We previously introduced the idea that a fiber’s ability to dissolve in water is a common and helpful way to distinguish between different fiber types. An ingredient’s ability to dissolve in water, in general, is meaningful because it informs a host of other critical information, such as its health impact and its behavior in a food matrix. Like all categorization systems, there are exceptions to using solubility as a differentiator, and occasionally other categorization systems are essential complements.
This section delves more deeply into the soluble/insoluble fiber classification system. We review the differences between soluble and insoluble fibers and explore examples of each type. We also look at two other important ways of grouping fibers together: Viscosity and fermentability. Finally, for each fiber type reviewed, we discuss how and why fibers with those particular properties interact with our health.
Recall that dietary fibers are technically carbohydrates. The term “carbohydrate” is broad and identifies simple sugars alone or when bound together in long molecular chains. When our bodies can break down those long chains (digested and metabolized), those carbohydrates are starches. When our bodies cannot break down those chains, those carbohydrates are dietary fibers. Resistant starches are exceptions to this categorization system as they are structurally similar to starches but considered fibers as they are poorly digested. The Codex Alimentarius and several countries define dietary fiber as carbohydrates that are neither digested nor absorbed in the small intestine and are more than three units of sugar in length.
Because our body cannot digest fiber, it enters our large intestine intact. Some fibers are fermented in the large intestine, while others pass through as stool. A particular fiber’s fate depends on three primary characteristics: solubility, viscosity, and fermentability. We’ll expand on each of these terms shortly!
Even though (or, perhaps, because) dietary fiber is resistant to digestion, it plays unique and vital roles in human health. Some of these roles include:
Soluble fibers dissolve in water, while insoluble fibers do not. To visualize this, imagine filling two glasses with water. Add sugar to one glass and stir; add sand to the other and stir. We all know the result: The sugar will fully dissolve while the sand will swirl around and ultimately settle to the bottom. The sugar is soluble, and the sand is insoluble.
Whether a fiber dissolves in water has important implications for how our bodies manipulate it. Typically, if fiber does not dissolve in water (is insoluble), our gut microbiome will have a more difficult time consuming (fermenting) that fiber. Insoluble fibers tend to significantly influence stool regularity but can do so in opposing ways. Wheat bran, for example, is a commonly used insoluble fiber ingredient that acts as a laxative, but when ground to a very fine powder, it behaves as a constipator. Insoluble fibers increase stool bulking and shorten transit time, which dilutes toxins and results in their rapid elimination from the body. Oat hulls, which AgriFiber uses to make its insoluble oat fiber MFO, are also an effective stool bulking and softening agent.
Soluble fibers are more accessible to our gut bacteria. If a soluble fiber selectively supports health-beneficial gut microbes, it is said to be prebiotic, and will evoke some benefits associated with good gut health (we cover the science behind prebiotics in more detail here!). Some soluble fibers confer positive health outcomes unrelated to interactions with our gut microbes. Beta-glucans are a fantastic example of this, with research documenting their ability to reduce LDL (“bad”) cholesterol levels. AgriFiber’s soluble fiber is both a prebiotic and a class of fiber (called arabinoxylan) shown to positively regulate blood glucose levels.
Soluble and insoluble fiber are often found in the same foods at different ratios—grains, vegetables, fruits, legumes, and nuts and seeds are the most common sources.
Formally, viscosity is defined as a fluid’s resistance to flow. For our purposes, we can think of viscosity as how thick or sticky a liquid is. Intuitively, water has a very low viscosity, maple syrup and honey are thicker and have a higher viscosity, but not as thick as liquid cement, which has an extremely high viscosity.
Whether or not, and to what extent, a fibrous solution is viscous is another common way food scientists and dietitians distinguish amongst different types of fiber. Microcrystalline cellulose isn’t viscous, arabinoxylans are moderately viscous (like AgriFiber SFC), and pectin is highly viscous.
Solubility and viscosity are imperfectly correlated. In general, insoluble fibers are less viscous than their soluble counterparts. Going back to our analogy of sand and sugar in water, we know that if we keep adding sugar, the water will eventually turn into syrup, but adding more sand into the glass doesn’t make the water any thicker. An important exception to this rule of thumb relates to the size of the fiber chain. As a reminder, fibers are chains of sugar molecules. Despite their solubility, some short-chain soluble fibers do not contribute viscosity. This is notably the case for two synthetic fibers—fructooligosaccharides (FOS) and galactooligosaccharides (GOS). AgriFiber’s insoluble corn fiber, MFC, is another anomaly: Our patented process produces an insoluble fiber that is extremely efficient at adding viscosity to a solution. The chemistry behind this is beyond the scope of this article, but if you’re curious, please reach out—we love talking fiber!
