The muscles of the walls of the small and large intestine push and pinch the digesting food, mixing it with enzymes, bringing it into close contact with the villi which absorb the nutrients, and ensuring the undigested food remains pass easily through the system. These muscles require something to push against. If the food is always soft or semi liquid, these muscles lose their strength, pockets and bulges form in the intestine walls where debris accumulates and waste becomes difficult to eliminate.
Fibre gives bulk to the food in the system and ensures that these muscles maintain their strength and can control the movement of food. The fibre also ensures that no pockets of debris are left behind and the whole digestive system operates efficiently and is self cleansing.
The most useful fibre comes from several portions a day of fresh fruit and vegetables, and from whole grain cereals. Added fibre is not nearly as useful and wheat fibre is not a useful addition to the diet. Wheat fibre is too abrasive and can obstruct the absorption of minerals.
Small children should not be fed on high fibre foods other than fresh fruit and vegetables.