Fiber has been recognized for centuries as a healthful dietary ingredient. Ever since I was a kid, I can remember my mother telling me to eat fiber. But fiber has come a long way since my mom tried to hide it in my cereal or pancakes in the form of wheat germ or wheat bran, with the taste and consistency of sawdust. That worked out about as well as when she tried putting cod liver oil in my orange juice.

But what really is fiber? Basically, fiber consists of the indigestible portions of plants, and it includes multiple categories – gums, waxes, pectins, mucilage, cellulose, hemi-cellulose, lignins, and oligosaccharides. Because fiber is not digestible, it is also not absorbed. As fiber passes through the digestive tract, it reaches the large intestine where it undergoes fermentation, which is what allows it to provide all sorts of beneficial byproducts.

There are two main types of fiber – soluble and insoluble fiber. Insoluble fiber includes cellulose, hemi-cellulose, and lignins – they don’t dissolve in water. Examples of soluble fiber include gums, pectin, and mucilage, which combine with water and become gummy.1 Because soluble fibers are more fermentable they can provide more beneficial byproducts, while insoluble fibers are great bulking agents that improve the mechanics of elimination and help prevent constipation.

Health benefits of fiber include:

  • It increases fecal bulk
  • It decreases transit time
  • It supports healthy blood sugar by slowing its absorption into the bloodstream
  • It decreases cholesterol by decreasing its absorption and enhancing its elimination
  • It traps toxic substances and eliminates them from the body
  • And, what is emerging to be one of its most important functions – it interacts positively with the gut flora.

Fiber as a prebiotic

The interaction between dietary fiber and the intestinal microbiota (bugs in the gut) significantly impacts many metabolic processes of the body. And this is something my mother didn’t know about fiber – scientists didn’t even know this in the 60s – which really places my mom ahead of her time. When discussing fiber in this context it is often referred to as a prebiotic – the food that feeds the good bugs or supports the probiotic.

But how does all this work? We know some of the answers, while other answers are still being discovered. The bacteria in the large intestine ferments the fiber, resulting in the formation of health-promoting substances like short-chain fatty acids (SCFAs) – the most well-known one being butyric acid (or butyrate).

SCFAs protect the mucus membranes in the GI tract, while prolonged lack of fiber damages these protective mucus membranes. Butyrate provides the main energy source for the cells of the large intestine, helping keep them healthy.

SCFAs also enter the circulation where they act as signaling molecules that benefit immunity and metabolism. In the case of a low-fiber diet, metabolism does not favor production of beneficial SCFAs, but instead results in less favorable metabolites that can be pro-inflammatory and carcinogenic.

Diets high in fiber (or what is sometimes referred to as microbiota-accessible carbohydrates or MACs) contribute to more microbial diversity in the gut – more different types of bugs – which is a good thing.

On the flipside, diets high in sugar and fat and low in fiber result in less microbial diversity, which in turn is associated with inflammation and chronic conditions such as inflammatory bowel disease, autoimmune diseases, colorectal cancer, allergies, and obesity.2  Regarding obesity, several human studies point to low fiber intake resulting in low microbial diversity, which  in turn contributes to increased risk for weight gain.

How diverse is your microbiota? You can find this out and much more with a Gutbio test from Onegevity Health.

Low-fiber diets have long-term effects

In a mouse study, the effect of a long-term, low-MAC diet led to significant decreases in microbial diversity over three generations that could not be reversed by adding MACs.3 It appears that eliminating fiber for short periods has no long-term effects on the microbiome. On the other hand, long-term elimination, which occurs with chronic consumption of a Western diet high in fat and sugar and low in fiber, can literally cause extinction of some beneficial bacterial strains.

What are the best dietary sources of fiber?

Vegetables, fruits, legumes, whole grains, nuts, and seeds are good fiber sources. Whole grains, vegetables, and some fruits are good sources of insoluble fiber, while soluble fiber is found in beans, peas, lentils, apples, and oats. Fiber doesn’t have to be harsh on a sensitive gut. For example, one avocado has 9 grams of fiber. Here are some fiber amounts in some other common foods:4

  • 1 cup cooked black beans – 17 grams
  • 1 cup cooked lentils – 16 grams
  • 1 cup cooked garbanzo or pinto beans – 11 grams
  • 2 tablespoons chia seeds – 10 grams
  • 1 cup guava – 9 grams
  • 1 cup collard greens – 8 grams
  • 1 cup raspberries – 8 grams
  • 1 cup butternut squash – 7 grams
  • 1 cup blackberries – 7 grams
  • ¼ cup almonds – 7 grams
  • 1 medium-sized pear – 6 grams
  • 1 cup oatmeal – 5 grams
  • 1 cup barley – 5 grams

How much fiber is enough?

The jury is still out on this one. Current recommendations from the American Heart Association are 25-30 grams daily – the average U.S. adult intake is 15 grams. Even amounts of 30 grams daily might not be enough to significantly impact biomarkers associated with disease risk. Some studies indicate that 50 grams or more daily might be necessary to make a significant health difference.2

This amount might not be easily achieved with diet alone, suggesting additional supplemental fiber might be indicated or a protein-multivitamin-metabolic formula with 12 grams of fiber per serving. If you have not been eating a lot of fiber but wish to increase your intake, then it’s recommended that you do so gradually, particularly if you have a GI condition such as IBS.

Changes to FDA labeling requirements for 2020 – how they affect fiber

By now you might have heard about significant changes in the FDA’s dietary supplement labeling requirements due to go into effect in 2020. Thorne got ahead of the curve starting in 2018. This blog will help you navigate the changes if you are unfamiliar with them. One aspect that had not been finalized until recently was what substances the FDA would consider as being dietary fiber.

One ingredient in Thorne products – larch arabinogalactan – was previously designated as a dietary fiber but is currently not on the list.5 Therefore, although we are confident in the health benefits of arabinogalactans, they will not be included in the fiber amount listed on the product labels of Arabinex and FiberMend. This will result in a decrease in total fiber content listed on these product labels.


  1.  Dhingra D, Michael M, Rajput H, Patil R. Dietary fibre in foods: a review. J Food Sci Technol 2012 Jun;49(3):255-266.
  2.  Makki K, Deehan E, Walter J, Bäckhed F. The impact of dietary fiber on gut microbiota in host health and disease. Cell Host Microbe 2018 Jun 13;23(6):705-715. 
  3.  Sonnenburg E, Smits S, Tikhonov M, et al. Diet-induced extinctions in the gut microbiota compound over generations. Nature 2016;529:212-215.
  4.  Surprising sources of dietary fiber. [Accessed 12/27/19]
  5.  FDA unveils new dietary guidance: good news for inulin, polydextrose, some gray areas remaining. [Accessed 12/27/19]