A close relative of maltose is a molecule known as isomaltose (typically found in items such as beer and honey). The biggest difference between maltose and isomaltose is that isomaltose is joined together by an α-1,6 chemical bond, rather than an α-1,4 chemical bond. Scientists suspected that by adding a certain enzyme (transglucosidase) to high maltose syrup, they could change the bonds from α-1,4 to α-1,6, thereby making it more resistant to being broken down by the enzymes, as described above, when compared to maltose. Again, while this sounds excellent in theory, it is not necessarily what happens in our bodies. In fact, isomaltose (and thus, IMO syrups used in some of these products) is broken down by certain enzymes on the brush border of the small intestine.[2] Though the α-1,6 bond breaks down slower compared to the α-1,4 bond, these IMO syrups, which often use a blend of di-and oligosaccharides, ultimately metabolize into small amounts of glucose and maltose[2] and thus should be viewed as a slow digesting carbohydrate rather than a true fiber.
One study compared the glycemic response of SCF to the glycemic response of glucose in 12 healthy adults during a randomized, controlled, crossover study.[6] The findings of this study revealed that SCF had a significantly lower incremental glucose and insulin response than that of the glucose control. Additionally, another study observed a significant lowering effect on postprandial (during or after food consumption) blood glucose and insulin (coinciding with an increase in fat oxidation) upon consumption of 55 grams of SCF in 18 overweight adults, compared to a full calorie control.
Even so, keto followers may experience a rise in LDL cholesterol, sometimes called “bad” cholesterol because too much of it can lead to a buildup of plaque in the arteries, which can increase the risk of heart disease. And that’s where fiber can help. However, many high-fiber foods, like beans, fruits, vegetables, nuts, seeds, and whole grains, are also high in carbs, so they're limited on the keto diet.
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Thus far, we have established what IMO is and how its structure can differ in regard to its carbon bonds. The real question is, “What are the metabolic responses of products that contain these IMOs?” The glycemic index of IMO is very low,[3] however, it has been shown to be nearly completely digested (83 % or more) by enzymes on the small intestinal border.[3] Thus, IMOs should not necessarily be classified as a true fiber but rather as a low glycemic carbohydrate like steel cut oatmeal, at about 3.3 calories per gram.
Dr. Ryan P. Lowery is the CEO of Ketogenic.com, author of The Ketogenic Bible, President of the Applied Science and Performance Institute and KetoPhD™. His mission  is to spread awareness around the Ketogenic Lifestyle and its’ many benefits beyond body composition. He earned his BS and MS in exercise physiology and exercise and nutrition science from the University of Tampa and completed his doctorate work at Concordia University in Health and Human Performance with a focus on “The Effects of a Well-Formulated Ketogenic Diet and Exogenous Ketone Supplementation on Various Markers of Health and Body Composition in Healthy and Diseased Populations.” Over his career, Ryan has published over 150 papers, abstracts, and book chapters on human performance and sports nutrition and has dedicated his life to educating the masses. In his free time, Ryan enjoys spending time with his best friend, Scoot the Keto Pup, jet skiing, and traveling around the world. The way to his heart is through a good glass of wine and Keto desserts.
Lastly, one of the advertised benefits of IMO is possible prebiotic activity. Prebiotics are critical, as they feed the beneficial bacteria in our digestive system, specifically in the large intestine. These bacteria have several amazing functions, such as lowering body fat, improving insulin sensitivity, and lowering depression. Two “gold standard” prebiotics in the industry are inulin and fructooligosaccharides (FOS). Inulin and FOS are non-digestible carbohydrates that robustly increase beneficial bacteria. The challenge, however, is that both inulin and FOS, due to their rapid digestibility by intestinal bacteria, result in low gastric tolerance, and, ultimately, gastric distress. Additionally, inulin and FOS, when added to protein bars or other goods, may degrade over time into individual sugar units. Regardless, one study comparing inulin to IMOs, found that the prebiotic activity in inulin is 14 times greater than that of IMOs.[4] This is logical because, as you recall from above, approximately 70% to 90% of IMOs are digested. As such, only a small portion of these prebiotic fibers make it to the large intestine, in which two out of three studies have demonstrated that this small portion may indeed have some prebiotic effects.