By: Dr. Ryan Lowery
We’ve all seen it on food labels: “Only 2 net carbs” or “Low net carbs.” But what does this truly mean? What are net carbs, and why does it matter? Are all net carbs created equal, or are we stretching those claims a bit too much? After reading through this article, I think you will agree 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 whom 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 actually has on the body. The purpose of this article is to help educate both companies and consumers on what accurately constitutes a net carb and how different fiber sources impact critical biological responses involving glucose and insulin.
Walk around any fitness expo, or even down the “snack bar” aisle of a grocery store, and you are bound to see many varieties 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 gas stations. Companies have mastered the ability to create something that is pleasing to both the eye and the pallet (i.e., flavors like chocolate chip cookie dough, birthday cake, chocolate brownie, peanut butter, etc.), yet provides ample protein while “low” in carbohydrates. 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 (i.e., market ran by competing industries) that is flooded with these “healthier and high-protein” alternatives, what truly separates one product from another?
First off, there’s the 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 will not be running out to buy it. In my opinion, most companies have nailed this aspect down to some degree. The majority of bars, cookies, or other low-carb snacks that I have tried 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 number of net carbs. This now prompts the 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 nutrients that are not digested by gastrointestinal enzymes. While true fibers are digested, they are not digested in the small intestine like normal carbohydrates, but rather are digested (fermented) by bacteria in the large intestine. True fibers should only be digested by the bacteria in the large intestine. Referring back to the previous example regarding the fitness expos, you can certainly “smell,” and often experience which high fiber bars have some “true” fiber based on the fermentation and digestion.
There are two general categories of dietary fiber: soluble and insoluble. Fibrous foods typically contain both soluble and insoluble fibers. As a society, we understand the importance of fiber, including the benefits related to lowering body fat, decreasing the prevalence of diabetes, improving insulin sensitivity, decreasing the risk of heart disease and increasing satiety, as well as the beneficial 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 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 around 20 grams of carbohydrates, yet maybe 15 of those grams are from “fiber.” The result is five grams of net carbs, right? Not so fast. . . if a Type I diabetic were to consume that bar, cookie, or brownie with the five grams of net carbs, there should not be a need for insulin since, theoretically, there is minimal glucose (blood sugar) entering the system from those five net carbs, which shouldn’t require an insulin response. Unfortunately, theory and outcome do not always match.
IMOs can be made in several ways, but they are primarily derived from a sugar called maltose. IMO is promoted as a prebiotic, dietary fiber with a light sweetness profile. Its functional properties (i.e., moisture retention, low viscosity) make it well-suited for nutrition bars, cookies, candies, and the like. In order to fully understand IMOs and how the body processes them, we first need to understand how starches are digested in the body. Starches, also known as polysaccharides, are long and sometimes branched chains of glucose molecules. Initially, starch digestion begins in the small intestine with an enzyme called α-amylase. A -amylase breaks these long glucose chains into much shorter chains, called oligosaccharides, which are composed of anywhere from two 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 (monosaccharides) which are then absorbed.
One of the most common disaccharides (two monosaccharides joined together) is maltose. Maltose is generated when two glucose molecules are linked to one another by an α-1,4 chemical bond (1st carbon is bound to the 4th carbon, 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 (broken down).
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.
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.
One of the first studies to examine IMO syrups (2) had six 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 30 min post-ingestion. Additionally, insulin rose to nearly parallel levels 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 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.
Is Soluble Corn Fiber (SCF) a True Fiber?
The increased awareness regarding the importance of fiber, in addition to its distinct metabolic effects, has resulted in a surge of companies switching to an alternative fiber known as soluble corn fiber (SCF). Interestingly, SCF 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: corn syrup is exposed to a suite of enzymes for at least 48 hours, some of which are found in the brush border of your small intestine, as well as the 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 (3, 5 ). The glycemic index for SCF is approximately 25, which is very low (3).
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.
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 correlated with an increase in calcium absorption upon the consumption of 12 grams of SCF per day for three weeks (7, 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 SCF 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 study in our lab.
Below are three key variables we looked at:
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 digest the fiber through a process called “fermentation.” In doing so, the bacteria produce hydrogen ions (H+) that circulate into the bloodstream, through our lungs, and is then exhaled outward.
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 (9). In addition, while insulin was elevated during the IMO condition, it actually tended to 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 research may argue that IMOs should not be classified as a “true fiber.” Rather, IMOs should be viewed 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 four 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 affects you.
4. Oku T, Nakamura S. Comparison of digestibility and breath hydrogen gas excretion of fructo-oligosaccharide, galactosyl-sucrose, and isomalto-oligosaccharide in healthy human subjects . Eur J Clin Nutr 2003;57:1150-1156.
7. Bouhnik Y, Raskine L, Simoneau G, Vicaut E, Neut C, Flourie B, Brouns F, Bornet F. The capacity of nondigestible carbohydrates to stimulate fecal bifidobacteria in healthy humans: a double-blind, randomized, placebo-controlled, parallel-group, dose-response relation study. Am J Clin Nutr 2004;80:1658-1664.
8. Kaneko T, Kohmoto T, Kikuchi H, Shiota M, Iino H, Mitsuoka T. Effects of isomaltooligosaccharides with different degrees of polymerization on human fecal bifidobacteria. Biosci Biotech Biochem 1994;58:2288-2290.
9. Lowery, R. P., Wilson, J. M., Barninger, A., Sharp, M. H., Irvin, C., Stefan, M., … & Wagner, R. (2018). The effects of soluble corn fibre and isomaltooligosacharides on blood glucose, insulin, digestion and fermentation in healthy young males and females. Journal of Insulin Resistance, 3(1), 6.