Dietary Fiber

Fiber is one of those nutrition quirks that most people do not fully understand, but know that it is “good”. Well, that is not necessarily false, but fiber, as with everything, has its drawbacks as much as it has its positives. In this article, we will educate ourselves on what fiber is exactly, what is its function, possible negatives, how much to take, and much more – as usual, we should walk away from this read (or watch, if you decide to watch the video instead) feeling far more educated on the topic.

What is fiber?

Fiber is characterized as the parts of a plant that are indigestible by the human gastrointestinal system; in simpler terms, it is an indigestible carbohydrate [1]. It is a non-starch, therefor non-glucose, polysaccharide (made up of multiple non-starch units) [2]. Due to this description, this is why fiber is most often found in plant related foods.

As you may be aware, there are two types of fiber – soluble and insoluble fiber.

Fiber Solubility

Soluble fiber is fiber that draws in water. Soluble fiber has the unique characteristic in that it, usually, swells as water and nutrients come in contact with it; the consequences of such a reaction are discussed in the next section [3][6]. Now, as for energy use, which is most typically associated with soluble fiber, there is some debate if fiber offers any real caloric role [7]. Some attach a numerical value of 2 calories per gram of fiber, but this number is debated and more consistently labeled as fiber offering no caloric amount (discussed later)[3][7].

However, while soluble and insoluble fibers are often classified as different, that difference is not as cut and dry as one might expect. Differences are based on what solvents fiber is exposed to, with some becoming more soluble in water, and others being more soluble in other solvents [5]. Also, the linkages of monomers to make up various fibers offers an explanation for why certain fibers are more or less soluble in water, as well; some fibers made up of the same monomers are typically insoluble while those with various types of monomers making up the molecule tend to be more soluble [5]. This difference may be due to the ability for some fiber monomers to pack together more readily leading to increased insolubility [5].

What is the function of fiber?

Fiber has a long list of touted benefits, but as research surfaces, there is some debate if fiber, itself, offers these benefits or if the foods that are consumed to acquire this fiber are the real reason for these known health benefits [6]. However, regardless if fiber has a direct impact on all these touted health benefits or not, fiber does have some proven, direct relationships with certain health situations – let us examine some of these health effects (there are more than listed here).

Cardiovascular Disease

Cardiovascular disease, leading to stroke and myocardial infarction (heart attack), is the leading cause of the death in the United States [8]. Fiber has been associated, strongly, with reduced levels of cardiovascular disease [6][9][10]. The higher the intake of fiber rich foods, the lower risk someone is in suffering from cardiovascular disease issues as fiber rich nutrition has shown a 29% reduction in heart disease risk and a 26% reduction in stroke risk compared to low fiber dense nutrition intake [11]. The mechanism is assumed to be due to a reduction of low density lipoprotein (LDL) cholesterol, which is often simplistically considered “bad” cholesterol, among a few other blood markers like blood pressure [6][13]. However, before we go off exclaiming fiber as the king of cardiovascular health, there are a few points to be made.

First, while it is true that fiber rich foods aid, significantly, in bettering our health versus cardiovascular disease, that may not be a complete story and may be giving fiber undo full credit. Studies that have examined fiber intake, isolated, in an attempt to see its impact on cardiovascular disease have been inconclusive and unable to see a direct link [6][11][12]. So, secondly, what assumptions can we make from this? No definitive ones other than fiber may help, but we do know that fiber rich foods do help. This is likely do to the fact that most fiber rich foods are also packed with protective and healthful micronutrients and a diet rich in micronutrients is far healthier, overall, than a diet low in micronutrients, which is highly associated with low fiber. Low fiber denotes low overall micronutrient, which is associated with bad health, which means heightened cardiovascular disease (in this case) – the reverse is true, of course.


As for diabetes, we have a similar story as we do with cardiovascular disease risk. In terms of looking at non-controlled epidemiological studies, fiber does seem to have a positive impact on diabetes risk and management [6][14]. However, this impact is yet to be completely elucidated. It is possible fiber slows the digestion of carbs leading to a slower release of carbohydrates in the small intestine where carbohydrates are absorbed into the body; this would mean that a more time sensitive release may lead to less dramatic spikes in blood sugar and may aid in regulation. However, it is also possible that, because of association, higher fiber consumers are also higher micronutrient consumers and therefor typically have a healthier outlook on their nutrition, and the fiber itself may, again, not have a direct impact, but may be a positive associate to micronutrient dense foods – the real directors of better diabetes management and prevention.

Gastrointestinal Disease

Gastrointestinal disease is a broad field that encompasses many different disease states from colon cancer, gastroesophegeal reflux disease (GERD), gallbladder disease, diverticulitis, irritable bowel disease, among others [11]. Of course, it would be several articles in themselves to cover each one individually, so for the purpose of this article, we will simply look at the body of work and how fiber has an effect on these diseases.

It seems that for some of these diseases we need further research to say definitively that fiber plays a role in diminishing their prevalence, but fiber, at least from a mechanistic standpoint, may have some influence in gastrointestinal health. This makes sense if we understand that fiber has a huge, direct impact on gastric emptying (stomach clearance of food stuff) and moving food stuff through the intestinal tract [11]. The use of fiber is, with some evidence, thought to decrease the symptoms and prevalence of intestinal ulcers, gastroesophegeal reflux disease, diverticulitis, constipation (discussed further in the following section), and may help in irritable bowel syndrome, as well as inflammatory bowel disease [11]. Soluble fiber is thought to decrease the concentration of irritants in food stuff and insoluble-like fibers are thought to increase the speed of the digestive process [11].


