Let’s face it: for some people, following a low-carb or ketogenic diet is easy. They ditch the bread, pasta, potatoes, candy, and other starchy and sugary foods and never look back. They’re totally content sticking with fatty proteins, non-starchy vegetables, dairy foods, and nuts and seeds. For others, though, including keto-friendly sweet treats can make it easier to stick to a lower-carb way of eating for the long term. But can you trust the labels? What if some of the ingredients aren’t quite so keto-friendly? What about the added fibers used to lend sweetness and bulk to foods marketed as “keto”? Are these totally benign, or could they be interfering with you reaching your goals with this way of eating?

First Rule of Keto Club

The first rule of tweaking your diet is: “If it ain’t broke, don’t fix it.” So, however you’ve constructed your keto or low-carb diet so far, if you’re happy with the results you’re getting, keep on truckin’! If you’re satisfied with your physical and mental health, and your energy levels (or as ancestral health authority Robb Wolf often says, “How you look, feel, and perform”), then you don’t need to change anything. If you’ve got a few favorite go-to keto nutrition authorities and they’ve said something that makes you want to change something about your diet, there’s no harm in experimenting. But you are the ultimate authority on your body, and you know best how something affects you.

But what if something you’ve been eating isn’t working so well for you? If keto bars, cereals, cookies, and the like are a regular part of your diet, could the added fibers these typically contain be a problem for you?

Why Added Fibers?

There are a few different reasons why food products marketed as “keto” often contain added fibers. For starters, these various fibers contribute bulk and texture, so in some cases they’re a fundamental part of the physical structure of the product. Another reason is that some of them have a sweet taste, so they’re also used as sweetening agents. (For example, fructo-oligosaccharide, a prebiotic fiber, is about half as sweet as regular sugar.¹) And third, food manufacturers have long since caught on that many people doing keto diets go by “net carbs” rather than total carbs when counting their carbs for the day. (Net carbs is the total grams of carbohydrate in a serving of food minus grams of fiber and sugar alcohol.) The higher the fiber in a food, the lower the net carb count—and the more attractive to the growing numbers of customers looking for just these kinds of things.

Okay, sounds good so far. But are there any drawbacks to cramming a bunch of extra fiber into a food product? Is there anything a savvy keto dieter should keep in mind before making these things a staple of their diet?

Two main concerns come to mind with regard to added fibers used in keto food products: effects on blood sugar, and effects on the gastrointestinal (GI) system. We’ll address both of these, but let’s start with some general info on fiber.

The US Food and Drug Administration (FDA) provided its first official definition of dietary fiber in 2016, in the rules and regulations regarding nutrition and supplement facts labels. They specified that dietary fiber is a nondigestible carbohydrate in both soluble and insoluble forms, and includes both the fibers intrinsic to whole, intact foods as well as isolated and synthetic fibers that are added during manufacturing. The FDA identified at least 15 of these isolated and synthetic nondigestible carbohydrates, many of which you’ve probably seen in ingredient lists on keto food products²:

• cellulose
• pectin
• guar gum
• locust bean gum
• hydroxypropylmethylcellulose
• β-glucan
• psyllium husk
• mixed plant cell wall fibers
• arabinoxylan
• alginate, inulin, and inulin-type fructans (fructooligosaccharides)
• high-amylose starch/soluble corn fiber (resistant starch 2, RS2)
• galactooligosaccharides
• polydextrose
• resistant maltodextrin/dextrin

Effects of Added Fiber on Blood Sugar

It’s important to realize that here we’re focusing specifically on fibers that are added as ingredients in food products, and not the fiber that’s intrinsic to whole food. For example, inulin or soluble corn fiber as opposed to the fiber that occurs naturally as part of the food matrix in things like broccoli, almonds, or blackberries. It’s possible that added fibers might not have the same effect as fibers that are intrinsic to a food. One of the beneficial effects of dietary fiber is that it slows the digestion of food, which may help lessen the glycemic impact of a high-carbohydrate meal (how much and how quickly it raises blood sugar). But think about it: flaxseeds are high in fiber, but how likely is it that taking a tablespoon of ground flaxseeds along with a glass of orange juice is going to make much of a difference in someone’s blood glucose?

