We’ve all seen it on the labels: 2 net carbs or low net carbs. But what does this truly mean? What are net carbs and why does this matter? Are all net carbs created equal or are we stretching those claims a bit too much? After reading through this article, I think that you will agree with me on the latter, such that there is a pressing need to educate on the precise definition of a “net carb” and what exactly constitutes a true “fiber.” This topic is very personal to me. I have family members who are severely overweight, some of which are diabetic, and others who are dealing with a multitude of autoimmune diseases. The only thing that upsets me more than misleading supplement facts (an article for another day) is misleading information that is placed on nutritional labels which can often leave the consumer unaware of the metabolic response that food truly has on the body. The purpose of this article is to help educate both companies and consumers on what truly constitutes a “net carb” and how different “fiber” sources impact critical biological responses involving glucose and insulin.

Low-Carb Products

Walk around any fitness expo or even down the “snack bar” isle at your store – and you are bound to see numerous different types of low carb, high protein bars, cookies, candies, and everything in between. Protein bars are in the mainstream right now, and they seem to be everywhere from the local grocery store to the airport and even at gas stations. Companies have mastered the ability to create something that is pleasing to both the eye and the pallet (i.e. flavors including: chocolate chip cookie dough, birthday cake, chocolate brownie, peanut butter, etc.) yet provides an ample protein and a “low” carbohydrate count. If you attend any fitness or food-related expo, you are very aware that the booths with the longest lines are the ones that are sampling their latest protein bars or “high protein, low carb” treats (cookies, brownies, ice creams, etc.). Nonetheless, in a red ocean market that is flooded with these “healthier and high-protein” alternatives, what truly separates one product from another?

First off, there is taste. Consumers want to have their cake and eat it too. At the end of the day, if the sweet indulgence tastes more like a bar of chalk, then there is a high-probability that consumers likely will not be running out to the stores to buy it. In my opinion, most of the companies have nailed this aspect down to some degree. A majority of bars, cookies, or other low carb snacks that I have experienced actually taste really good. However, even if a product can meet the consumer standards with respect to taste and quality, the true separation occurs at the level of “fiber” source. The buzz words “high fiber” and low net carbs are exploding in today’s society. Thus, companies are attempting to find ways in which they can add fiber to their products thereby boosting their nutritional profile and simultaneously decreasing the amount of net carbs. This now prompts question, are all fiber sources nutritionally the same and if not, what does this mean for the consumer?

What is Fiber


Dietary fiber refers to the nutrients in the diet that are not digested by gastrointestinal enzymes. While “true fibers” are digested, they are not digested in our small intestine (like normal carbohydrates), but rather they are digested (fermented) by bacteria in our large intestine. “True fibers” should only be digested by the bacteria in our large intestine. Referring back to our previous example regarding the fitness expos, you can certainly “smell” and often experience, which high fiber bars have at least some sort of “true” fiber based on the fermentation and digestion.

There are two general categories of dietary fiber: soluble and insoluble. For more information on these check out our article on fiber. Most foods that we consume contain both soluble and insoluble fibers. As a society, we understand the importance of fiber including the benefits related to: lowering body fat, decreasing prevalence of diabetes, improving insulin sensitivity, decreasing the risk of heart disease and increasing satiety as well as the beneficial (“good”) bacteria in our digestive system (1). Unfortunately, less than 5 % of Americans actually meet the 30 gram per day recommended intake. To help increase fiber consumption, an increasing number of companies have developed a host of delicious, low carb, high fiber treats. Despite this, it is important to understand how our bodies biologically process two of the most common “fibers” on the market that are used in these “treats”: Isomaltooligosaccharides (IMOs) and Soluble Corn Fiber (SCF).

Isomaltooligosaccharides (IMOs): A “True Fiber?”

In most cases, if you grab a low carb snack at random from the grocery store shelf and look at the label, a common nutrient profile contains 15-25 grams of carbohydrates, yet 10-20 grams of those are from “fiber.” The result is 5-10 grams of net carbs right? Not so fast… now, if a Type I diabetic were to consume that bar, cookie or brownie with the 5-10 grams of net carbs, there should not be a need for insulin since theoretically, there is no glucose or insulin response. Unfortunately, theory and outcome do not always match.


IMOs can be made in several ways, but they are primarily derived from the sugar called maltose. IMO is promoted as a prebiotic, dietary, fiber with a light sweetness profile. Its functional properties (moisture retention, low viscosity) make it well-suited for nutrition bars, cookies, candies, and everything in between. In order to fully understand IMOs and how the body processes them, we first need to understand how starches are digested in the body. Initially, starch digestion begins in the small intestine with the enzyme called α-amylase (salivary and pancreatic). Alpha-amylase breaks these glucose polymers into much shorter chains, called oligosaccharides which are composed of anywhere between 2 to approximately 10 glucose units. Following this, specific enzymes on the brush border of the small intestine break down these oligosaccharides even further into individual glucose units which are then absorbed.

