Researchers then follow them for years and tally up how many subjects get sick in each group. It's considered the most definitive type of study, but even it isn't fool proof. Say fiber only helps in men and you have women in your study. Or let's say, fiber doesn't help unless you give, people eat 50 grams a day, but in your study you only told them to eat 25 grams a day. Or let's say, it's only cereal fiber that matters, and you had people eating fruits and vegetables.
Epidemiology is a slow, often painful process, full of false starts and dead ends.
To draw a conclusion about any particular food, you need to look at many studies and see if there's a pattern. No study is definitive. And the way that scientists are trying to proceed and make progress to some degree clashes with people's need for information that they can apply right now. Which brings us back to whether we should keep filling-up on fiber. He concluded that a high-fiber diet does help protect against colon cancer.