Soluble vs. Insoluble Fiber for Gut Health

Dr. David Wiss

July 9, 2021

Gut Health

The United States Department of Agriculture (USDA) recommends consuming between 25-38 grams of fiber per day from food, not supplements. Estimates suggest that Americans (on average) consume less than half of that. This data is alarming to those of us who study nutrition, medicine, and public health. This conversation has picked up traction as we begin to understand the importance of the gut-brain axis in mental health. 

Some of our ancestors consumed closer to 100 grams/day from regular consumption of whole plant foods. As refined carbohydrate consumption has increased due to the rise in ultra-processed foods, fiber consumption has decreased. The population over-consumes refined grains (e.g., bread, pasta, cereal) and under-consumes intact whole grains (e.g., faro, barley, millet). Americans drink too much juice and do not eat enough fruit (a good source of fiber). Many people skimp on beans, nuts, and seeds, and instead get their protein from animal sources that lack fiber entirely.

Why is this important? Many experts believe that the rise in refined sugars and decline in plant fibers is responsible for many of the chronic diseases that currently plague the health care system (e.g., type-2 diabetes and other forms of metabolic disease) [1]. It is still unclear if the rise in sugar or the decline in fiber is the more significant driver. We believe that chronic low fiber (rather than just added sugars) is a culprit, as it can contribute to a “leaky gut” (increased intestinal permeability). 

We know that low-fiber intakes are responsible for gut dysbiosis (microbial imbalance), which can, in turn, lead to a host of other adverse conditions. For example, a low-fiber diet can promote the expansion and activity of colonic mucus-degrading bacteria, leading to erosion of the colonic mucus barrier [2]. This may explain the rising incidence of colorectal cancer among people under 50 in the US [3]. A recent population-based study in Japan showed that dietary fiber was inversely associated with lower all-cause mortality [4]. In Korea, it has been shown that higher intakes of seaweed and mushroom fiber were associated with a lower likelihood of depression [5]. 

Links between fiber intake and mental health have begun to emerge in the literature. This is not surprising given that the gut is now understood to be our “second brain.” Known benefits of dietary fiber include:

  • Balanced blood sugar

  • Balanced cholesterol levels

  • Supports heart health

  • Supports a diverse gut microbiome

  • Supports the “satisfaction factor” of a meal

  • Keeps things moving through the digestive tract

Does the type of fiber matter? The Korean study indicated that the specific source of dietary fiber might be an important factor in modulating depression [5]. Concerning gut microbiota alterations, soluble fiber appears to be more critical than insoluble fiber [6].

Soluble fiber dissolves in water and becomes a gel-like substance in the gastrointestinal tract, broken down by bacteria in the large intestine. The fermentation in the colon by gut bacteria creates other compounds referred to as “postbiotics” (e.g., short-chain fatty acids) with a wide range of health benefits. Foods high in soluble fiber are some of the best anti-inflammatory foods to eat. Good sources of soluble fiber include:

  • Oats

  • Peas

  • Beans

  • Apples

  • Citrus Fruit

  • Carrots

  • Barley

Insoluble fiber does not dissolve in water and remains mostly unchanged as it moves through the digestive tract. Insoluble fiber absorbs fluid and adds bulk to the stool. It speeds up the movement and processing of waste, preventing constipation. The most common types of insoluble fiber are cellulose and lignin, which are naturally found in nuts, seeds, whole grains, some vegetables, and especially the skins of produce (eat the kiwi skin!). While fruits and vegetables are known to reduce inflammation, remember that whole grains and beans have more fiber than fruits and vegetables. 

It is also important to remember that most foods contain a combination of soluble and insoluble fiber rather than one or the other (much like dietary fats found in food). An exception is fiber isolates, which are used in dietary supplements. Single fibers do not have the same health and gut benefits as the fiber matrixes found in food. This is partially attributable to the benefits of polyphenol antioxidants in whole plant foods [7–11]. Fiber and polyphenol supplements can certainly be helpful but should not supplant the consumption of whole foods, which is the main tenet of anti-inflammatory eating. 

Both fibers are critical for optimum health. We have found that increasing soluble fibers benefits gastrointestinal health and improves mood and mental health. Examples include the daily consumption of beans or drinking chia seeds soaked in water. The best evidence-based anti-inflammatory diet to date is the Mediterranean Diet. In addition to high fiber, this approach to eating is also high in omega-3 fatty acids

In many cases, probiotics will be helpful to facilitate this transition from a low to a high-fiber diet. Digestive enzymes might also be helpful but are not ideal long-term. Notably, some people overconsume fiber at the expense of other nutrients. Overeating fiber can also be counterproductive for health. Many people who become health enthusiasts can “go off the deep end” here and can even develop eating disorders as a result. So there is no need to become obsessed with counting grams, but it is wise to become informed about your fiber intake for optimal mental health. 

Fiber may be the one indispensable component in the future of nutritional psychiatry. Let’s “dial it in” together and watch your mood improve! Remember, when changing digestive processes, sometimes it gets worse before it gets better. We believe in feeding beneficial microbes. This is one reason why dietary change can be so challenging. Thankfully, you don’t have to do it alone! Are you ready to dive into your gut with Wise Mind Nutrition? 

References

1. Lustig RH. Ultraprocessed Food: Addictive, Toxic, and Ready for Regulation. Nutrients. 2020;12(11):3401.

2. Desai MS, Seekatz AM, Koropatkin NM, Kamada N, Hickey CA, Wolter M, et al. A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility. Cell. 2016;167(5):1339-1353.e21.

3. Siegel RL, Miller KD, Sauer AG, Fedewa SA, Butterly LF, Anderson JC, et al. Colorectal cancer statistics, 2020. Ca Cancer J Clin. 2020;70(3):145–64.

4. Katagiri R, Goto A, Sawada N, Yamaji T, Iwasaki M, Noda M, et al. Dietary fiber intake and total and cause-specific mortality: the Japan Public Health Center-based prospective study. Am J Clin Nutrition. 2020;

5. Kim C-S, Byeon S, Shin D-M. Sources of Dietary Fiber Are Differently Associated with Prevalence of Depression. Nutrients. 2020;12(9):2813.

6. Morrison KE, Jašarević E, Howard CD, Bale TL. It’s the fiber, not the fat: significant effects of dietary challenge on the gut microbiome. Microbiome. 2020;8(1):15.

7. Leri M, Scuto M, Ontario ML, Calabrese V, Calabrese EJ, Bucciantini M, et al. Healthy Effects of Plant Polyphenols: Molecular Mechanisms. Int J Mol Sci. 2020;21(4):1250.

8. Číž M, Dvořáková A, Skočková V, Kubala L. The Role of Dietary Phenolic Compounds in Epigenetic Modulation Involved in Inflammatory Processes. Antioxidants Basel Switz. 2020;9(8):691.

9. Nazzaro F, Fratianni F, Feo VD, Battistelli A, Cruz AGD, Coppola R. Polyphenols, the new frontiers of prebiotics. Adv Food Nutrition Res. 2020;94:35–89.

10. Vauzour D, Rodriguez-Mateos A, Corona G, Oruna-Concha MJ, Spencer JPE. Polyphenols and Human Health: Prevention of Disease and Mechanisms of Action. Nutrients. 2010;2(11):1106–31.

11. Aravind SM, Wichienchot S, Tsao R, Ramakrishnan S, Chakkaravarthi S. Role of dietary polyphenols on gut microbiota, their metabolites and health benefits. Food Res Int. 2021;142:110189.