This was a question that peaked my curiosity, so I dug into the research.
Most of the studies around digestion kinematics focus on ruminants (typically cows), which isn’t too useful for us.
Luckily, I found one study that looked at the digestion speed of fiber in chimpanzees and humans.
Study Summary: Fiber Transit Time in Humans
What they did: They fed 24 adult males various types of fiber, marked with blue plastic pellets so amounts in stool could be tracked.
Feel free to read the whole study, but the relevant part that we care about is this table that shows the mean transit times of each type of fiber.
In particular, we care about the left section – “Mean transit time”.
Each of these 3 columns represents a different tracking method, but all produced similar results. Pick any of the 3 columns (Pellets, PEG2, Cr-mordant) to look at.
It took anywhere from 38.9 hours to 61.1 hours for the fiber to digest.
That number is the mean, which means it can take even longer depending on the individual.
That’s our answer to the original question that prompted this post, and if that’s all you’re looking for, feel free to move on.
What Affects Fiber Digestion Time
The reason I was first interested in this question was thinking about the possibility of getting too much fiber after going vegan.
Even when I fasted for up to a day, my gut still didn’t feel empty, which surprised me since I always heard that food was digested withing 8-10 hours or so. Perhaps most of the nutrition is extracted and it’s moved on to your gut, but fiber and other remnants are still there, which this research clearly shows.
Regardless of fiber source, you’re looking at it staying in your body for at least a day and a half. Beyond that, the digestion time varies based on:
- Type of fiber – As the results show, the coarse bran (think large unprocessed flakes of fiber from wheat) actually takes less time to digest than fine bran (more processed and smaller). Cellulose (from plant walls) takes less time than fine bran.
- Mixture of fiber – When you eat a food with fiber in it, it’s usually a combination of many things that are considered “dietary fiber”. Some digest faster than others. For example, cabbage fiber takes longer to digest than just cellulose, because it contains other types of fiber as well.
- Amount of fiber – For the most part, serving size won’t affect the transit time too much. But as you increase your fiber past a certain intake, especially if it’s new to your gut, your total digestion time will increase. This could be an issue if you’re eating a lot of foods high in fiber.
Wrapping up, fiber stays in your system around 39 to 61 hours.