In most cause hypotheses, fibre just doesn't rank at all. I mean, of course everyone should eat more fibre so that's good, but it isn't a credible cause for the rise of colon cancer rates.
Most destructive to the fibre hypothesis is that fibre intake has actually increased over the past few decades, at the same time that youth colon cancer rates crept up[1]. I don't know what notion people have about common Western food practices decades ago, but in the 1980s, for instance, fibre was basically non-existent in the common diet: No one ate multigrain anything. Everything was loaded with sugar, trans fats and simple carbs (we literally considered that healthy, with hilarious products like Snackwells).
[1] Twice as many young people, per capita, are facing colon cancer now than in the 80s. Overall colon cancer rates are down significantly now, however this is most certainly due to the massive reduction in smoking and other behavioural issues that catch up to you after many decades, but kids today are in no way eating less fibre than kids in the 1980s.
> There are a large multitude of studies that show a negative correlation between CRC and fiber intake even when adjusting for diet, exercise, BMI, income, alcohol consumption etc…
Not really. Take any behaviour or practice that is common among the healthier cohort and it's easy meat for bullshit study results. It's why the nutrition field in particular is full of contradictory studies because "adjusting for" is carrying a lot of unmerited credibility.