This is honestly a limitation of nearly all nutrition research -- it's based on observational data. Part of the reason we expect fiber to reduce mortality, rat

This is honestly a limitation of nearly all nutrition research -- it's based on observational data. Part of the reason we expect fiber to reduce mortality, rather than simply being a marker of other factors correlated with mortality, is that we can identify physiological mechanisms. For example, for cardiovascular mortality, fiber reduces LDL cholesterol / ApoB which lowers heart attack and stroke risk.