One of the most common disaccharides (two monosaccharides joined together) is maltose. Maltose is generated when two glucose ( what your blood “sugar” is) molecules are linked to one another by an α-1,4 chemical bond (1st carbon is bound to the 4th carbon, therefore making it easily digestible). The type of bond involved in saccharide linkage is critical as it determines its ability to become hydrolyzed by the enzymes we described above. As such, the α-1,4 chemical bond, as listed in the above example (maltose), has the ability to become hydrolyzed.

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, we could change the bonds from α-1,4 to α-1,6 and thereby, making it more digestive resistant by the enzymes in the pancreas 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 digested by certain enzymes on the brush border of the small intestine (2). Though the α-1,6 bond is slower digesting when compared to the α-1,4 bonds, 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.”

IMO Syrups in Products


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 (35), 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.

One of the first studies to examine IMO syrups (2) had 6 subjects consume 25 grams of IMO syrup. These researchers found that glucose levels increased from 109 mg/dL pre-ingestion to a peak of 136 mg/dL at 30 min post-ingestion. Additionally, insulin rose to nearly parallel with that of glucose from 4.8 μU/mL pre-ingestion to nearly 32 μU/mL at 30 min post-ingestion. These values clearly indicate that some digestion is occurring. Furthermore, these researchers found that IMO was about 83% as digestible as maltose under resting conditions and about 69% as digestible after the exercise period. Taken together, this suggests that a large majority of the carbohydrate in the IMO syrup was, in fact, digested, absorbed, and metabolized.

Lastly, one of the advertised benefits of IMO is possible prebiotic activity. Prebiotics are critical as they feed the healthy (“good”) bacteria in our digestive system. 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 “good” bacteria. The challenge, however, is that both inulin and FOS, due to their rapid digestibility by “good” 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, as you recall from above, that 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.

Is Soluble Corn Fiber (SCF) a “True Fiber?”

The increased awareness regarding the importance of fiber, in addition to their distinct metabolic effects, have resulted in a surge of companies switching to an alternative fiber known as Soluble Corn Fiber (SCF). Interestingly, Soluble Corn Fiber has been available on the US market since 2007 and is used in foods and beverages across the Americas, Europe and Southeast Asia. SCF is produced through an extensive process: To begin, corn syrup is exposed to a suite of enzymes for at least 48 hours; some of these enzymes are found in the brush border of your small intestine and pancreas (5). Notably, a large majority of the corn syrup contains easily digestible carbohydrates; however, a small portion is, in fact, not digestible. At the end of this enzymatic exposure, a stream of digestion-resistant carbohydrates remains and is subsequently filtered several times. The resulting product is a “true fiber” that contains a mixture of α 1-6, α 1-4, α 1-2, and α 1-3 glucosidic linkages, which, as mentioned above, contribute to its low digestibility.

Both animal and human studies have shown that SCF resists digestion in the small intestine and passes into the large intestine where it is fermented (6,7) The glycemic index for SCF is extremely low and is approximately 25 (7).


One study compared the glycemic response of SCF to the glycemic response of glucose in 12, healthy, adults during a randomized, controlled, crossover, study (7). 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.

Taken together, the above studies prompt the question: “What is the prebiotic activity of SCF?” If it is a “true” fiber per our definitions above, then SCF should have a beneficial effect on gut bacteria. A study performed in 24 adolescents noted an increase in beneficial bacteria (e.g. Bacteroides, Butyricicoccus, Oscillibacter and Dialister). Furthermore, this was also correlated with an increase in calcium absorption upon the consumption of 12 grams of SCF per day for three weeks (8). An additional study which administered 8, 14, and 21 grams of SCF over 14 days found that good bacteria (e.g. bifidobacteria) increased and peaked at 8 grams per day. This value is nearly identical to inulin, which is considered the “gold standard.” Despite its nearly parallel effects to inulin at 8 grams/day, research has demonstrated that SCF is 3-4 times more tolerable than inulin due to its slower rate of digestibility by the gut bacteria! In fact, 26 grams of SCF barely increased GI symptoms relative to a placebo!


How Do Soluble Corn Fiber and IMOs Compare to One Another?

