A study out of the University of North Carolina found no association between dietary fiber intake and diverticulosis in comparing the group that ate the highest amount, 25 grams—three times the amount of the lowest fiber intake group. They concluded that a low-fiber diet was not associated with diverticulosis. The university sent out a press release: “Diets high in fiber won’t protect against diverticulosis.” The press picked it up. Study finds high-fiber diet may not protect against diverticulosis. Went all over the paleo blogs, and even medical journals; an important paper calling into question the fiber theory of the development of diverticulosis. Other editorials, though, caught the critical flaw. To understand this, let’s turn to another dietary deficiency disease: scurvy.

Medical experiments on prisoners at the Iowa State Penitentiary showed that clinical signs of scurvy start appearing after just 29 days without vitamin C. Experiments on pacifists during World War II showed the same thing—that it takes about 10mg of vitamin C a day to prevent scurvy. So, imagine going back in time a few centuries, when they were still trying to figure scurvy out. Dr. James Linde had this radical theory that citrus fruits could cure scurvy. What if an experiment were designed to test this crazy theory, in which sailors were given the juice of either one wedge of lemon, or three wedges of lemons a day? If a month later on the high seas there was no difference in scurvy rates, one might see headlines like this. The printing press pamphleteers would all be touting the study that found that a low-vitamin C diet is not associated with scurvy.

See, a wedge of lemon only yields about 2mg of vitamin C, and it takes 10mg to prevent scurvy. So, they would have been comparing 2mg a day to like 7mg a day—one vitamin C deficient dose to another vitamin C deficient dose. No wonder there would be no difference in scurvy rates. We evolved eating so many plants that we likely averaged around 600mg of vitamin C a day. That’s what our bodies are biologically used to getting. What about fiber? How much fiber are we used to getting? Over 100 grams a day. The highest fiber intake group in the North Carolina study was eating only 25 grams, which is less than the minimum recommended daily allowance, which is about 32 grams. They didn’t even make the minimum. So they compared one fiber-deficient diet to another fiber-deficient diet—no wonder there was no difference in diverticulosis rates.

The African populations, where they had essentially no diverticulosis, ate diets consisting in part of very large platefuls of leafy vegetables—similar, perhaps, to what we were eating a few million years ago. They were eating plant-based diets containing 70 to 90 grams of fiber a day.

Most vegetarians don’t even eat that many whole plant foods, though some do. At least they hit the minimum mark, and have less diverticulosis to show for it. This was a relatively small study, though. 35 years later, 47,000 people were studied, confirming that consuming a vegetarian diet and a high intake of dietary fiber were both associated with a lower risk of both hospitalization and death from diverticular disease. And they had enough people to tease it out. Compared to those eating a single serving of meat a day or more, those who ate less than half a serving appeared to have a 16% lower risk; pescetarians—no meat except fish—down 23%, though not in and of themselves statistically significant, but eating vegetarian was 35% lower risk, and those eating strictly plant-based appeared to be at 78% lower risk.

As with all lifestyle interventions, it only works if you do it. High-fiber diets only work if they’re actually high in fiber.

To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Katie Schloer.

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