review. Adv Nutr. 2022;13(2):492–529. doi:10.1093/advances/nmab119

Disclaimer: This article is for educational purposes only and does not replace medical advice. Individual needs vary, and personalised guidance from a qualified healthcare professional is recommended for tailored support.