I see this claim all the time, everywhere, when fiber and constipation are discussed. It has always mystified me, as someone who has struggled with constipation. I’ve usually found that “bulk” was more of a problem than a solution. Where this claim came from, I have no idea. We have known for many decades that fiber is beneficial for “regularity,” and can help manage constipation. I’m guessing that someone thought this up as an explanation, and in the absence of a better one, it became dogma.
What are the real ways that fiber helps regulate gut motility?
The principal way that fiber helps is by supporting gut microbes that modulate the gut nervous system, and thus motility. Zheng and colleagues (2025) recently published a review of what we know about the effects of microbes on constipation. They highlight the key role of certain groups of microbes that ferment fibers in normal bowel motility. The mechanism mostly relies on interactions between microbial products—such as short-chain fatty acids (like butyrate)—and serotonin, which regulates gut motility.
So, while fiber can help stool pass by absorbing water and softening it, the major benefits of fiber rely on the products of specific gut microbes, which in turn rely on dietary fiber.