Carbohydrates are an extremely vast range of simple or complex molecules that include:

• Sugars (mono and disaccharides): these are the simplest components of food. The most important is glucose, the foundation molecule for the structure of all living beings. Common table sugar (beet sugar) or sucrose, for example, is made up of one molecule of glucose and one of fructose (the sugar contained in fruit).
• Starch (polysaccharide): this is the main component of most foods including grains (corn, wheat, barley, etc.), nuts (chestnuts, hazelnuts, etc.), tubers and roots (potatoes, tapioca, etc.), and legumes (chickpeas, peas, beans, etc.).
• Glycogen (polysaccharide): this is the "fuel" of the animal kingdom. It is contained in the muscles and, as an energy reserve, in the liver, albeit in modest amounts. It is transformed into glucose so that it can be "burned" during anaerobic muscle activity.
• Cellulose or fibre (polysaccharide): serves as a support structure in the plant kingdom. In general, it is not significant in terms of energy contribution for monogastric animals (humans, dogs, cats, chickens, etc.), although it is very important for guaranteeing the proper functioning of the gastrointestinal tract.

With the exclusion of cellulose, the metabolisable energy from the elements described above (starch, sugars, glycogen) is as follows:
1 gram = 3.5 calories

The following limiting factors for the use of sugars and starches in the diet should be kept in mind:

• Starch must always be cooked in order to be digested; different cooking methods (extrusion, flaking, blowing, etc.), provide different digestibility coefficients. Even the diameter of the starch granules affects digestibility: the smaller they are the greater the digestibility.
• Simple sugars (sucrose, fructose, etc.) are generally less tolerated than starches. Abuse of these elements causes water to be pulled into the intestine by osmotic effect, leading to diarrhoea.
• Glycogen is contained in animal tissues and blood in such small amounts that it is not significant in terms of the energy it contributes to the diet. (Moreover, it is transformed into lactic acid following the death of the tissue).

Cellulose is not digested directly by dogs and cats, although some micro-organisms in the large intestine can destroy it in various ways. Bacterial fermentation produces short-chain acetic, propionic and butyric acids, which do not effect the energy balance of the animal but are important for the epithelial cells of the gastrointestinal tract.

Some of the fibre is, however, necessary to ensure intestinal peristalsis (the movement of the smooth muscle in the intestine), which may cause constipation when lacking.



Diets containing fibres that are highly fermentable cause the production of irregular faeces, while those that are moderately fermentable ensure the proper functioning of the gastrointestinal tract and decrease energy density.