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Dietary Fiber: Insoluble vs. Soluble

Dietary Fiber: Insoluble vs. Soluble

Kathleen M. Zelman, MPH, RD, LD

Confused about fiber? You’re not alone. Dietary fiber is a misunderstood nutrient. Many people know it is important, but not much more than that. This article fills you in on the two main types of fiber – soluble and insoluble -- where to find them, and the health benefits they provide.

Dietary fibers are found naturally in the plants that we eat. They are parts of plant that do not break down in our stomachs, and instead pass through our system undigested. All dietary fibers are either soluble or insoluble. Both types of fiber are equally important for health, digestion, and preventing conditions such as heart disease, diabetes, obesity, diverticulitis, and constipation.

Soluble vs. Insoluble Fiber

Soluble fiber dissolves in water. Insoluble fiber does not. To some degree these differences determine how each fiber functions in the body and benefits your health.

Soluble fibers attract water and form a gel, which slows down digestion. Soluble fiber delays the emptying of your stomach and makes you feel full, which helps control weight. Slower stomach emptying may also affect blood sugar levels and have a beneficial effect on insulin sensitivity, which may help control diabetes. Soluble fibers can also help lower LDL (“bad”) blood cholesterol by interfering with the absorption of dietary cholesterol.

  • Sources of soluble fiber: oatmeal, oat cereal, lentils, apples, oranges, pears, oat bran, strawberries, nuts, flaxseeds, beans, dried peas, blueberries, psyllium, cucumbers, celery, and carrots.

Insoluble fibers are considered gut-healthy fiber because they have a laxative effect and add bulk to the diet, helping prevent constipation. These fibers do not dissolve in water, so they pass through the gastrointestinal tract relatively intact, and speed up the passage of food and waste through your gut. Insoluble fibers are mainly found in whole grains and vegetables.

  • Sources of insoluble fiber: whole wheat, whole grains, wheat bran, corn bran, seeds, nuts, barley, couscous, brown rice, bulgur, zucchini, celery, broccoli, cabbage, onions, tomatoes, carrots, cucumbers, green beans, dark leafy vegetables, raisins, grapes, fruit, and root vegetable skins.

How Much Dietary Fiber Do You Need?

Most Americans get only about 15 grams of fiber per day in their diet. But the 2005 Dietary Guidelines for Americans recommends about 25 grams for women under 50 and teenage girls. Teenage boys and men under 50 (who consume more calories than women) require upwards of 30-38 grams of dietary fiber daily. 

Don’t worry about what kind of fiber you are taking in unless you are seeking a specific health benefit, such as eating more soluble fiber to lower cholesterol. Instead, focus on eating a healthy diet rich in fruits, vegetables, whole grains, legumes, nuts, and seeds. This will provide a variety of soluble and insoluble fibers and all of the health benefits.

As you increase the fiber in your diet, you may experience more intestinal gas. Increasing fiber gradually will allow your body to adapt. Because some fibers absorb water, you should also drink more water as you increase fiber.

Tips to Get More Fiber in Your Diet

Meeting dietary fiber goals can be a challenge, especially if you’re used to eating lots of processed and refined foods. Use these tips to help work more fiber in your diet:

  • Eat more whole fruit instead of fruit juice. 
  • Read labels. Look for the word "whole” before any grains on the ingredient list and check the number of grams of dietary fiber on the nutrition facts panel of packages to select high-fiber foods. 
  • Start your day with a bowl of bran or other high-fiber cereal that contains at least five grams of fiber per serving.
  • Snack on raw vegetables. 
  • Add legumes, seeds, and nuts into soups, salads, and stews. 
  • Replace refined white bread, pasta, and rice with whole-grain products. 
  • Eat a vegetarian meal at least once a week.

Fiber supplements can also help you meet your fiber needs. But experts warn that the health benefits of supplements have not been studied as extensively as dietary fibers and may not have the same effect as fiber in foods.