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:
- Sweet potatoes
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.
She advises her patients suffering from constipation to focus on adding more vegetables like these to their diets:
- Leafy greens
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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.
One study review 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 another review, 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.