Dietary Fiber (sometimes called roughage) is the indigestible portion of plant foods having two main components:

1.soluble (prebiotic, viscous) fibre that is readily fermented in the colon into gases and physiologically active byproducts

2.insoluble fibre that is metabolically inert, absorbing water throughout the digestive system and easing defecation.
It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed. Soluble fibre absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Insoluble fibre has bulking action and is not fermented,although a major dietary insoluble fibre source, lignin, may alter the fate and metabolism of soluble fibres.