Inulin is a fructan carbohydrate (i.e., a chain of fructose molecules) and soluble dietary fiber found naturally in some plants as a form of energy storage. While inulin can be taken as a supplement, it can also be consumed as part of the diet with foods such as Jerusalem artichokes, chicory, asparagus, garlic, onions, wheat, and bananas.^[1] Structurally, inulin is usually composed of a straight chain of fructose sugars that are connected by a type of bond (a β-(2,1)-glycosidic bond) that is resistant to human digestive enzymes.^[2] Because of this, inulin travels to the large intestine largely undigested and acts as a food source for microbes of the gut microbiome — primarily via bacterial fermentation. This process leads to the production of compounds capable of influencing human health — such as short-chain fatty acid (SCFAs), while also stimulating the growth of potentially beneficial bacteria.^[3]

Inulin can be categorized based on the length of its fructose chain, which also influences its physiochemical properties (e.g., solubility, fermentation rate).^[1] Long-chain inulin generally consists of 10 to 60 fructose units, while short-chain inulin has 2 to 10 fructose units and is referred to as a fructooligosaccharide (FOS) or oligofructose.^[1] Together, short- and long-chain inulin are called “inulin-type fructans,” which will be used synonymously with “inulin” on this page.^[4]

Inulin is best known for its prebiotic effects on the gut microbiome, stimulating the growth of bacteria generally thought to be beneficial for health. Most consistently, inulin has been found to increase the absolute abundance of Bifidobacterium species. It may also increase levels of Lactobacillus species and Faecalibacterium prausnitzii, and decrease levels of Clostridium species (some of which are potentially pathogenic), although these effects are less consistently found.^[3][5]

Regular supplementation with inulin may improve blood sugar regulation in people with type 2 diabetes or prediabetes),^[2][6] reduce LDL cholesterol,^[7] increase satiety,^[3] improve bowel movement frequency and consistency,^[8][9] increase the absorption of calcium and magnesium,^[3] and, in people with overweight or obesity, reduce C-reactive protein.^[10] While these are the effects most consistently observed in clinical trials, there is still a substantial amount of variability between studies, and the effects tend to be small in magnitude.

Inulin is generally considered safe, and the main side effects are gastrointestinal in nature due to the gas produced during bacterial fermentation. Possible side effects include flatulence, abdominal rumbling or cramping, bloating, and diarrhea.^[3][11] These side effects may be worse at higher doses and with short-chain inulin (oligofructose, FOS) due to faster fermentation rates.^[11] Starting at a lower dose and increasing slowly over time can help to improve tolerance.^[3]

One major limitation of inulin is how variable its effect can be between individuals. Many of the potential benefits of inulin are due to its interactions with the gut microbiome, but the microbes in the gut can vary greatly between different people, which could lead to inconsistent and unpredictable effects.^[3] A study in mice found that after six months of consuming a high-fat diet (intending to mimic a Western diet) enriched with inulin, most mice developed liver damage and liver cancer, which seemed to be related to the production of secondary bile acids by certain gut bacteria. The authors suggested that the health effects (both positive and negative) of fermentable fibers may depend on the unique gut microbial environment. Importantly, the amount of inulin consumed daily in this study was not clear, and this negative effect has not been observed in human clinical trials.^[12]

Most research suggests that inulin works primarily as a fermentable fiber which can undergo bacterial fermentation in the gut, leading to changes in the composition and function of the gut microbiome and the production of bioactive metabolites, like SCFAs.^[3] SCFAs are capable of influencing the health of our body in a multitude of ways, including by promoting a healthy gut environment and helping to regulate appetite, inflammation, and metabolism.^[4] While some clinical trials have reported increased levels of SCFAs in the blood or feces with inulin supplementation, most have actually failed to find an effect.^[3] Additionally, while inulin can certainly modulate the gut microbiome, connecting these changes to beneficial health outcomes in humans has proved challenging. Ultimately, further research is needed to better understand how inulin works.