Countless studies, clinical trials, and surveys have concluded the same—eating more fiber will help the average person live a longer and healthier life. Not all types of fiber offer the same benefits, however, and knowing how to spot the differences can ensure a healthier and happier you.
For the longest time, I didn’t really know what fiber was. I knew it was supposed to be good for me and I knew it was supposed to help me poop. That was the extent of my knowledge. I’ve since learned how different types of fiber affect me differently and believe anyone stands to gain from that knowledge.
- Solubility, Gel-formation, and fermentation rate affect how beneficial fiber is for health.
- Some fiber is better than others for certain health goals like lowering cholesterol or boosting metabolism
- Increased dietary fiber lowers risk for all disease (all-cause mortality)
- A high fiber + low-fat diet is FDA-approved for treating cancer
- Psyllium is a type of fiber that treats diarrhea and constipation and also helps probiotic gut bacteria grow.
Research estimates that, in the United States, only 10% of people eat the recommended 38 grams of fiber per day (R). I frequently read and/or listen to heated exchanges between nutritional experts debating the merits of one diet compared to another: keto vs. carnivore; vegan vs. paleo; Mediterranean vs. vegetarian—it’s never-ending. However, the one thing they all seem to agree on is that eating a lot of fiber is good.
Categorizing Dietary Fiber
The characteristics of fiber determine their health benefits. Fibers that do not form a gel when exposed to water will not offer health benefits like lowering cholesterol or improving cardiovascular health. This can be true of some fibers that form very low-viscosity gels as well.
If you are after these types of health benefits, clinical data dictates you used soluble, nonfermenting, gel-forming fiber (psyllium, guar gum, fruit pectin) It’s important to keep in mind that many studies have used blends of different fibers when measuring associated health outcomes of supplementation (R). Another vote for diversity on the dinner plate.
Soluble vs. Insoluble Fiber
Fiber comes in two different types: soluble and insoluble. This terminology is related to how different types of fiber interact with water. If fiber can dissolve in water it’s considered soluble—otherwise, it’s considered insoluble.
Soluble fiber attracts water and ultimately becomes a gel during normal digestive processes. This process helps slow down the movement of food through the upper digestive tract in such a way that it helps increase the absorption of nutrients.
Insoluble fiber doesn’t get metabolized during its trip throughout digestive tracts. It mostly adds to the overall bulk of one’s poop and helps to “push” things through. The majority of dietary fiber is insoluble.
These are the two broadest types of fiber but not the only characteristics worth noting. Nutritional science also distinguishes fiber by the following attributes (R):
- Fermentation Rate
- Gel Formation
Viscosity & Fermentation Rates
Solubility is only one defining characteristic of fiber. The ability to become a gel and fermentation ability are also important factors to consider. Below is a quick description of what these characteristics mean and why they’re important:
Fermentation Ability: Fibers that ferment lose much of their water content and provide little constipation-relieving benefit. Fermentable fiber is of particular benefit for the support of colonic bacteria, namely bifidobacterium (R).
Viscosity (gel formation): Fibers that are water-soluble transform into a gel state during digestion. Higher-viscosity fibers (thicker gels) slow food transit times and have a more pronounced influence on glycemic control, cholesterol, and nutrient absorption. Fibers that are both vicious and non-fermentable can help alleviate constipation.
One of the most alarming studies I ever read involved researchers feeding human subjects plastic ground up into different sizes. The goal was to find out if particle size had any influence on bowel frequency. In this case plastic—being 100% ingestible—was a functional proxy for indigestible fiber.
Researchers found that larger particle sizes increased both stool size and bowel movement frequency (R). In fact, the plastic demonstrated a similar laxative effect to that of wheat bran! When considering fibers, those that are either A.) insoluble or B.) non-fermentable are likely to have the largest particle size.
General Types of Fiber
It’s easier to consider different types of fiber knowing a bit about viscosity, fermentation, and solubility. Combinations of these characteristics determine how fiber behaves in the body. I’ve gathered that the following four combinations are those with the greatest degree of supportive research (PDF):
Type 1: Insoluble + Poorly Fermented
This type of dietary fiber makes it all the way through your digestive tract without changing much. Plastic would qualify as insoluble fiber. This fiber will works as a laxative when particle sizes are large enough—otherwise, it’s just going to make your poops bigger. Examples: Wheat bran, cellulose, plastic.
