Soluble vs. Insoluble Fiber: How to Know What’s Right for You if You Have IBS

Fiber is an important part of your daily diet. That’s especially true for people living with irritable bowel syndrome (IBS), a gastrointestinal condition marked by stomach cramps, diarrhea, and constipation. Because the body reacts differently to soluble and insoluble fiber, each type can help or hurt, depending on the IBS symptoms you're experiencing at any given time.

The Differences Between Soluble and Insoluble Fiber

Experts liken fiber to an on-off switch as far as IBS is concerned. Soluble fiber slows things down in the digestive tract, helping with diarrhea, while insoluble fiber can speed things up, alleviating constipation.

“Soluble fiber is hydrophilic so people can think of soluble fiber as being a magnet to water,” says Melissa Majumdar, RD, a senior bariatric dietitian for the Brigham and Women's Center for Metabolic and Bariatric Surgery in Boston.

By attracting water, soluble fiber removes excess fluid, which is how it helps decrease diarrhea. Majumdar recommends that her patients with IBS who are dealing with diarrhea increase their intake of these soluble fiber-rich fruits and vegetables:

• Apples
• Oranges
• Pears
• Strawberries
• Blueberries
• Peas
• Avocados
• Sweet potatoes
• Carrots
• Turnips

Oats, beans, bran, and barley are also good sources of soluble fiber.

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Insoluble fiber, on the other hand, does not dissolve in water, so it stays intact as it moves through your digestive system. “This is something that can be helpful for constipation because it adds bulk to the stool and can get things moving, almost like a laxative effect,” says Majumdar, who is also a spokesperson for the Academy of Nutrition and Dietetics.

She advises her patients suffering from constipation to focus on adding more vegetables like these to their diets:

• Zucchini
• Broccoli
• Cabbage
• Leafy greens
• Cauliflower
• Blackberries

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Other foods rich in insoluble fiber include flaxseed, chia seeds, whole grains, bran, brown rice, cereals, and rolled oats.

Fiber supplements can also help you increase your intake, but Majumdar says that people should turn to this only if they can’t get enough fiber in their diets.

“Some of my patients are limited in their diets and can’t get enough fiber to meet what their body needs, so I would go to a supplement in those cases,” she says.

A meta-analysis published in September 2014 in The American Journal of Gastroenterology evaluated the use of dietary fiber supplementation in 14 randomized, controlled clinical trials involving 906 people living with IBS. The authors concluded that fiber supplementation — especially with psyllium, a soluble fiber — was effective in improving symptoms of IBS when compared with a placebo.

According to a review published in September 2017 in the International Journal of Molecular Medicine, dietary fiber supplementation appears to be safe, although if introduced to the body too rapidly, it can lead to unwanted side effects like abdominal bloating.

Still, Majumdar cautions that supplements are considered functional fiber, which means they may not be as beneficial as a whole food. Foods that have labels touting “added fiber” are also forms of functional fiber and should be met with some skepticism.

“Though not harmful, we don’t know that those are beneficial necessarily because they don’t have the same nutrients and biochemicals that a whole food would have,” she says.

Increasing Your Fiber Intake for IBS Symptom Relief

While dietary fiber can improve the function of your digestive system, increasing your intake all at once can leave you feeling bloated and gassy when your body's not used to high amounts.

If you want to increase your fiber intake to better control IBS symptoms, Majumdar recommends adding fiber one meal at a time, then waiting a few days to a week to see how the body reacts. If all is well, you can continue adding more fiber to your diet.

“The first thing I would do is break down each meal and see where there are places to add fruits and vegetables,” she says.

For example, instead of eating a pastry for breakfast, try Greek yogurt with fruit, nuts, and flaxseed instead. For lunch and dinner, try adding salads, sides of fruits and vegetables, and whole grains like brown rice, quinoa, and farro.

A good rule of thumb is to fill up half your plate with fruits and vegetables, Majumdar says. Also, replace refined grains with whole grains. Instead of white bread, refined cereals, and white rice, choose whole-grain breads, bran muffins, oatmeal, whole-grain cereals, and brown rice.

Remember to make these changes gradually for an easier transition.

And don’t forget to drink plenty of water. “Fiber can’t do its job without water. It can cause more GI distress if it’s not married with fluid,” Majumdar says.

Finally, Majumdar notes that fiber isn’t the only factor in IBS symptoms. She recommends talking to your doctor about your diet and trying elimination diets for periods of time to identify which foods are triggering your symptoms.

Additional reporting by Ashley Welch