Does fiber speed up or slow down digestion?

The question comes up because people have heard both answers. Fiber slows digestion, which is why oatmeal keeps you full. Fiber speeds digestion, which is why bran is what your grandmother took. Both are true. They are true in different parts of you.

At the top end, fiber slows things down. The soluble, gel-forming kind in oats, barley and beans makes a meal leave the stomach and get absorbed more gradually. That blunts the rise and crash in blood sugar that leaves you rummaging in a cupboard at four in the afternoon, and it is most of why a bean-heavy lunch holds you until dinner and a sandwich does not. A 1978 experiment showed the effect directly: adding viscous fiber to a glucose drink flattened the blood-sugar curve, and the flatter curve tracked how viscous the fiber was.

At the bottom end, fiber speeds things up. Further down, it adds bulk and holds water, and a sluggish bowel moves faster. It does not push a regular one into overdrive. It tends to bring both extremes toward normal, which is the part of the story that was worked out by a surgeon who counted stools.

Denis Burkitt was an Irish-born surgeon whose name is on the lymphoma he described in Uganda. He spent his later career on a different puzzle: why the bowel diseases filling Western hospitals were rare where people still ate traditional high-fiber diets. With Alec Walker, a South African researcher who had been timing gut transit for years, and the surgeon Neil Painter, he recorded more than a thousand transit times and weighed what came out. Unrefined diets produced large, soft stools that moved through quickly. Refined Western diets produced small, firm ones that took days. The paper was published in 1972.

The next year another group put twenty people on extra bran or a high-fiber diet and timed them. The slow ones sped up, from about 3.8 days to 2.4. The unusually fast ones slowed down. Fiber pulled both toward the middle. A 2016 review of ninety-three later studies put a number on it: where transit started at 48 hours or more, each extra gram of fiber a day shortened it by about half an hour, and where it was already faster than that, fiber did not change it.

Fifty years on, that part of Burkitt’s work has held up well. So has a lower rate of diverticular disease in the people who eat the most fiber: in a British cohort followed for a decade, the highest fiber intakes went with about forty percent less risk of being admitted to hospital with it.

The cancer part is more complicated, and I would rather say so than let the good half of the story carry the weak half. Large population studies find about ten percent less colorectal cancer for every extra ten grams of fiber a day. But the two big trials that gave people bran or a high-fiber diet did not stop polyps coming back in people who had already had them removed. Polyps over three or four years are not cancer over decades, and the trials were small next to the cohorts. The honest summary is that fiber probably lowers the risk, that the case rests on observation rather than experiment, and that nobody should eat oats expecting them to undo a colonoscopy finding.

What fiber also does, and this matters for the muscle half of eating well, is fill you up. On a plate where half or more is plants, fiber is the reason you can eat until you are full and still lose fat rather than muscle. That is the mechanism The LP3 Diet is built on, and chapter two tells the rest of this story.