Fiber: Soluble vs. Insoluble

The gut used to be an afterthought. It was a means to an end, a way to procure energy for our bodies to run as well as simply enjoy our food; a way to make disease worse or to improve it. At one time, few people gave much thought to how the gut must be maintained to provide a positive impact on individual well-being; even fewer people understood the role dietary fiber played in that maintenance. 

But times have changed, and now many individuals understand the importance of dietary fiber intake.

 

What is fiber?

Fiber—the edible but indigestible parts of plants—creates widespread and complex effects on a person’s health, helping to tightly regulate the processes of food intake and elimination.

To understand the role of fiber in promoting gut health, one has to get acquainted with some of the major categorical differences between fiber types. The most recognizable difference in dietary fiber is the distinction between soluble and insoluble fiber.

The primary difference between these fiber types is the ability to dissolve in water.

That quality means that the two different fiber types have distinguishing characteristics about their origin, chemical structure, outward appearance, and function within the body. Let’s take a detailed look at the similarities and differences between soluble and insoluble fiber. 

 

But first, a brief review of our digestive tract

To understand the roles of soluble and insoluble fiber in the digestive tract, it will help to have a little anatomy review. 

Recall that the digestive system consists of one big tube from the mouth to the anus and includes the esophagus, stomach, small intestine, and large intestine. Along the way, you also find the pancreas, gallbladder, and liver. A useful, and perhaps imperfect, analogy might come from one of the river systems found in the US.

In essence, our main digestive tube is like the Mississippi River, the pancreatic duct the Ohio River, and the common bile duct (the drainage tube for the gallbladder and liver) the Missouri River. The small intestine would be the relatively small area where the Ohio and Missouri drain into the Mississippi, resulting in a fertile supply of nutrients. 

So why the throwback to 7th-grade geography class?

The small intestine is where digestive enzymes break foods down into compounds that can be used for building or for fuel, which promptly get absorbed into the bloodstream. Our intestines are the primary area of action for dietary fiber and act like similarly to the silt deposits in the lower half of Ol’ Man River. Like river water irrigating and bringing nutrients to the surrounding soil, molecules like fats, carbohydrates, proteins, and minerals all readily get soaked up from the walls of the small intestine.

But not all parts of plants can get absorbed. The non-absorbed plant starches are dietary fiber. 

Soluble Fiber

Soluble dietary fiber dissolves in water and forms a thick gel in the intestinal tract. The gel forms due to the presence of fibers like pectin and plant gums (some of them, like xanthan gum and guar gum, are commonly found on lists of food ingredients). Soluble fiber is found in many types of foods: 

● Oats 

● Oat bran

● Rice bran

● Strawberries

● Citrus fruits

● Dried legumes

● Potatoes 

Psyllium, a type of fiber available as a supplement, is mainly stocked with soluble fiber. 

 

What does soluble fiber do?

As food makes its way through the small intestine, soluble fiber binds to fats and sugars, preventing their absorption through the walls of the small intestine and into the bloodstream. Instead, these newly formed fiber complexes get fermented and broken down into two essential nutrients: 

● Food for the healthy bacteria that live in the gut downstream of the small intestine (these foods for bacteria are called prebiotics); 

●     Short-chain fatty acids, which are the primary nutrient source for the cells of the large intestine.

This action has the effect of lowering blood sugar, as well as cholesterol. Thus soluble fiber is thought to be able to help people control elevated blood sugar, high cholesterol, and cardiovascular disease. 

 

Insoluble Fiber

Insoluble dietary fiber does not dissolve in water. In addition to being additional food for healthy bacteria, insoluble fiber draws water up from the surrounding environment, making stools bulkier and easier to pass through the intestines into the rectum. Foods which contain insoluble fiber include: 

● Wheat bran

● Outer layers of fruits and vegetables

● Cereal

● Whole grains

● Seeds

 

What does insoluble fiber do?

These insoluble fiber types are usually a coarse powder by the time they reach the large intestine, prompting our colon cells to add mucus to the water that has already been drawn in. This process is called “bulking” and allows for a smoother exit of feces.

It might seem like dietary fiber doesn’t have a particularly glamorous job. Still, there are immense benefits to a high-fiber diet…

Regular bowel movements: high fiber regulates fecal water content. It makes for soft, voluminous stools, rather than the constipating kind (too little water) or loose stools (too much). 

Fewer bowel complications: Diets high in fiber are believed to result in fewer cases of colorectal cancer, fewer hemorrhoids, and may reduce the chance for diverticular disease (small pouches in the colon that are prone to infection). 

Healthier gut bacteria: Fiber acts as a prebiotic, a food source for the “good” bacteria in the gut. 

Reduces blood sugar: Soluble fiber complexes with sugars and blocks their absorption.

Reduces cholesterol: through a similar mechanism, soluble fiber binds cholesterol and disallows it from being absorbed. 

The Institute of Medicine recommends 38 grams of daily fiber for men 50 and under 30 grams for men 51 and older, 25 grams for women 50 and under, and 21 grams for women 51 and older. 

 

With just a brief look in your fridge or your pantry, you can see for yourself how you can boost your soluble and insoluble fiber intake.

 

 

References

National Institute on Diabetes and Digestive and Kidney Diseases. Your Digestive System and How It Works. December 2017. Accessed April 13, 2020.

Weickert MO, Pfeiffer AFH. Impact of Dietary Fiber Consumption on Insulin Resistance and the Prevention of Type 2 Diabetes. J Nutr. 2018;148(1):7–12. 

Soliman GA. Dietary Fiber, Atherosclerosis, and Cardiovascular Disease. Nutrients. 2019;11(5):1155. 

Dreher ML. Whole Fruits and Fruit Fiber Emerging Health Effects. Nutrients. 2018;10(12):1833. 

Institute of Medicine (US) Standing Committee. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington (DC): National Academies Press (US); 2005.