As food travels through our intestines, its viscosity impacts our body’s interaction with it. Beta-glucan’s viscosity profile is thought to influence its clinically proven ability to lower cholesterol. It is also hypothesized that additional viscosity in the gut slows transit time and increases satiety.
Fermentability dictates whether fiber is consumed by the bacterial populations in our gut (for more information about the science of prebiotics and the microbiome, click here!). Like viscosity, solubility and fermentability are imperfectly correlated. More fermentable fibers are soluble than insoluble, but exceptions exist. Methylcellulose, a type of synthetic fiber, is soluble but not fermentable.
Fiber must be fermentable to be considered prebiotic, and the ability for fiber to ferment (to be consumed by gut microbes) is required to take advantage of the health benefits that emerge from the gut microbiome. Inulin is an example of a prebiotic fiber that is highly fermentable. Arabinoxylans, like AgriFiber SFC, are also fermentable.
Beware: Too much of a good thing isn’t always good. Inulin is rapidly fermented in the gut, resulting in bloating and gastric distress. AgriFiber SFC and other structurally complex fibers are fermented more slowly, which means they provide all the health benefits of inulin without the stomachache.
In short: No. Both insoluble and soluble, viscous and non-viscous, and fermentable and non-fermentable fibers have pros and cons and can be beneficial in the right circumstances.
The USDA recommends consuming at least 14 grams of total dietary fiber for every 1000 calories consumed to support overall health and reduce the risk of several chronic diseases such as colon cancer, hypertension, and diverticulitis—this recommendation is not dependent on the type of fiber being consumed. Of course, in nature, these types of fiber are not isolated but intermingled, with every kind of fiber reinforcing others.
Understanding the different fiber classification systems can help us become more educated readers of food labels and better understand how different foods impact our health. Remember that these classification systems aren’t perfect—if a food claims to have “added prebiotics,” don’t assume those prebiotics are soluble. Of course, if you have any questions, we are here to help.
The information contained herein is considered to be accurate. However, no warranty is expressed or implied regarding the accuracy of the information, the results to be obtained from the use thereof, or that any such use will not infringe upon any intellectual property. We cannot anticipate all conditions under which this information may be used. We accept no responsibility for results obtained by the application of this information. Please note that none of the information herein is confirmed by any governmental agency, unless otherwise specified.
We previously introduced the idea that a fiber’s ability to dissolve in water is a common and helpful way to distinguish between different fiber types. An ingredient’s ability to dissolve in water, in general, is meaningful because it informs a host of other critical information, such as its health impact and its behavior in a food matrix. Like all categorization systems, there are exceptions to using solubility as a differentiator, and occasionally other categorization systems are essential complements.
This section delves more deeply into the soluble/insoluble fiber classification system. We review the differences between soluble and insoluble fibers and explore examples of each type. We also look at two other important ways of grouping fibers together: Viscosity and fermentability. Finally, for each fiber type reviewed, we discuss how and why fibers with those particular properties interact with our health.
Recall that dietary fibers are technically carbohydrates. The term “carbohydrate” is broad and identifies simple sugars alone or when bound together in long molecular chains. When our bodies can break down those long chains (digested and metabolized), those carbohydrates are starches. When our bodies cannot break down those chains, those carbohydrates are dietary fibers. Resistant starches are exceptions to this categorization system as they are structurally similar to starches but considered fibers as they are poorly digested. The Codex Alimentarius and several countries define dietary fiber as carbohydrates that are neither digested nor absorbed in the small intestine and are more than three units of sugar in length.
Because our body cannot digest fiber, it enters our large intestine intact. Some fibers are fermented in the large intestine, while others pass through as stool. A particular fiber’s fate depends on three primary characteristics: solubility, viscosity, and fermentability. We’ll expand on each of these terms shortly!
Even though (or, perhaps, because) dietary fiber is resistant to digestion, it plays unique and vital roles in human health. Some of these roles include:
Soluble fibers dissolve in water, while insoluble fibers do not. To visualize this, imagine filling two glasses with water. Add sugar to one glass and stir; add sand to the other and stir. We all know the result: The sugar will fully dissolve while the sand will swirl around and ultimately settle to the bottom. The sugar is soluble, and the sand is insoluble.
Whether a fiber dissolves in water has important implications for how our bodies manipulate it. Typically, if fiber does not dissolve in water (is insoluble), our gut microbiome will have a more difficult time consuming (fermenting) that fiber. Insoluble fibers tend to significantly influence stool regularity but can do so in opposing ways. Wheat bran, for example, is a commonly used insoluble fiber ingredient that acts as a laxative, but when ground to a very fine powder, it behaves as a constipator. Insoluble fibers increase stool bulking and shorten transit time, which dilutes toxins and results in their rapid elimination from the body. Oat hulls, which AgriFiber uses to make its insoluble oat fiber MFO, are also an effective stool bulking and softening agent.
