I suppose it's fair to assume that bowel epidemiology of bowel disease = epidemiology of the food we eat. In the old days, one or two countries with hugely different diet and hugely different disease burdens (Japan and stomach cancer, Nigeria and bowel cancer) gave pointers to disease causation. Nowadays that would count as the public health equivalent of anecdotal evidence, and so a more systematic approach is required. It's a well known fact that I'm deeply suspicious of systematic reviews; too many of them are like sausage machines with no brainpower applied at any stage in the process. (And as blogged a couple of weeks ago, I'm not alone in that opinion.)
That said, this review of global trends in inflammatory bowel disease does at least seem to be aware of the problem, and applied sensible exclusion criteria before studies were allowed to enter the review. Something is definitely going wrong with what we eat, but it'll be difficult to find out exactly what the problem is.