A diet of fiber-rich foods, such as fruits and vegetables, reduces the risk of developing diabetes, heart disease and arthritis. Indeed, the evidence for fiber’s benefits extends beyond any particular ailment: Eating more fiber seems to lower people’s mortality rate, whatever the cause. That’s why experts are always saying how good dietary fiber is for us. But while the benefits are clear, it’s not so clear why fiber is so great. “It’s an easy question to ask and a hard one t