Updated: Mar 28
What is fiber?
They are simply known as dietary fiber, a non-digestible carbohydrate found in foods.
It’s split into two broad categories based on its water solubility:
Soluble fiber: dissolves in water and can be metabolized by the “good” bacteria in the gut (think Metamucil)
Insoluble fiber: does not dissolve in water
Perhaps a more helpful way to categorize fiber is as fermentable versus non-fermentable, which refers to whether friendly gut bacteria can use it or not. There is also a lot of overlap between soluble and insoluble fibers. Good bacteria can digest some insoluble fibers in the intestine, and most foods contain both soluble and insoluble fibers. The recommended intake of fiber daily is men should consume 38 grams, and women should consume 25 grams per day.
Why Is Fiber Good for You?
There is growing evidence that adequate fiber intake benefits your digestive system and reduces your risk of chronic diseases. Many of these benefits are mediated by your gut microbiota — the millions of bacteria that live in your digestive system. However, not all fiber is created equal. Different types have different health effects.
Fiber feeds “good” gut bacteria.
The microbiota that lives in the human body outnumbers the body’s cells by 10 to 1. The microbiota lives on the skin, mouth, and nose, but most of the microbiota lives in the gut, primarily the large intestines. Five hundred to 1,000 different microbiota species live in the intestine, totaling about 38 trillion cells. This gut microbiota is also known as gut flora. This is not a bad thing. There is a mutually beneficial relationship between you and some of the microbiota that lives in your digestive system. You provide food, shelter, and a safe habitat for the microbiota, and in return, the microbiota takes care of some things that the human body cannot do on its own. These aspects are blood sugar regulation, immune system function, and even brain function.
You may wonder what this has to do with fiber. Like any other organism, the microbiota needs to eat to get the energy to survive and function. The problem is that most carbs, proteins, and fats are absorbed into the bloodstream before they make it to the large intestine, leaving little for the gut flora. This is where fiber comes in. Human cells don’t have the enzymes to digest fiber, so it reaches the large intestine relatively unchanged. However, intestinal microbiota does have the enzymes to digest many of these fibers. This feeds the microbiota or “good” bacteria in the gut, and their food is known as prebiotics.
Prebiotics promote the growth of “good” gut bacteria, which can positively affect health. The friendly microbiota produces nutrients for the body, including short-chain fatty acids such as acetate, propionate, and butyrate, of which butyrate appears to be the most important. These short-chain fatty acids can feed the cells in the colon, leading to reduced gut inflammation and improvements in digestive disorders such as irritable bowel syndrome, Crohn’s disease, and ulcerative colitis.
When the microbiota ferments the fiber, they also produce gases. Therefore, high fiber diets can cause flatulence and stomach discomfort in some people. These side effects usually go away with time as your body adjusts.
Fiber can reduce blood sugar spikes.
High fiber foods tend to have a lower glycemic index than refined carb sources, stripped of most of their fiber. However, scientists believe that only high viscosity, soluble fibers have this property. This is important, especially if you’re following a high-carb diet. In this case, the fiber can reduce the likelihood of the carbs raising your blood sugar to harmful levels.
If you have blood sugar issues, you should consider reducing your carb intake — especially your intake of low fiber, refined carbs such as white flour, and added sugar.
Fiber can reduce cholesterol.
Viscous, soluble fiber can also reduce your cholesterol levels. However, the effect isn’t nearly as impressive as you might expect. A review of 67 controlled studies found that consuming 2–10 grams of soluble fiber per day reduced total cholesterol by only 1.7 mg/dl and LDL (bad) cholesterol by 2.2 mg/dl on average (Brown et al., 1999). However, this also depends on the fiber consumed. Some studies have found impressive reductions in cholesterol with increased fiber intake (Gato et al., 2013, Arvill et al., 1995). Some studies show that those who eat more fiber have a lower risk of heart disease.
Fiber can reduce constipation.
One of the main benefits of increasing fiber intake is reduced constipation. Fiber helps absorb water, increase the bulk of stool, and speed up the movement of stool through the intestine. Some studies show that increasing fiber can improve symptoms of constipation, but other studies show that removing fiber improves constipation. The effects depend on the type of fiber.
In one study in 63 individuals with chronic constipation, a low-fiber diet fixed their problem. The individuals who remained on a high fiber diet saw no improvement (Ho et al., 2012). However, in general, fiber that increases the water content of your stool has a laxative effect. In contrast, fiber that adds to the dry mass of stool without increasing its water content may have a constipating effect.
Soluble fibers that form a gel in the digestive tract and are not fermented by gut bacteria are often effective. An excellent example of a gel-forming fiber is psyllium. Other fiber types, such as sorbitol, have a laxative effect by drawing water into the colon. Prunes are a good source of sorbitol. Choosing the correct type of fiber may help your constipation but taking the wrong supplements can do the opposite. For this reason, you should consult a healthcare professional before taking fiber supplements for constipation.
Fiber might reduce the risk of colorectal cancer.
Colorectal cancer is the third leading cause of cancer deaths globally (WHO, 2022). Many studies have linked a high intake of fiber-rich foods with a reduced risk of colon cancer (Kunzmann et al., 2015). However, whole, high fiber foods like fruits, vegetables, and whole grains contain various other healthy nutrients and antioxidants that may affect cancer risk too.
Therefore, it’s difficult to isolate the effects of fiber from other factors in healthy, whole-food diets. Yet, since fiber may help keep the colon walls healthy, many scientists believe fiber plays an important role (Zeng et al., 2014).
So what does all this mean…
Dietary fiber has various health benefits. Not only does it feed your gut bacteria, but fermentable fiber also forms short-chain fatty acids, which nourish your colon walls. Additionally, soluble fiber may reduce your appetite, lower your cholesterol levels, and decrease the rise in blood sugar after high-carb meals. If you’re aiming is for a healthy lifestyle, try to get a variety of fiber types from whole fruits, vegetables, and grains.
Arvill A, Bodin L. Effect of short-term ingestion of konjac glucomannan on serum cholesterol in healthy men. Am J Clin Nutr. 1995 Mar;61(3):585-9. doi: 10.1093/ajcn/61.3.585. PMID: 7872224.
Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr. 1999 Jan;69(1):30-42. doi: 10.1093/ajcn/69.1.30. PMID: 9925120.
Gato N, Kadowaki A, Hashimoto N, Yokoyama S, Matsumoto K. Persimmon fruit tannin-rich fiber reduces cholesterol levels in humans. Ann Nutr Metab. 2013;62(1):1-6. doi: 10.1159/000343787. Epub 2012 Nov 17. PMID: 23171573.
Ho, K. S., Tan, C. Y., Mohd Daud, M. A., & Seow-Choen, F. (2012). Stopping or reducing dietary fiber intake reduces constipation and its associated symptoms. World journal of gastroenterology, 18(33), 4593–4596. https://doi.org/10.3748/wjg.v18.i33.4593
Kunzmann AT, Coleman HG, Huang WY, Kitahara CM, Cantwell MM, Berndt SI. Dietary fiber intake and risk of colorectal cancer and incident and recurrent adenoma in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr. 2015 Oct;102(4):881-90. doi: 10.3945/ajcn.115.113282. Epub 2015 Aug 12. PMID: 26269366; PMCID: PMC4588743.
World Health Organization (2022) Cancer Retrieved from https://www.who.int/en/news-room/fact-sheets/detail/cancer.