Fiber is an essential nutrition topic for the beginning of any discussion about digestive health. A high fiber diet helps maintain normal bowel function, or regularity, and has been shown to reduce risk of heart disease, type 2 diabetes and lower the risk of certain cancers. Fiber also improves satiety—the feeling that one has had enough to eat. For many years the daily fiber recommendation was at least 25 grams, while in practice the average American diet was closer to 10 grams. So why is there a disconnect, and why is this still a hot topic?
Fiber is the undigested part of the plant, or more specifically, an undigested carbohydrate. Eating more fiber means eating more plant-based foods. Whole grains, nuts, seeds, legumes, fruits, and vegetables are all sources of fiber—not all are created equal or have as much fiber content by the time they are eaten. Many plant foods are processed to add or remove parts. This often involves elimination of fiber content. As such, eating fewer processed foods is also associated with increasing fiber in the diet; choosing brown rice instead of white rice is a good example.
There are two main fiber classifications: soluble and insoluble. Both are present in most plant foods and beneficial to the human body, but amounts vary per food type. Celery for example, is predominantly high in insoluble fiber. Oats are higher in soluble fiber. Soluble fiber is further defined by types that form gels and/or are fermentable. This becomes important when a fiber restriction or modification is made to the diet.
Soluble fiber dissolves in water to form a gel in the stomach. This gel aids in lowering blood cholesterol and glucose levels by limiting absorption of fats and sugars. The gel traps them and prevents uptake into the bloodstream. In addition to oatmeal, beans, lentils, blueberries, and apples are great sources of soluble fiber.
Insoluble fiber comes from the structural component of plants (stalks and skins), which provide cellular support, strength, and construction. In the human body, insoluble fiber adds bulk to food, and it takes longer (and uses more energy) to digest. In short, it makes people feel fuller for greater periods of time. Insoluble fiber is frequently referred to as a “colon cleanser” as it keeps the digestive system moving and helps prevent constipation. Getting in higher levels of insoluble fiber is associated with a reduction in colon cancer (1) and reduced incidence of diverticulitis (2). While insoluble fiber does not dissolve in water, staying hydrated is an important part of maintaining a healthy digestive system. Insoluble fiber is aided by water in the colon to form softer, easy to pass stools. Skins of fruits and vegetables, seeds, whole wheat products, and leafy greens are all sources of high insoluble fiber.
Fiber and the Microbiome: How Fiber Works as a Fuel
In gel form, soluble fiber travels to the small intestine where it is used as a food source by bacteria. Fermentation occurs as the fiber is broken down, releasing energy that results in supportive compound products as well as discomfort-causing gases for some people.
The diversity of the microbes in our gut is key to a thriving microbiome as discussed in Trust your Gut. This microbiome, in turn, supports healthy mucosal lining of the colonic walls, creating a barrier between the digestive tract and the internal environment. Legumes and pulses (seeds obtained from a pod; i.e.,like beans, peas, and lentils) are excellent sources of fermentable fiber. The wider variety consumed, the stronger the microbiome.
As we learn more about the role fiber plays in our health, and now the microbiome, recommendations have increased to upward of 30 grams a day for adults(3). The best approach is to “start low, and grow slow” to avoid intestinal distress while increasing your daily intake. Adding five grams per day (examples include an apple, half of an avocado, one cup of broccoli) is a good goal until reaching the desired amount may be a manageable approach.
References
Mirrafiei, A., Jayedi, A., & Shab-Bidar, S. (2023). Total and different dietary fiber subtypes and the risk of all-cause, cardiovascular, and cancer mortality: a dose-response meta-analysis of prospective cohort studies. Food & function, 14(24), 10667–10680. https://doi.org/10.1039/d2fo04024g
Diverticular Disease - NIDDK. (July 11, 2024) National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/digestive-diseases/diverticulosis-diverticulitis#:~:text=Diverticulosis%20is%20a%20condition%20that%20occurs%20when,complications%2C%20doctors%20call%20this%20condition%20diverticular%20disease.
McKeown, N. M., Fahey, G. C., Jr, Slavin, J., & van der Kamp, J. W. (2022). Fibre intake for optimal health: how can healthcare professionals support people to reach dietary recommendations?. BMJ (Clinical research ed.), 378, e054370. https://doi.org/10.1136/bmj-2020-054370
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