Type 2: Soluble + Non-Viscous + Fermentable
This type of fiber dissolves when eaten, helps creates useful nutrients and can provide prebiotic benefit. It does not offer any significant benefit to gut motility directly but may through the support of probiotic bacteria. Example: Inulin.
Type 3: Soluble + Viscous + Fermentable
This type of fiber slows the speed by which foods move through the upper digestive tract. This improves glycemic control, lowers cholesterol, and helps absorb nutrients. It stimulates bile production and can also help you feel more satisfied while eating. It also serves as food for beneficial gut bacteria. No significant, direct, influence on gut motility. Example: Apple Pectin, Acacia Gum*.
Type 4: Soluble + Viscous + Non-Fermentable
This type of fiber also has a positive influence on glycemic control and cholesterol as comparable fermentable forms. Being non-fermentable, this fiber type retains the majority of its shape into the colon. This provides an osmotic effect and can help normalize stool consistency and supports favorable colonic bacteria. In other words, it helps you poop better and supports digestive health. Example: Psyllium.
Note: Research has shown mixed results in support of Acacia Gum as a soluble non-fermenting fiber. Differences in outcome may be described to the way the fiber was processed beforehand (ground to powder vs. simply dissolved in water.)
Fiber’s activity in the upper digestive tract (small intestine) is responsible for its benefits on cholesterol, blood sugar, and satiety regulation. For fiber to offer these benefits, research shows are must be gel-forming.
Fibers with higher viscosity offer more health benefits such as lowering cholesterol, stabilizing blood sugar, or improving immune health (R)(R). Research has also demonstrated that combining supplemental fiber with certain medications, such as statins, will help lower cholesterol even farther (R).
Fiber for Constipation
Fibers with larger particle sizes are likely to be better choices for relieving constipation. Non-fermentable + soluble fibers are also well-suited to help relieve constipation. Research shows psyllium to be a great choice for addressing constipation and even supports the growth of beneficial bacteria (R)!
Insoluble and soluble viscous fiber can provide a laxative effect. Keep in mind that only soluble viscous fibers are going to provide any benefit for cholesterol or glycemic control. The best all-around choice here is Psyllium.
Fiber for Diarrhea
Fiber that is slow to ferment or doesn’t ferment is what helps thicken things up and improve symptoms of diarrhea. Fibers like psyllium will help soften hard stools but will also help harden the soft or near-liquid stool. It’s kind of a one-size-fits-all solution (R)(R).
Fiber that is insoluble or has a high rate of fermentation should be avoided since they may make symptoms of diarrhea worse. Again, the best all-around choice here is Psyllium.
Health Benefits of Fiber
There are few compounds that have such overwhelming research to stifle debate among nutritionists. Fiber is one such compound. There is a direct relationship between how much fiber the average person eats, their quality of life, and their life expectancy. All things being equal, the best chance you have at living a longer and healthier life is to eat more fiber (R).
FDA Approved Cancer Treatment
Increased consumption is an FDA-approved health claim for lowering the risk of cancer (21 CFR 101.76). This is provided that one’s diet also falls into the category of being “low fat,” per the technical definition of the FDA (21 CFR 101.62). This means that eating more fiber and less fat will lower the risk for cancer, in the opinion of the FDA. That’s a bold claim.
I could write all day about the specific benefits of dietary fiber. Here’s a list of many common benefits of eating more fiber (R):
- Lower Risk for Heart Disease
- Lower Risk for Metabolic Disease
- Lower risk for obesity
- Lower risk for intestinal disease
- Lower blood pressure
- Improved glycemic control
- Regulates bowel movements
- Promotes healthy weight loss
- Improved Immune Function
As hard as it may be to believe—the list goes on, and on, and on. Fiber is good—everyone agrees. What they don’t agree on, and what I’m not sure anyone really knows, is whether whole food fibers offer specific benefits compared to isolated or synthetic fibers (like Metamucil.)
I covered a lot in this article. Consideration for dietary fiber can be as complex—or as simple—as you want it to be. Finding a blend of fiber supplements (make sure to buy from a quality brand) is a very useful approach to getting enough fiber. One can mix, match, blend, and even counter-balance existing dietary foods.
On the other hand—eating more fruits and vegetables at each meal will boost your fiber intake as well! Nutrition is a very personal and very complex science to consider. Every person has unique nutritional needs but there are core requirements that are relevant to us all. We all need fat, protein, and carbs to survive and—as discussed here—high amounts of dietary fiber!