Back to that in a bit. First, let’s take a closer look at fiber overall. Dietary fiber is a type of carbohydrate that is believed to have little to no impact on blood sugar and insulin because it’s not digestible and therefore doesn’t enter the bloodstream. The reason humans can’t digest fiber is because of the chemical bonds that make up the molecular structure of the different kinds of fiber. Our digestive enzymes are very selective and act only on certain kinds of food molecules or chemical bonds. For example, the enzyme sucrase splits apart sucrose (“table sugar”) into glucose and fructose. The lactase enzyme breaks apart lactose (“milk sugar”) into glucose and galactose. We don’t have digestive enzymes capable of breaking the bonds in most dietary fibers, so fiber passes through the GI tract to the large intestine.^1,3

If fiber isn’t absorbed, then it should have zero impact whatsoever on blood glucose. Turns out this is a bit difficult to verify in humans. Research is surprisingly sparse on individual responses to these added fibers, and research on the effect of these fibers specifically in the context of a low-carb or ketogenic diet is practically nonexistent. Most studies involve adding fiber to high- and moderate-carb diets or individual meals, and they’re intended to see if the added fiber helps reduce the glycemic impact or improves biomarkers related to type 2 diabetes or insulin resistance.^4-9 (They usually do on the whole, although findings are mixed.^10,11) Results from those studies don’t tell us a whole lot about the effect of adding fiber to low-carb or ketogenic meals, where we’re more interested in seeing if blood glucose is raised.

To say this another way, research findings from people eating high-carb diets might not be equally applicable to people eating very low-carb diets. If fiber is filling/satiating and helps to slow the absorption of glucose, then some of the purported “benefits” of fiber—feeling full sooner, eating less food, and having a smaller blood glucose excursion after a meal—are not necessarily positive aspects of fiber, itself, but rather, the fact that these effects may be mitigating the negative effects of eating a higher carb diet! (Particularly among people who already have diabetes or pre-diabetes.)

The authors of a 2020 review looking at the effects of isolated and synthetic fibers on markers of metabolic diseases concluded that for these added fibers to have an impact powerful enough to be relevant, the doses would have to be relatively high over a long period of time, and most importantly, the best effect would come when these are used instead of, rather than in addition to, digestible carbohydrates.¹⁰ (Interestingly, in things like low-carb breads and bars, that’s precisely how these are used.)

The best way to see if added fibers are affecting your blood glucose is simply to test. You can use a glucometer to check your blood sugar before consuming something that contains these fibers, and then test again, maybe at 15 and 30 minutes after eating the test food, and then at the 1- and 2-hour marks. This is easier said than done, though. As mentioned earlier, these added fibers are consumed in things like keto bars, cookies, ice cream, cereal, chocolate, etc.—items that also typically contain sugar alcohols. So if you did see a significant change in blood sugar, it would be difficult to identify what the responsible factor was: the fibers or the sugar alcohols. With the exploding popularity of keto, though, there are food options available now that are rich in added fibers but have no sugar alcohols at all or are much lower in them, like keto breads, crackers, and wraps.

All Fibers are Not Created Equal

In theory, fiber is not broken down and absorbed at all. But the more food scientists tinker with things, the more variable people’s responses might be. For example, you might see “resistant tapioca starch” on an ingredient label. This is created by manipulating the starch molecule in order to make it “resistant” to digestion so that it has a much lower impact on blood sugar. Note that the impact is lower, not nonexistent. The many different types of fibers vary in the amount of digestible carbohydrates they contain.¹² One study determined that resistant maltodextrin from tapioca starch was about 86% fiber but had a glycemic index of 59 in healthy subjects.¹³ (This is considered a “moderate” glycemic index.) Subjects were given a large dose of the resistant tapioca starch—larger than would typically be included in a food product—but even so, as much as 30-70% of “resistant” starch can be metabolized, so for some people, even a relatively small amount of this ingredient can affect blood sugar.^13,14 Like other fibers, resistant starch might have its best effect on lowering post-meal glucose and insulin levels when it replaces conventional digestible starch, rather than being added to high-starch meals as a supplement.¹⁵

Plus, there’s a lot—a lot—of individual variability in glycemic responses to food, including resistant starch. In a 2015 study conducted in Israel, 800 subjects were observed for a week, during which they consumed nearly 47,000 meals in total.¹⁶ They wore continuous glucose monitors, which provided over 1.5 million glucose readings over a total of 130,000 hours. The results showed that the very same foods led to wildly different glycemic responses among individuals. This raises serious questions about the validity of glycemic index and glycemic load. Obviously, these are not fixed values if people have such different responses. How quickly and how high someone’s blood sugar rises after consuming a food apparently is not solely determined by the properties of the food, itself, but rather, by how an individual’s unique genetics and biochemistry prime them to handle that food.¹⁷ (Not to mention a host of lifestyle factors, such as sleep, that affect carbohydrate tolerance.¹⁸) One person’s low-glycemic high-fiber bran muffin could be another’s sugar-frosted breakfast cereal. So if someone else’s blood sugar barely moves in response to resistant starch or some other fiber, that doesn’t mean yours won’t.

