Material derived mainly from plant cell walls which cannot be digested enzymically, but which can be partially broken down by intestinal bacteria to produce volatile fatty acids that can then be used as a source of energy. Roughage consists of soluble fibre which reduces levels of blood cholesterol and increases the viscosity of the intestinal contents, and insoluble fibre (cellulose and cell walls). Foods with a high fibre content include wholemeal cereals and flour, root vegetables, nuts and fruits.