Carbohydrates is a term that covers molecules comprised of carbon, oxygen and hydrogen. Carbohydrates are predominantly of vegetable origin, with the exception of glucose in the blood, glycogen in muscles and liver, and lactose in milk. All vegetables contain carbohydrates, ranging from sucrose in beet pulp to the most indigestible fibre in tree bark.
Cats an dogs can live without carbohydrates in their food, as the body can synthesise the carbohydrates the body needs from amino acids ingested as protein. The intake of carbohydrates does, however, greatly improve the body's functioning ability.
While glucose, sucrose, lactose and starch have the sole function of providing energy, their botanical origin and how well they are cooked influences their digestion. Diets containing poorly cooked starch can cause diarrhoea. Fibre, which is also a carbohydrate, is very good for gastro intestinal transit and for the balance of bacterial flora in the gut. This is true of fructo-oligosaccharides (FOS) and mannan-oligosaccharides (MOS) for instance.