Fiber’s Many Gut Benefits

New research suggests that dietary fiber impacts gut health in some surprising ways.

Fiber may be one of the most important factors in maintaining gut health, and a healthy gut is a key player in protecting us from disease. “Ninety-five percent of Americans are lacking in this key nutrient,” says Mascha Davis, MPH, RD, a spokesperson for the Academy of Nutrition and Dietetics. “It’s the ‘roughage’ or indigestible portion of a plant. Whole grains, nuts, seeds and fruits and vegetables are high in fiber.”

Lots of studies over the years have tried to tease out the impact of the fiber in our diets on digestive health. Here is what we know to date:


Here are some common high-fiber foods to help you meet your gut- and health-protecting dietary fiber goals:

  • 100 percent bran cereal
  • Beans (navy beans, split peas, lentils, kidney beans, refried beans)
  • Whole grains (oats, bulgur, barley)
  • Fruits (prunes, guava, Asian pear, berries)
  • Vegetables (artichoke hearts, cooked frozen spinach, Brussels sprouts, squash)
  • Nuts (almonds, pistachios, pecans, peanuts)

Source: University of Oregon Linus Pauling Institute

Constipation. Fiber may be best known for its role in fighting constipation. “There are two types of fiber and both are important for fighting constipation,” says Davis. “Insoluble fiber adds bulk to the stool and helps move things through the GI tract. Soluble fiber absorbs water so the stools are softer and move through faster.”

Most fiber-rich foods have some of each. Bran (the outer coating on whole grains), fruits, and vegetables are great constipation fighters, but fiber supplements of cellulose and psyllium have also been found to be effective. Just be sure to drink plenty of fluids when increasing fiber intake.

Diverticular disease. It’s not uncommon for small pouches (called diverticula) to form in the wall of the colon. Most people with diverticulosis, as this condition is called, have no symptoms, but 15 to 20 percent may develop diverticulitis, a painful inflammation of these pouches.

While not all research agrees that a high intake of dietary fiber prevents diverticular disease, a study that followed over 46,000 men from 1986 to the end of 2012 found that a Western dietary pattern (high in red meat, refined grains, and high-fat dairy) was associated with a higher risk of developing diverticulitis than a diet high in fiber-rich fruits, vegetables and whole grains.

Colorectal cancer. Some large studies have suggested that fiber, especially from whole grains, may lower risk for developing colorectal cancer. While the American Cancer Society does not think there is sufficient evidence yet that fiber itself lowers risk, diets high in fiber-rich foods (like fruits, vegetables and whole grains) and low in red and processed meats apparently do.

Intestinal lining dysfunction. Like the nasal passages, the walls of our intestines hold our first line of defense against invading pathogens. Keeping the lining of our intestines healthy is essential to protecting our overall health, and fiber can help. The fiber we eat feeds many of the billions of microbes living in our intestines.

Keeping our microbial friends well fed and happy may be key to keeping our intestines healthy. When the microbes munch on fiber, they release short-chain fatty acids (SCFAs), which provide fuel for intestinal cells and help create an environment that’s more attractive to beneficial organisms, and less hospitable to pathogens like salmonella or E. coli.

Research Results: Fiber and Gut Health

When mice in two new studies were fed a low-fiber diet, the overall population of beneficial microbes shrank, and the make-up of that population changed. At the same time, the mucus layer of their intestine thinned, allowing the bacteria to get closer to the intestinal wall. Without an adequate mucus layer, even beneficial gut bacteria can trigger an immune response in the intestinal lining.

A hallmark of immune reactions is inflammation. After just a few days on a low-fiber diet, the mice in the studies developed chronic inflammation. After a few weeks, they began putting on fat and developing higher blood sugar levels. When mice on a generally un-healthy high-fat diet were given fiber, they fared much better.

“The diversity and quantity of the good bacteria in our gut is very much influenced by diet,” says Davis. “People who eat more plant foods have been shown to have increased diversity and number of beneficial gut flora,”

Bottom Line: Boost Fiber-Rich Plants

“It’s really important to get plenty of fiber from fruits and veggies, whole grains, and other plant sources,” says Davis. Whether it’s helping your gut operate smoothly, or feeding those beneficial gut microorganisms, the gut-health-boosting fiber in delicious, nutritious plant foods is just one more reason to enjoy them in you diet.

—Judith C. Thalheimer, RD, LDN


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