To date, no studies have directly compared SCF and IMOs head to head. Fortunately, here at the Applied Science and Performance Institute (ASPI), our passion is to test these ideas on site and report our results directly to you in real time! To tackle this question head-on, we have conducted a pilot experiment in our lab. Below are three key variables we looked at:

  1. Blood glucose response of SCF vs. IMOs.
  2. Insulin Response of SCF vs. IMOs.
  3. Breath Hydrogen* response of both SCF and IMOs.

*Breath Hydrogen is an assay that indicates in “real-time” whether or not a particular nutrient is being digested. Upon consumption of a standard carbohydrate (e.g. rice), you can see that it is broken down in the small intestine and subsequently, blood glucose rises. If the carbohydrate is not digested in the small intestine, it moves into the large intestine. This indicates that it is a “true fiber.” In the large intestine, bacteria digests the fiber through a process called “fermentation.” In doing so, the bacteria produce hydrogen ions (H+) that circulate into the blood stream, through our lungs, and consequently, we exhale them outward.
In our pilot experiment here at ASPI, we monitored a subject consuming either IMOs or SCF respectively and then tracked the variables listed above for 150 minutes following consumption.

Below are the Preliminary Results:




As observed by the graphs above, in contrast to the IMOs, in which blood glucose rapidly increased to 125 mg/dL, SCF did not elicit any blood glucose response. In addition, while insulin was elevated during the IMO condition, it tended to actually go down in the SCF condition! Despite the results from the blood glucose and insulin responses, the Breath Hydrogen assay will distinguish which is a “true fiber.” Our data below clearly indicates that SCF, indeed, passes into the large intestine as indicated by the large breath hydrogen response. In stark contrast, IMOs do not.


Conclusion on Net Carbs and Fiber

Taken together, previous research in combination with our lab’s current preliminary research, we may argue that IMOs should not be classified as a “true fiber.” Rather, IMOs should be looked at as a very low glycemic carbohydrate source, much like steel cut oats. In essence, if you see a low carb snack that has 20 grams of IMO fiber, it is likely that approximately 16 grams of this fiber will act as a slow digesting carb and 4 grams will act as an indigestible fiber. Those who are on a Ketogenic Diet should be aware of these fibers and proceed with caution when consuming them in large amounts, as they could raise both blood glucose and insulin levels. A more Ketogenic-friendly fiber is SCF which has been demonstrated to act as more of a “true fiber.” Additionally, SCF is very tolerable in the gut despite its profound prebiotic activity. It is important to keep in mind that everyone is metabolically different, so if you are consuming food items with these fibers in them, be sure to monitor blood glucose and ketone readings to find how each of these fibers personally affect you.


  1. Slavin, J. (2013). Fiber and prebiotics: mechanisms and health benefits. Nutrients, 5(4), 1417-1435.
  2. Kohmoto, T., Tsuji, K., Kaneko, T., Shiota, M., Fukui, F., Takaku, H., ... & KOBAYASHI, S. (1992). Metabolism of 13C-isomaltooligosaccharides in healthy men. Bioscience, biotechnology, and biochemistry, 56(6), 937-940.
  3. Kohmoto, T., FUKUI, F., TAKAKU, H., MACHIDA, Y., Masaaki, A. R. A. I., & MiTSUOKA, T. (1988). Effect of isomalto-oligosaccharides on human fecal flora. Bifidobacteria and Microflora, 7(2), 61-69.
  4. Oku, T., & Nakamura, S. (2003). Comparison of digestibility and breath hydrogen gas excretion of fructo-oligosaccharide, galactosyl-sucrose, and isomalto-oligosaccharide in healthy human subjects. European journal of clinical nutrition, 57(9), 1150-1156.
  5. Timm, D. A., Thomas, W., Boileau, T. W., Williamson-Hughes, P. S., & Slavin, J. L. (2013). Polydextrose and soluble corn fiber increase five-day fecal wet weight in healthy men and women. The Journal of nutrition, 143(4), 473-478.
  6. Cervantes-Pahm, S. K., Kim, B. G., & Stein, H. H. (2009). Digestible energy in resistant starch and dietary fiber sources fed to pigs. J. Anim. Sci, 87, 547.
  7. Kendall, C. W., Esfahani, A., Hoffman, A. J., Evans, A., Sanders, L. M., Josse, A. R., ... & Potter, S. M. (2008). Effect of novel maize-based dietary fibers on postprandial glycemia and insulinemia. Journal of the American College of Nutrition, 27(6), 711-718.
  8. Whisner, C. M., Martin, B. R., Nakatsu, C. H., McCabe, G. P., McCabe, L. D., Peacock, M., & Weaver, C. M. (2014). Soluble maize fibre affects short-term calcium absorption in adolescent boys and girls: a randomised controlled trial using dual stable isotopic tracers. British Journal of Nutrition, 112(03), 446-456.

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