Polysaccharides

The most common polysaccharides in nature

Carbohydrates of food interest can be divided into three categories: monosaccharides; oligosaccharides; polysaccharides. Monosaccharides include the simplest molecules, directly absorbable without the need for a digestive process: the most widespread are glucose, fructose (present in fruit), galactose (which does not exist freely in food, but is produced during the enzymatic digestion of lactose in the intestine), mannose (which is found bound to many proteins).

Oligosaccharides are made up of a few units of monosaccharides: the most common are disaccharides (with two monosaccharide molecules), including sucrose (formed by a glucose molecule plus a fructose), that is the common kitchen sugar; maltose (formed by two molecules of glucose), contained in cereals; lactose (formed by a molecule of galactose and glucose), the only source of which is milk, human or animal. Polysaccharides are polymers produced by the aggregation of more than ten molecules of monosaccharides: those of greater importance for nutrition are starch, glycogen, cellulose, all consisting of long molecules of glucose linked together in different ways. Starch is the most important carbohydrate in the vegetable kingdom and is the main source of carbohydrates for human consumption (cereals, potatoes, legumes). Glycogen is a polysaccharide of the animal kingdom, contained in the liver and in the muscles as a form of carbohydrate deposition: it has little food importance. Cellulose is the skeleton of vegetable fibers (woody and fibrous part of all plants): that contained in food is eliminated almost entirely with faeces, to which it gives volume and consistency; only herbivorous animals possess enzymes to digest it.

Nutritional properties of polysaccharides

• ENERGETIC FUNCTION: they represent the main source of energy for rapid use and low cost.
• PLASTIC FUNCTION: they are constituents of nucleic acids, nucleotide coenzymes, glycolipids, glycoproteins, support and protection structures.
• REGULATING FUNCTION of the metabolism as they determine a saving in the use of proteins for energy purposes.
• ANTIKETOGENIC FUNCTION: in the case of carbohydrate deficiency there is the formation of ketone bodies and metabolic acidosis.