You know what baffles me?
Why we ever decided to teach people with IBD to think about food in terms of “soluble” versus “insoluble” fiber.
I’ll be honest. I’ve used those terms plenty of times with my patients.
But fiber is so much more complex than that.
Solubility is just one property. It’s not the whole picture.
How fiber behaves in the body depends on particle size, texture, viscosity, fermentability, food preparation, and more.
So why do so many IBD food lists say “avoid (or limit) insoluble fiber”?
It comes down to roughage.
Roughage refers to large, coarse, intact particles: thick fruit and vegetable skins, whole nuts and seeds, raw fibrous vegetables, and coarse whole grains.
These particles can mechanically irritate the gut and stimulate motility by drawing water and mucus into the colon.
In people with healthy digestive tracts, that can be beneficial — promoting bulkier, more regular bowel movements.
But for those with active inflammation, diarrhea, abdominal pain, or cramping, those coarse particles can worsen symptoms.
In people with narrowing or stricturing disease, they may even increase the risk of obstruction.
Roughage is really about particle size and texture.
When foods are well-cooked, blended, ground, pureed, or finely milled, they no longer have the same mechanical effect in the gut.
A soft, finely milled whole wheat bread technically still contains insoluble fiber, but it no longer behaves like coarse roughage — so it may be tolerated similarly to white bread.
That distinction matters because many people unnecessarily avoid foods they may actually tolerate well.
Soluble fiber isn’t one thing either.
I recently came across an article on dietary recommendations for IBD that simply stated soluble fiber is “beneficial.”
Beneficial how?
Some soluble fibers are highly fermentable and fuel beneficial gut microbes, though fermentation can also increase gas production.
Some form a gel in the small intestine that helps slow transit and supports blood sugar and cholesterol management. Fewer stay viscous long enough to travel through the colon, where that gel structure can help form stool.
Not all soluble fibers are fermentable, and not all are viscous — which is why we need to be more specific about the foods we recommend and what we’re hoping they accomplish.
Most whole foods contain a mix of both soluble and insoluble fiber anyway, which makes applying these recommendations to everyday eating even harder.
Telling someone to “choose soluble fiber” or “avoid insoluble fiber” doesn’t give them much practical guidance.
At least when it comes to roughage, the better frame is texture and preparation.
Not everyone with IBD needs to avoid it — but for those with active symptoms, inflammation, or risk of obstruction, reducing coarse particle size often matters more than counting fiber grams or avoiding a specific fiber “type.”
➡️ “Avoid insoluble fiber” is often really shorthand for avoiding roughage: large, coarse particles.
One more thing worth mentioning about roughage
It’s not just a plant food issue.
Some animal foods, like sausage casings, chewy shellfish, and gristly cuts of meat, contain no fiber at all, but can still be difficult to break down and may increase risk of obstruction in someone with a stricture or narrowing.
Again, the better frame is texture.
Does the food break down easily when chewed? If not, it may still pose a risk, regardless of what the fiber content says.
➡️ If you or your patients are at risk for obstruction, low-fiber isn’t the same as low-roughage.