In terms of bathroom regularity, which is a healthy sign, as we do not want a backed up colon causing constipation, fiber does have a direct, immutable impact [6]. The reason fiber is so potent is due to its impact on stool (poop) weight, which encourages the passing of poop through the colon; this is accomplished by soluble-like fiber taking in water and increasing in viscosity and size [6]. Not only that, insoluble-like fiber goes through a fermentation performed by large intestine bacteria, which also increases the size of the stool [6][11][16]. So, yes, fiber definitely decreases constipation in the correct doses.


Satiety is a bit tricky when it comes to dietary fiber. There is no confusion that dietary fiber certainly increases satiety and decreases hunger cues, at least in the short term [6][11][17]. These mechanisms are described as being influenced by stomach distension caused by fiber’s solubility and viscosity – the latter being more important of the two [6][17]. Not only that, the simple act of increased chewing from fiber’s tough structure may lead to increased stomach distension as gastric acids are further produced in the stomach [6]. As we know from the diabetes section, fiber also helps control glucose release into the blood stream by decreasing the time of absorption while also decreasing gastric emptying (stomach contents being released into the intestinal tract for absorption) and this also leads to better satiety control [6]. Finally, while more soluble fibers that are viscous offer increased satiety, more insoluble fibers may provide a satiating benefit when food enters the intestinal tract by manipulating gut hormone feedback mechanisms [17].

So, all in all, we know that fiber does impact satiety positively; however, it seems the research is pointing toward soluble-like, high-viscous fiber and insoluble-like fiber are beneficial while soluble-like, low-viscous fibers are unhelpful [17]. In the end, however, these distinctions are relatively useless in most real world, applicable situations; so, consume fiber for higher satiety.

Calories & Nutrients

Fiber is a debated topic when it relates to nutrients and calories. While some attribute 1.5 – 2.0 calories per gram of fiber, others say fiber has no caloric content as, technically, it is not absorbed [4][7]. Now, making sense of this, fiber truly does not get absorbed by the small intestine like other carbohydrates and therefore, in traditional definition, offers no calories [7]. However, fiber does go through a process called fermentation in the intestinal tract and what fiber does go through the fermentation process is converted to short chain fatty acids, which are calorically viable [3][18].  It is also possible that due to the variety of the, still not fully understood, gut bacteria in the intestines that fiber may offer more or less calories to a person depending on the state of their intestinal bacteria and its ability to ferment [7].

On a slightly separate note, while fiber may or may not offer calories, fiber does lead to an impaired absorption of other nutrients in the small intestine as some nutrients can get stuck within the viscous nature of some soluble-like fibers [3][18]. This means that some caloric content of foods may not be absorbed due to higher fiber contents if that fiber is of a viscous nature; this is true for macro- and micro-nutrients alike [3][18].

Bacterial Health

Not a ton is known about this subject, yet. We do know fiber has a direct impact on gut bacteria due to its impact via fermentation; however, we do not know to what extent or what specific fibers have an impact on the health of these microorganisms [3]. Further research is necessary to make strong claims as to the impact of fiber on gut health beyond fermentation purposes.

Digestion and Absorption?

As we, hopefully, know by now, fiber is not truly digested or absorbed by the body. However, fiber is first ingested just as with any other carbohydrate, chewed, then swallowed into the stomach. There, depending on its solubility, it pulls in water. If it is a viscous fiber, it will look like a gel. After some time, it is, without being digested, dropped into the intestines where it is not absorbed. However, if the fiber is viscous, it may trap some nutrients within the gel [3]. Once it reaches the large intestine, if it is fermentable fiber, it may be fermented into short chain fatty acids and a variety of gases from carbon dioxide, methane, and hydrogen [3]. Then, the least fermentable fibers offer a last impact by increasing the size and weight of the stool, which activates a release of stool process and the stool (poop) leaves the body [3].


Recommendations for fiber intake are usually relatively standard, and for healthy populations within the late adolescent, young adult, and adult years, roughly 14g/1000 calories/day is a good point to aim for and is set as the acceptable intake by the government dietary guidelines [19]. Over consuming fiber can lead to significant discomfort as, again, fiber leads to a distention of the stomach, and more importantly, an increase in gassiness in the large intestine [20].

Sources of fiber?

A few sources of fiber are beans, peas, whole grains, bran cereal, fruits, and vegetables – basically, think sources of carbohydrates, for the most part (not all sources of carbohydrates are fiber rich, but fiber rich sources are mainly carbohydrates)[19][20]. There are, of course, other sources of fiber, but the point of this article is not to list every single detail.


Okay, so after this extensive explanation of fiber, its functions, dosages, and sources - what is the bottom line? It is an indigestible carbohydrate and although typically characterized between soluble and insoluble, it is more accurate to see the various fibers as a gradated scale of solubility and viscosity. Fiber has a definitive, direct impact on satiety, laxation and regularity, and diabetes while also offering possible benefits in cardiovascular disease, gastrointestinal diseases, and gut bacteria health. Fiber is indigestible and therefor is considered to offer no calories, but the fermentation process in the colon synthesizes some fibers to short chain fatty acids which could provide energy. Dosage is set at 14g/1000 calories/day and possible sources are beans, peas, whole grains, fruits, and vegetables (among others).

Writer: Nicolas Verhoeven
This is educational material only and not meant to be prescripton, consult your physician before making any changes.


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