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
May 6, 2010
Episode #093

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?


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.

What Foods Contain Insoluble Fiber?

The best food sources of insoluble fiber include:

  • wheat

  • corn

  • oat bran

One of the reasons that you’re encouraged to eat whole grains is because they include the bran and are therefore higher in fiber than refined grains.

Other good sources of insoluble fiber include:

  • nuts

  • flaxseed

  • the skins and peels of many fruits and vegetables, such as apples and potatoes

But the truth is that most foods contain a mixture of soluble and insoluble fiber. The inside of apples, for example, provide soluble fiber and the skins are mostly insoluble fiber.

How Much Soluble and Insoluble Fiber Do You Need?

The dietary recommendations for fiber are 25 gram per day for women and 38 grams per day for men. There is no guideline for how much of that should be soluble or insoluble. In the average diet, about three-quarters of the fiber is insoluble and one quarter is soluble. I think that reflects the fact that we tend to eat a lot of grain-based foods and not enough fruits and vegetables. 

Which Type of Fiber Should You Eat?

If you’re particularly concerned with keeping your blood sugar steady or your cholesterol down, you might want to focus on eating more legumes and oats, which are good sources of soluble fiber. If digestive health or constipation is a problem, you might want to emphasize foods that contain a lot of insoluble fiber, such as flaxseed, wheat, corn, and rice bran. But chances are you could do with more of both kinds of fiber. Most people get less than half the recommended amount.

Should You Take Fiber Supplements?

I suggest that you get most or all of your fiber from actual foods instead of fiber supplements, because high fiber foods contain a lot of other healthy nutrients as well.

And it’s not that hard to do.  For example, you’ll get a third of your fiber needs by eating the recommended five servings of vegetables and two to three servings of fruit (not juice!) each day. Four servings of whole grains provide another third. Three or four servings of nuts, seeds, or legumes will take you over the finish line—with a nice mix of soluble and insoluble fiber.

If you have a nutrition question for me, send an email to nutrition@quickanddirtytips.com or post it on my Nutrition Diva Facebook Page. If tweeting is more your thing, I also have a handy little Twitter account. 

You can also search the archives using the search box at the top of the page. There’s a good chance I might have already answered your question in a previous article.


Fiber image courtesy of Shutterstock

Related Tips