Carbohydrates

The dry matter of plants consists of 60-90% carbohydrates - the parts of plants in which energy is stored in different combinations of carbon, hydrogen and oxygen. They are classified as follows:

Monosaccharides: simple sugars, which in a single unit contain between 3 and 7 carbon atoms. In this group, three 6-carbon monosaccharides (also called "hexoses", C6H12O6) are the most important:

Glucose - present in most plant and animal tissues, also the main end product of starch digestion and glycogen hydrolysis in the liver. It is the primary form of carbohydrate which circulates in the blood and is utilized by the body cells for energy.
Fructose (fruit sugar) - found in ripe fruits, honey and some vegetables. Also a product of digestion or acid hydrolysis of sucrose in the body.
Galactose - one half of the disaccharide lactose, which is present in the milk of all mammals. Unlike glucose and fructose it is not found in an isolated form but broken down in the digestion process and converted into glucose by the liver.

Disaccharides: consist of two monosaccharides linked together:

Lactose - one molecule of glucose linked to one molecule of galactose. Present in the milk of all mammals and the only carbohydrate of animal origin of significance in the diet.
Sucrose (table sugar) - one molecule of glucose linked to one molecule of fructose. Found for example in sugar beets or sugar cane.
Maltose - two linked glucose molecules. Not commonly found in an isolated form, but produced in the body when starch is digested.

Polysaccharides: are long and complex chains of linked monosaccharides, like for example

Dextrins - the intermediate products of starch digestion.
Glycogen - the form in which carbohydrates are stored in the body.
Starch - the main sources of carbohydrates present in cereal grains, potatoes and other "starchy" parts of plants.
Dietary fiber - several forms of carbohydrates such as cellulose, hemicellulose, pectin, plant gums and mucilages. Unlike other carbohydrates, these are linked by a special kind of bonds that resist "normal" digestion but are partially broken down by the beneficial bacteria in the large intestine. More on this topic in the article on fiber.

Among other things, glucose is necessary for the nervous system to function properly and a limited amount is stored in the body ready for release into the blood if needed. Excess amounts are metabolized into fat for more "long term" storage - which may lead to obesity. The dog's metabolism is very proficient at hydrolizing glycogen from protein sources, so large amounts of carbohydrates are not required in the diet.

Common carbohydrate sources in dog foods are corn, rice, wheat, sorghum, millet, barley and oats. As long as they are properly processed (dogs do not produce the enzymes required to break down plant cell walls), they are readily metabolized. A moderate amount of quality grade grains contributes many nutrients as well as fiber. In many inexpensive dog food products, grains and grain byproducts are mainly used as a cheap (and incomplete) source of protein, not because of their carbohydrate content.