Soluble fibers are more accessible to our gut bacteria. If a soluble fiber selectively supports health-beneficial gut microbes, it is said to be prebiotic, and will evoke some benefits associated with good gut health (we cover the science behind prebiotics in more detail here!). Some soluble fibers confer positive health outcomes unrelated to interactions with our gut microbes. Beta-glucans are a fantastic example of this, with research documenting their ability to reduce LDL (“bad”) cholesterol levels. AgriFiber’s soluble fiber is both a prebiotic and a class of fiber (called arabinoxylan) shown to positively regulate blood glucose levels.
Soluble and insoluble fiber are often found in the same foods at different ratios—grains, vegetables, fruits, legumes, and nuts and seeds are the most common sources.
Formally, viscosity is defined as a fluid’s resistance to flow. For our purposes, we can think of viscosity as how thick or sticky a liquid is. Intuitively, water has a very low viscosity, maple syrup and honey are thicker and have a higher viscosity, but not as thick as liquid cement, which has an extremely high viscosity.
Whether or not, and to what extent, a fibrous solution is viscous is another common way food scientists and dietitians distinguish amongst different types of fiber. Microcrystalline cellulose isn’t viscous, arabinoxylans are moderately viscous (like AgriFiber SFC), and pectin is highly viscous.
Solubility and viscosity are imperfectly correlated. In general, insoluble fibers are less viscous than their soluble counterparts. Going back to our analogy of sand and sugar in water, we know that if we keep adding sugar, the water will eventually turn into syrup, but adding more sand into the glass doesn’t make the water any thicker. An important exception to this rule of thumb relates to the size of the fiber chain. As a reminder, fibers are chains of sugar molecules. Despite their solubility, some short-chain soluble fibers do not contribute viscosity. This is notably the case for two synthetic fibers—fructooligosaccharides (FOS) and galactooligosaccharides (GOS). AgriFiber’s insoluble corn fiber, MFC, is another anomaly: Our patented process produces an insoluble fiber that is extremely efficient at adding viscosity to a solution. The chemistry behind this is beyond the scope of this article, but if you’re curious, please reach out—we love talking fiber!
As food travels through our intestines, its viscosity impacts our body’s interaction with it. Beta-glucan’s viscosity profile is thought to influence its clinically proven ability to lower cholesterol. It is also hypothesized that additional viscosity in the gut slows transit time and increases satiety.
Fermentability dictates whether fiber is consumed by the bacterial populations in our gut (for more information about the science of prebiotics and the microbiome, click here!). Like viscosity, solubility and fermentability are imperfectly correlated. More fermentable fibers are soluble than insoluble, but exceptions exist. Methylcellulose, a type of synthetic fiber, is soluble but not fermentable.
Fiber must be fermentable to be considered prebiotic, and the ability for fiber to ferment (to be consumed by gut microbes) is required to take advantage of the health benefits that emerge from the gut microbiome. Inulin is an example of a prebiotic fiber that is highly fermentable. Arabinoxylans, like AgriFiber SFC, are also fermentable.
Beware: Too much of a good thing isn’t always good. Inulin is rapidly fermented in the gut, resulting in bloating and gastric distress. AgriFiber SFC and other structurally complex fibers are fermented more slowly, which means they provide all the health benefits of inulin without the stomachache.
In short: No. Both insoluble and soluble, viscous and non-viscous, and fermentable and non-fermentable fibers have pros and cons and can be beneficial in the right circumstances.
The USDA recommends consuming at least 14 grams of total dietary fiber for every 1000 calories consumed to support overall health and reduce the risk of several chronic diseases such as colon cancer, hypertension, and diverticulitis—this recommendation is not dependent on the type of fiber being consumed. Of course, in nature, these types of fiber are not isolated but intermingled, with every kind of fiber reinforcing others.
Understanding the different fiber classification systems can help us become more educated readers of food labels and better understand how different foods impact our health. Remember that these classification systems aren’t perfect—if a food claims to have “added prebiotics,” don’t assume those prebiotics are soluble. Of course, if you have any questions, we are here to help.
The information contained herein is considered to be accurate. However, no warranty is expressed or implied regarding the accuracy of the information, the results to be obtained from the use thereof, or that any such use will not infringe upon any intellectual property. We cannot anticipate all conditions under which this information may be used. We accept no responsibility for results obtained by the application of this information. Please note that none of the information herein is confirmed by any governmental agency, unless otherwise specified.