Gastrointestinal Effects

Moving on from blood sugar, what about the GI effects of added fibers? It’s seen as a good thing that these fibers are not digested or are only partially absorbed in the small intestine, but for this very same reason, they can have negative effects lower down in the GI tract. The low digestibility of these fibers means they have little to no impact on blood glucose in most people. But because they’re not absorbed, they are passed along to the colon, where they serve as a food source for the bacteria that inhabit the gut. As mentioned earlier, humans lack the digestive enzymes to break apart these fibers, but our colonic bacteria break them down via fermentation. The result, for many people, is bloating, gas, and flatulence.¹⁹ Whether these are positives or negatives depends on your own level of discomfort, and whether you’re regularly in close proximity to other people!

Fibers also contribute to fecal bulking—that is, to a larger mass of stool.²⁰ Conventional thinking says this is a good remedy for constipation, but larger, bulkier stools aren’t automatically a good thing. For example, if you have constipation, larger, bulkier stools are the last thing you want. (If your stool is already stagnating and not passing through your colon in a timely manner, why would you want there to be even more of it?) An intriguing study in subjects with constipation of unknown cause showed that following a zero-fiber diet led to increased frequency of bowel movements and complete resolution of several symptoms, like bloating and straining.²¹ A diet low in fiber might contribute to constipation for some people, but for others, increasing dietary fiber can make things worse.²²

The news isn’t all bad, though. There might be some beneficial effects for colonic and overall health from consuming these fibers. The fermentation process generates short-chain fatty acids (SCFAs), which lowers the pH of the colon (makes it more acidic) and may help reduce the risk for colon cancer, improve mineral absorption, and reduce the growth of harmful bacterial strains in the colon.^19,23 Plus, the SCFAs produced might have effects of their own, including being an energy source for colon cells, and having beneficial effects on the brain and central nervous system.^23,24 Some of the supposed benefits of a higher fiber diet might be attributable to the butyrate rather than to the fiber, itself. (If “butyrate” reminds you of beta-hydroxybutyrate, one of the ketones your body produces on a ketogenic diet, pat yourself on the back! They are structurally similar and while they’re not the same molecule, they have some beneficial properties in common and there might be synergies that occur when the body has both.^26,27 Small amounts of butyrate are also found in butter.)

Summary

If you’re concerned about the effect foods with a lot of added fiber are having on your blood sugar, get a glucometer (available at drugstores or online in the US), and test. This won’t give you any data on insulin, but it’s better than nothing, and it can give you helpful information to go on when deciding which kinds of fiber you’re comfortable including in your diet on a regular basis, and which ones you might prefer to avoid altogether or to have only once in a while.

And go with your gut, literally and figuratively. If fiber-fortified keto-friendly foods cause you physical discomfort, stay away from them! Don’t be bamboozled by slick marketing hype when your own body is giving you the clearest feedback you need.

References:

1. Punit Kumar, Kashyap Kumar Dubey (2019). Chapter 10 – Current Perspectives and Future Strategies for Fructooligosaccharides Production Through Membrane Bioreactor. Editor(s): Pratyoosh Shukla. Applied Microbiology and Bioengineering, Academic Press. p.185-202. ISBN 9780128154076.
2. Alexander C, Swanson KS, Fahey GC, Garleb KA. Perspective: Physiologic Importance of Short-Chain Fatty Acids from Nondigestible Carbohydrate Fermentation. Adv Nutr. 2019;10(4):576-589.
3. Roberfroid M. Dietary fiber, inulin, and oligofructose: a review comparing their physiological effects. Crit Rev Food Sci Nutr. 1993;33(2):103-48. doi: 10.1080/10408399309527616. Erratum in: Crit Rev Food Sci Nutr 1993;33(6):553.
4. de Carvalho CM, de Paula TP, Viana LV, Machado VM, de Almeida JC, Azevedo MJ. Plasma glucose and insulin responses after consumption of breakfasts with different sources of soluble fiber in type 2 diabetes patients: a randomized crossover clinical trial. Am J Clin Nutr. 2017 Nov;106(5):1238-1245.
5. Kamalpour M, Ghalandari H, Nasrollahzadeh J. Short-Term Supplementation of a Moderate Carbohydrate Diet with Psyllium Reduces Fasting Plasma Insulin and Tumor Necrosis Factor-α in Patients with Type 2 Diabetes Mellitus. J Diet Suppl. 2018 Jul 4;15(4):507-515.
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