What is the Difference Between Soluble and Insoluble Fiber?

Soluble and insoluble fiber play different roles in promoting health and preventing disease. Find out what do they do and which foods contain them.

Monica Reinagel, MS, LD/N, CNS
5-minute read
Episode #93

I’ve written before about the health benefits of fiber.  There are two major categories of fiber: soluble and insoluble. Each plays a unique role in promoting health and preventing disease. Let’s take a closer look at how these two types of fiber work to keep you healthy and how to be sure you’re getting plenty of each.

What is Fiber?

First, let’s talk about fiber in general. Fiber comes from plant foods: fruits, vegetables, grains, nuts, and legumes. It’s a type of carbohydrate that the body can’t digest. Unlike starches and sugars, which are the other types of carbohydrates, fiber contributes no calories, or food energy. Instead of being broken down and absorbed into the bloodstream, fiber simply passes through the entire digestive tract.

Why Eat Fiber?

But that doesn’t mean it doesn’t do anything for you along the way. For one thing, food containing fiber takes up more space than food that doesn’t. That usually means that you feel fuller after you eat it. For example, you’ll feel more satisfied if you eat a meal that contains 300 calories and 10 grams of fiber, than if you eat a meal with same number calories but no fiber.

Not only will you feel more satisfied after you eat, but that feeling of satisfaction will last longer. That’s because when a meal contains fiber, the food moves more slowly from your stomach into your small intestine. Translation: It takes longer for you to feel hungry again after a high-fiber meal than after a low-fiber meal. 

Obviously, both of these effects (which are technically known as “increased satiety” and “delayed gastric emptying”) are helpful for those who are trying to watch their calorie intake. In fact, studies show that people who eat more fiber tend to be leaner and are less likely to gain weight over time. But fiber does some other beneficial things as well—depending on whether it is soluble or insoluble.

What is Soluble Fiber?

Studies show that people who eat more fiber tend to be leaner and are less likely to gain weight over time.

If you stirred some soluble fiber into hot water, it would dissolve. In your stomach, the soluble fiber you’ve eaten dissolves in the water from your food and/or digestive juices and makes a viscous liquid or gel. This gel can trap certain food components and make them less available for absorption.

In particular, soluble fiber interferes somewhat with the absorption of fats and sugars. Now, before you get too excited, let me clarify that soluble fiber doesn’t keep you from absorbing calories from foods high in fat and sugar—at least, not in any meaningful way. But its fat binding action can help reduce cholesterol. And by slowing down the absorption of sugar, it helps keep blood sugar levels steadier—which is helpful for managing and preventing diabetes.

What Foods Contain Soluble Fiber?

The best food sources of soluble fiber include:

  • legumes (peas and beans)

  • barley

  • oat bran

  • chia seeds

Most fruits and vegetables also provide some soluble fiber. Psyllium husk, which is a very popular fiber supplement, is also primarily soluble fiber.

What is Insoluble Fiber?

If you stir some insoluble fiber into hot water, it won’t dissolve. As soon as you stop stirring, it’ll just sink to the bottom. It will however, soak up a bunch of the water and puff up, the way a dry sponge expands as it soaks up water.

Now imagine this puffed up sponge moving through your intestines, and you’ll begin to get an idea what insoluble fiber does for you. Insoluble fiber is a very effective treatment and preventive for constipation and other digestive disorders like diverticulosis and irritable bowel syndrome.

Note: In some cases, people with acute digestive problems are advised to reduce the amount of fiber in their diet—at least temporarily—until things calm down. If you’re being treated by a physician, be sure to follow his or her recommendations about your diet.  The House Call Doctor has a handy guide to bowel problems, but be sure to consult your own doctor for your own specific needs.


About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.