Dietary fibre comes from the thick cell walls of plants. It is an indigestible complex carbohydrate. fibre is divided into two general categories: water-soluble and water-insoluble.

Soluble fibre has been shown to lower cholesterol. However, in many studies, the degree of cholesterol reduction was quite modest. For unknown reasons, diets higher in insoluble fibre (mostly unrelated to cholesterol levels) have been shown to correlate better with protection against heart disease in human trials. Soluble fibres can also lower blood-sugar levels, and some doctors believe that increasing fibre decreases the body's need for insulin—a good sign for diabetics.

Insoluble fibre acts as a stool softener, which speeds digestion through the intestinal tract. For this reason, insoluble fibre is an effective treatment for constipation. The reduction in "transit time" has also been thought to partially explain the link between a high-fiber diet and a reduced risk of colon cancer.