Isomalto-oligosaccharides (IMOs) are short chains of glucose molecules linked together in a way that makes them partially resistant to digestion. You’ll most commonly find them listed on protein bar and supplement labels, where they’ve been used as a sweetener and bulking agent, often marketed as a source of prebiotic fiber. The reality is more complicated than the packaging suggests: IMOs are largely digested and absorbed like regular carbohydrates, and their status as a true dietary fiber is disputed.
How IMOs Are Made
IMOs start as starch or maltose, which are then converted through an enzymatic process. A specialized enzyme breaks the bonds in maltose, releasing glucose units, and then reattaches those units in a different configuration. Specifically, it swaps the original bonds (called alpha-1,4 linkages) for alpha-1,6 linkages. This reshuffling is what gives IMOs their name and their partial resistance to digestion, since the primary starch-digesting enzyme in your gut only cuts alpha-1,4 bonds.
The resulting chains can range from two glucose units up to 14 or more. Commercial IMO syrups and powders typically contain a mix of chain lengths, along with varying amounts of leftover glucose and maltose. The purity of the product matters a lot: a syrup with mostly short chains and residual simple sugars behaves very differently in your body than a product with longer, purer chains.
Why They’re in Protein Bars and Supplements
IMOs became popular in the food industry for practical reasons. They provide about half the sweetness of table sugar, which makes them useful for adding mild sweetness and a syrupy texture to products like protein bars, energy balls, and baked goods without tasting overly sweet. They also act as a binding agent, helping hold bars together and keeping them soft. For manufacturers, IMOs offered a way to list “fiber” on the nutrition label while simultaneously improving taste and texture.
This dual role made IMOs extremely common in low-sugar and high-protein products throughout the mid-2010s. Many bars listed 10 to 15 grams of “fiber” per serving, with IMO syrup as the source.
The Fiber Controversy
The central problem with IMOs is that most of the product gets digested and absorbed in your small intestine, just like regular sugar. Clinical studies comparing IMO to pure dextrose (glucose) found that blood sugar and insulin responses were statistically indistinguishable. In one trial, 50 grams of IMO produced nearly the same blood glucose curve over two hours as 50 grams of dextrose. The only measurable difference was that blood sugar dipped slightly lower in the two-to-four hour window after consuming IMO, suggesting a small portion reached the colon. Researchers estimated that roughly 90% of commercial IMO is digested in the small intestine, with only about 10% making it to the large intestine where fiber fermentation happens.
This matters because dietary fiber, by definition, is not digested and absorbed in the small intestine. The FDA defines dietary fiber as non-digestible carbohydrates that have been shown to provide a specific health benefit, such as lowering blood glucose, reducing cholesterol, or improving bowel regularity. Added fibers that don’t appear on the FDA’s approved list require a citizen petition backed by scientific evidence of a beneficial effect. IMOs are not on that approved list. After scrutiny from regulators and independent researchers, many manufacturers reformulated their products, replacing IMOs with fibers like soluble corn fiber or allulose.
In practical terms, if you’re counting net carbs or relying on the fiber content listed on a bar made with IMO, you’re likely consuming more digestible carbohydrate than the label implies.
Prebiotic Effects of the Undigested Portion
The small fraction of IMO that does reach your colon can feed beneficial gut bacteria. Lab and animal studies show that IMOs promote the growth of Bifidobacterium, Lactobacillus, and Bacteroides species while reducing populations of less desirable microbes like certain Proteobacteria. These beneficial bacteria, in turn, produce short-chain fatty acids including acetate, propionate, and butyrate, compounds that nourish the cells lining your colon and play roles in immune function and metabolism.
Animal studies have also found that IMOs can increase populations of Akkermansia and Faecalibacterium, both associated with healthier metabolic profiles. In rats fed high-fat diets, IMO supplementation helped modulate gut bacteria in ways that reduced markers of metabolic dysfunction. However, these prebiotic benefits depend heavily on how much undigested IMO actually reaches the colon. With commercial products where 90% is absorbed in the small intestine, the prebiotic dose arriving in the colon is modest compared to established prebiotic fibers like inulin or fructo-oligosaccharides.
Longer-chain IMOs with purer alpha-1,6 linkages are more resistant to digestion and show stronger prebiotic potential, but these are not what most commercial food products have historically contained.
Digestive Tolerance
One genuine advantage of IMOs is that they’re well tolerated at typical serving sizes. Because most of the product is absorbed before reaching the colon, it causes less gas and bloating than many other fiber supplements. Health Canada notes that gastrointestinal symptoms like flatulence, bloating, soft stool, or diarrhea may occur at intakes above 30 grams per day. The threshold for diarrhea specifically is estimated at about 1.5 grams per kilogram of body weight, which works out to roughly 105 grams for a 150-pound person. For context, a typical protein bar contains 10 to 15 grams, so you’d need to eat several in a day to run into trouble.
Calories and Blood Sugar Impact
Because IMOs are mostly digested and absorbed as glucose, they carry a caloric load close to that of regular carbohydrates. Clinical data confirms they produce glycemic and insulin responses comparable to dextrose when matched gram for gram. This makes them caloric sweeteners in practice, not the low-calorie fiber alternative they were once marketed as.
If you’re managing blood sugar or counting calories, treat IMOs listed on an ingredient label as you would any other digestible carbohydrate. Products that have replaced IMOs with ingredients like soluble corn fiber, chicory root fiber, or allulose generally offer a more accurate nutrition label and a genuinely lower glycemic impact.
How to Spot IMOs on a Label
IMOs appear on ingredient lists under several names: isomalto-oligosaccharides, isomaltooligosaccharide, IMO syrup, IMO fiber, or the brand name VitaFiber. If you see any of these near the top of an ingredient list on a product claiming high fiber content, it’s worth checking when the product was formulated. Many companies reformulated between 2018 and 2021 in response to the fiber labeling concerns, but older formulations or smaller brands may still use IMOs as their primary fiber source.