What's the Difference Between Insoluble and Soluble Fiber, According to a Dietitian

Fiber is one of those do-it-all nutrients that most of us aren't getting enough of. Over 90% of adults in the United States do not eat enough fiber each day, despite its health benefits, according to the 2020-2025 Dietary Guidelines for Americans.

Present mainly in plants—vegetables, fruits, whole grains, legumes, nuts and seeds—fiber is a type of carbohydrate that's primarily undigested and passes through the digestive tract. Fiber comes in two forms: soluble and insoluble, which both provide distinct benefits.

Most plant-based foods come with both types of fiber in varying amounts. Research like a 2022 review in Food Science and Human Wellness shows that together, soluble and insoluble fiber form a mighty team to support a healthy gut and reduce the risk of chronic conditions, such as diabetes, heart disease and some types of cancers. Bulking up your diet with fiber has also been found to support weight loss.

If you want to increase your fiber intake, read on to learn about the differences between soluble and insoluble fiber and what foods contain each type of fiber. (Hint: Don't peel your apples.)

What Is Soluble Fiber?

According to the National Library of Medicine's resource MedlinePlus, soluble fiber is a type of fiber that absorbs water, forming a gel in the digestive tract and making stools easier to pass.

Benefits of Soluble Fiber

Because of its ability to absorb water and swell up, soluble fiber helps slow down the digestion of carbohydrates—and other nutrients—preventing sudden spikes in blood glucose levels. This slower digestion also helps keep you feeling fuller longer, which is especially helpful when it comes to weight loss.

The viscous soluble fiber may lower your cholesterol level by acting like a sponge that binds to cholesterol and fat in your food, carrying them out of your body through your stools.

And that's not all. A 2023 review in Nutrition Journal suggests that eating soluble fiber might lower blood pressure. This may be in part because eating foods with this fiber improves satiety, which can lead to weight loss and lower blood pressure, but further research is needed to pinpoint the reason why.

If you live with irritable bowel syndrome, eating soluble fiber may help manage your symptoms, such as diarrhea and loose stools, by making your stools more dense. If you have IBS, be sure to add fiber to your diet slowly—too much, too soon may cause even more distress. And drinking enough water alongside your fiber helps, too.

What Foods Have Soluble Fiber?

You can find soluble fiber in a wide range of plant-based foods, such as:

• Legumes: split peas, beans, lentils, edamame
• Whole grains: oats, barley
• Vegetables: Brussels sprouts, sweet potatoes, carrots, peas
• Fruits: oranges, avocados, apples, pears
• Seeds: psyllium, chia, flax

What Is Insoluble Fiber?

Contrary to soluble fiber, insoluble fiber is just what it sounds like—a type of fiber that does not absorb water. Rather than forming a gel-like soluble fiber, insoluble fiber adds bulk to the stool and draws water to the colon, making stools soft and easy to pass.

Benefits of Insoluble Fiber

Because insoluble fiber bulks up the stool and makes it softer and easier to pass, it limits the amount of time the stool is sitting in the colon. This means that any chemicals in the foods you're eating won't be sitting around as long, which reduces the chances of them causing harm. This may lower the risk of colorectal cancer.

Combined with soluble fiber, insoluble fiber may reduce your risk of type 2 diabetes, per a 2020 study in the Journal of Diabetes Investigation.

And a 2023 review in Nutrients suggests that the benefits of insoluble fiber may go beyond just the fiber itself and its effects on bulking up the stool. Scientists looked deeper at the evidence and found that the powerful plant compounds in fiber may be important contributors to disease prevention and improved health in addition to the fiber itself.

What Foods Have Insoluble Fiber?

Insoluble fiber is the tough and chewy parts of vegetables and fruits, such as apple skins, carrot peels, broccoli stems and asparagus stalks. In addition to being found in produce, you can also find insoluble fiber in whole-grain breads and cereals, wheat bran, oat bran and nuts.

How Much Fiber Should You Eat Every Day?

There are no specific recommendations on the amount of each type of fiber you should eat each day. The 2020-2025 Dietary Guidelines for Americans recommends daily fiber intake based on age and gender. If you are pregnant, your fiber needs also change.

Generally speaking, aiming for 25 grams a day is a great goal and 30 grams or more is even better.

Women:

• Ages 19 to 30: 28 grams
• Ages 31 to 50: 25 grams
• Over age 50: 22 grams

Pregnant Women:

• Ages 19 to 30: 28 grams (first trimester), 34 grams (second trimester), 36 grams (third trimester)
• Ages 31 to 50: 25 grams (first trimester), 31 grams (second trimester), 34 grams (third trimester)

Men:

• Ages 19 to 30: 34 grams
• Ages 31 to 50: 31 grams
• Over age 50: 28 grams

Enjoying a range of fiber-rich foods ensures you consume both types of fiber. Instead of meticulously tracking fiber intake, pay attention to the signs that you might be getting too much or too little fiber—both of which can cause gas, bloating and abdominal cramps. Eating too little fiber can also cause constipation and consequently hemorrhoids due to difficulty in passing stools.

The Bottom Line

In short, soluble fiber forms a gel and acts as a sponge to help sweep fat and cholesterol out of your intestinal tract, while insoluble fiber bulks up your stools. Both types of fiber are essential for supporting a healthy digestive system and overall wellness. If you're increasing your fiber intake, take it easy and progressively increase the amount over time. It's also important to increase your intake of fluids, such as plain water, to support healthy digestion and help pass the stool out of the body.