"Mainstream health authorities are constantly telling us to eat more fiber for health, particularly whole grains, fruit and vegetables. Yet the only clinical trial that has ever isolated the effect of eating a high-fiber diet on overall risk of death, the Diet and Reinfarction Trial, came up with this graph:
Oops! How embarrassing. At two years, the group that doubled its fiber intake had a 27% greater chance of dying and a 23% greater chance of having a heart attack. The extra fiber was coming from whole grains. I should say, out of fairness, that the result wasn't quite statistically significant (p less than 0.05) at two years. But at the very least, this doesn't support the idea that increasing fiber will extend your life. I believe this the only diet trial that has ever looked at fiber and mortality, without also changing other variables at the same time."
See The Whole Health Source for the full post which includes thoughtful follow up comments and responses.
It is interesting how people with undiagnosed celiac are often told to increase their intake of dietary fiber when they seek medical attention for their stomach problems and often this leads to their getting even sicker as they consume whole wheat bread instead of white bread. Since whole wheat bread has a lower gluten content than white bread, perhaps this is due to the fact that celiac disease is a malabsorption syndrome and the increase in fiber causes people to absorb even fewer nutrients. Also, the undigested fiber can feed bad bacteria, leading to further gut problems. Fermenting seems to allow you to absorb the nutrition of whole grains better.