How Fiber Benefits Your Body

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Verywell / Alexandra Shytsman

Like other carbohydrates, fiber is made up of many glucose molecules. However, fiber doesn't break down into glucose before it gets to the colon, and may not be digested there either. Here's what fiber does inside the human body and the benefits this can provide.

Fiber and the Digestive Tract

To understand the role that fiber plays in health, it's helpful to look at how it acts as it works its way through the digestive tract.

The Stomach

Insoluble fiber—which is found in wheat bran, beans, and nuts—isn't broken down easily in our digestive tract. It also tends to move out of the stomach quickly, unless there is fat, protein, or soluble fiber to slow it down.

Conversely, soluble fibers (such as peas, oats, and citrus fruit) slow stomach emptying, particularly when consumed with lots of fluid and some fat. This tends to promote feelings of fullness or satiety. It can also decrease the glycemic effect of a meal.

The Small Intestine

The small intestine has three main functions with regard to fiber. They are to move the fiber toward the colon, to start to reduce larger molecules into smaller ones, and to absorb the fiber-containing food's nutrients through the intestinal wall.

As far as how fiber moves through the small intestine, it's a similar situation as in the stomach. The presence of insoluble fiber tends to speed up "transit time" while soluble fiber (which turns into a gel-like substance during digestion) adds bulk.

The Colon

In the colon, fiber becomes a sort of "scrub brush," helping to clean out any buildup and bacteria, and contributing to greater colon health. It also helps keep your stools soft, providing both regular and easier waste elimination.

The Gut Microbiome and Fiber

The organs involved with food digestion (the stomach and intestines) are often referred to as the gut microbiome. Within this microbiome exists between 300 and 500 species of bacteria, providing a total of almost two million genes.

When dietary fiber is consumed, the microbes in the gut use it to produce short-chain fatty acids (SCFAs). These SCFAs are then absorbed by the body, where they either act as a catalyst for another microbe or help regulate the metabolic process.

Evidence is building that SCFAs are important in keeping the colon healthy and preventing conditions such as ulcerative colitis, colon cancer, and diverticular disease. They may also help regulate cholesterol and, to some extent, insulin responses.

Additional Fiber Benefits

Besides reducing the glycemic effect of meals and contributing to colon health, there is further evidence that fiber may benefit us in other ways. It seems to help lower cholesterol and triglycerides, for instance. It may also help prevent and/or treat:

  • Certain types of cancer, especially colon cancer
  • Type 2 diabetes
  • Heart disease
  • Ulcers, particularly in the beginning of the small intestine (duodenal ulcers)

Types of Fiber That Feed a Healthy Gut

Different types of fiber provide a wider variety of "bacteria food." This, in turn, produces different kinds of short-chain fatty acids and other health-promoting products, so it's important to get a variety of fibers in our foods.

The fiber types that are most amenable to fermentation are the soluble ones (gums, pectins, etc.). These are found in berries, beans, flaxseed, plums, apples, and oats, as well as in some fiber supplements, such as those using psyllium and guar gum.

Insoluble fiber (found in such foods as vegetables, the bran of grains; e.g. wheat bran, nuts, and seeds) isn't available for much fermentation. But it is still important in the colon.

Because of its tendency to "speed things along," fermentation of insoluble fibers takes place along the entire length of the colon—including near the colon's end, where the majority of colon cancers occur. Without insoluble fiber, most of the fermentation would take place at the top of the colon, reducing this benefit.

A Word From Verywell

Fiber's trek through the digestive tract offers many benefits, with both soluble and insoluble forms important to overall health. If you aren't sure how to best incorporate more fiber into your diet, working with a dietitian or nutritionist may help.

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