A study of 2,000 people found that higher fibre intake was linked to diverticulosis
That study took a single snapshot, so it cannot show which came first. People with bowel problems are often told to eat more fibre. Studies that measured fibre before anyone developed the condition find that more fibre goes with lower risk, not higher.
Fibre is recommended almost universally, yet when researchers actually looked inside the colons of about 2,000 people, more fibre and more bowel movements went with more diverticulosis, not less. If the standard advice were as solid as it is presented, you would not expect that. A finding like this at least deserves to be taken seriously rather than waved away because it is inconvenient.
The study shows a correlation at one moment in time. It cannot show cause, and the direction may well run the other way. The study usually cited examined around 2,000 people having colonoscopies. It found that those with diverticula (small pouches in the colon wall) reported eating more fibre and having more frequent bowel movements. It asked about diet at the same visit that found the pouches. Many people with gut symptoms have already been told to eat more fibre, so a link at that point is what you would expect even if fibre did nothing harmful.
The better test is to record diet first and follow people for years. Large prospective cohorts have done this, including the Health Professionals Follow-up Study, EPIC-Oxford and the Million Women Study, and meta-analyses have pooled them. They consistently find that higher fibre intake is associated with a lower risk of diverticular disease. That design is still observational, but it removes the question of which came first, which is exactly the weakness of the cross-sectional result.
Two things often get blurred here. Diverticulosis, the pouches themselves, is common with age and usually causes no symptoms. Diverticulitis is inflammation of those pouches. A low-residue diet is sometimes used for a short time during an acute flare. That is a treatment for an episode, not evidence that fibre causes the condition. The old advice to avoid seeds and nuts was dropped after follow-up data found no increased risk.
The study is not worthless. It is one weak piece of evidence, and it is outweighed by stronger ones. Calling it a fact that fibre causes colon disease asks it to carry far more than it can. That is especially so while holding larger, longer studies to a stricter standard.
Inferring that one thing caused another from the fact that they occurred together or one after the other. Correlation and sequence are not causation.
How it applies here: It treats a link found in a single snapshot as evidence that fibre causes diverticulosis. In that study, diet and diagnosis were measured at the same time, so the likelier explanation that people with gut problems had already raised their fibre intake cannot be ruled out.
Presenting only the evidence that supports a conclusion while leaving out evidence that tells against it.
How it applies here: It leans on one small cross-sectional study that points one way. Meanwhile it sets aside prospective cohorts and meta-analyses that measured fibre before diagnosis and point the other way, and it rejects larger studies as 'only observational' while relying on a weaker observational design itself.