The fibre is represented by residual cells in edible plants (polysaccharides, lignin) resistant to digestive enzymes humans.

The soluble fiber is formed mainly of polysaccharides that differ among themselves for the length and branching chains carboniose, as well as for the presence of different functional groups and their solubility is determined by molecular weight and structure.
 The soluble fiber interacts with the content of the intestinal lumen through mechanisms osmotic; according to its molecular weight, the chemical structure and its components, the fiber increases the viscosity of intestinal contents interfering with its speed transit.

The insoluble fiber, consisting primarily of cellulose, retains water and gas in the intestinal lumen, increasing the mass faecal and producing effects attributable to a volumetric and mechanical (stimulates the speed of transit): it interferes so negligible on the viscosity of luminary content and is poorly fermentability

The most important characteristic of fibre is the ability to "tie the 'water' means the soluble fiber, being made up of polysaccharides low molecular weight and polar constituents, has a high capacity for hydration; form into the mass suitable gelled (ie a solution viscose), which may have a weight more than one hundred times higher than the original.
. The insoluble fiber form a mass with lower weight, equal to 10-15 times the initial.

FUNCTIONS OF FIBER FOOD
 – Prevention and treatment of 'obesity: increase the sense of satiety
Improving the functionality and intestinal disorders associated with it (constipation)
–The introduction of fibre in the diet has been associated with reduced risk for major chronic degenerative diseases, particularly cancer colon-rectum (partly explained by the dilution of any carcinogens and the reduction of their contact time with the mucosa), diabetes and cardiovascular disease (in part to a reduction in blood levels of cholesterol) (National Research Council, 1989).
—Modulating action on the peaks of blood glucose , particularly important in non-insulin dependent diabetes (foods rich in fiber have a low glycemic index probably because the fiber interferes with the process of absorption of nutrients)
– Action on cholesterol lowering (total cholesterol and LDL)
- Effects probiotics through the growth of bacterial species acting probiotic (bifidobacteria) that is alive and viable micro-organisms that have a positive effect into the human favoring the balance of intestinal microflora.

The fibers are contained in:
• Fruits and vegetables> cellulose, hemicellulose, lignin, pectin.
• Cereals> cellulose, hemicellulose, lignin.
. • Semi (legumes and nuts)> cellulose, hemicellulose, pectin, mucilages.