How Can Fiber Make Both Diarrhea and Constipation Better?!

There’s one specific kind that can play double doody.

By Julie StewartReviewed by Brijen J. Shah, MD, AGAF

March 25, 2025

Whether you rush to the toilet on the reg or struggle to drop a deuce, you’ve probably heard the same advice: Eat more fiber.

But how does the same recommendation hold true for two opposite GI problems? While some kinds of fiber work better than others for certain issues, there is one very particular type that actually can come in clutch for both. It’s called viscous soluble fiber, and research suggests it can help people on both ends of the poop spectrum.

“Viscous fiber is really good for both constipation and diarrhea, because it can soften stool as well as slow down the movement of that stool,” Emily Pollard, RDN, LDN, a dietitian who specializes in chronic gut issues. But…how exactly can the same stuff firm up your poop in one situation, and loosen it another? We tapped the experts to explain how.

First: Not all fiber is created equally.

A quick fiber refresher. Fiber is a type of carbohydrate—one of the three macronutrients our bodies need to function properly. There are two general forms: soluble (which comes from the inner flesh of plants and dissolves in water) and insoluble (which generally comes from the skin of plants and does not dissolve in water).

Soluble fiber can then be broken down into two specific kinds, too: viscous (which forms a gel-like substance when mixed with water, and is found in foods like kiwis, prunes, chia seeds, and oatmeal, as well as psyllium fiber supplements like Metamucil) and nonviscous (found in foods like onions and chicory root). In general, when we think about soluble fiber, we’re generally thinking about the viscous kind.

Here’s a quick breakdown of how much fiber you can expect from some common foods:

Old-fashioned oats: 4 g fiber (2 g soluble) per 1/2 cup dry serving.
Chia seeds: 9.7 g of fiber (0.7 to 1.5 g soluble) per ounce. By soaking chia seeds, you transform some of their insoluble fiber into a more soluble form, says Pollard.
Prunes: 3.3 g fiber (1.8 g soluble fiber) per four to six prune serving.
Yellow peaches: 2.6 of fiber (1 g of soluble fiber) per medium fruit.
Kiwi: 2.2 g of fiber (0.5 g of soluble fiber) per single fruit.
Apples: 4 g of fiber (1.2 g soluble) per raw medium-sized piece of fruit. When you cook apples, you break some of their insoluble fiber down into a more soluble form, says Pollard.

So how does fiber help constipation and diarrhea?

In the double doody realm, viscous soluble fiber is the star. As the carb travels through your digestive tract, it absorbs water, which explains its shape-shifting properties. “The presence of that fiber will influence how water itself is retained or absorbed in the stool,” says Pollard. This dual effect is especially helpful for people with mixed-type irritable bowel syndrome (IBS) that switches between constipation and diarrhea, Darren Brenner, MD, a gastroenterologist at Northwestern Medicine Digestive Health Center and professor of medicine at Northwestern University, tells SELF.

Think of your poop like a sponge. Diarrhea is like a really oversaturated, filled-to-the-brim one. “Adding soluble fiber to a wet ‘sponge’ leads to absorption of the water, bulking and adding texture,” Dr. Brenner says. When some of the water is sucked out of the stool, your poop becomes firmer and thicker, which helps it slow down through your system.

Left to its own devices, runny stool can blast through your large bowel in just an hour or two. Formed poop, on the other hand, takes longer to enter and exit—say, closer to a day—which can put the brakes on your toilet trips.

Okay, so that’s how fiber helps with diarrhea. But what if you’re dealing with the opposite—constipation, in which there’s not enough moisture to move things along. The same analogy works here too. When you’re constipated, your poop is like a hard, dry sponge.

“Adding soluble fiber to a dry ‘sponge’ enlarges the surface area and weight of the sponge,” Dr. Brenner says. This allows your poop to draw in more water from your digestive tract, “softening the sponge,” he says. As a result, your poop now has enough moisture in it so the contractions of the muscles lining your gut can effectively propel it forward.

Either way, this type of fiber makes your stool heftier and easier to pass. It’s a bulking agent, whether it’s drawing water into something hard to soften it, or absorbing water when there’s too much, says Dr. Brenner.

“I always tell my patients, if things are too hard, it will soften them,” he says. “If they’re too loose and too watery, it’ll absorb the fluid and bulk them up.”

If you’re wondering how making poop bigger can help here, know that softness is key in constipation. The definition of it is stool that’s hard to release, usually resulting in three or fewer poops per week—the size of poop doesn’t really factor in there. In fact, some research suggests people with and without constipation have similarly sized poops when they do go. (It’s just that constipated people don’t go as often.)

While viscous soluble fiber comes in clutch for both constipation and diarrhea, that doesn’t mean other types of fiber are slouches in the bathroom department, either—they just have more specialized roles. For example, nonviscous soluble fibers, also known as fermentable fibers, don’t have the same water-absorbing effect as their viscous cousins. But when your gut bacteria break down those fermentable fibers, they create byproducts that can spur contractions in the muscles lining your gut. This helps stimulate bowel movements, making this type of soluble fiber a boon if you want to go more often.

Insoluble fibers, found in foods like wheat bran or corn, don’t absorb water either, but they can also aid constipation. These fibers mildly irritate your large bowel, triggering the release of water and mucus to move things along.

Here’s how to use this info to make your toilet trips a little smoother.

Even if everything gut-wise is going well, you probably still need to eat more fiber. After all, its benefits extend beyond your belly—it can help you protect your heart by reducing cholesterol and blood pressure, among other perks—and most people aren’t getting enough of it: About 90% of women don’t eat the recommended amount of 25 grams of fiber per day, per the US Dietary Guidelines.

Thankfully, we have you covered here: These little ways to eat more fiber can help you gradually add more of the nutrient to your day. And gradual is the key word. No matter what type of fiber we’re talking about, adding too much to your diet all at once can trigger GI symptoms like bloating and gas as your gut adjusts to the shift, says Dr. Brenner. Try to bump up your soluble intake by about two to four grams per day, max, until you hit about 10 grams of soluble fiber, he says. (A psyllium fiber supplement can help you get there, Dr. Brenner says).

While you’re increasing your fiber, make sure to fill up your water bottle on the reg too, says Pollard: Staying hydrated plays an important role in softening the consistency of your stool too, making it easier to pass.

If you’ve upped your fiber and your water intake and are still experiencing poop problems, it’s time to loop in a pro. Sometimes, gut problems stem from a disruption in the gut-brain connection or a disturbance in your balance of good gut bacteria, says Pollard. A registered dietitian nutritionist can help you suss out those other issues.

And if chronic diarrhea or constipation interfere with your ability to get through a work day or do things you enjoy, talk with your primary care physician—or see a gastroenterologist–for more help, says Dr. Brenner. This holds especially true if it comes with red flag symptoms, like extreme pain, bleeding when you poop, or severe heartburn. These could signal a range of health issues that require further treatment, so getting them checked out is super important.