Carbohydrates almost always come from plant sources such as cereal grains. Carbohydrates, per se, are not a required nutrient and the body's need for glucose and other carbohydrates can easily be met by breaking down triglycerides (fatty acids) and amino acids. While an animal could survive without carbohydrates, they are helpful in adding bulk, variety and taste to the diet. The source of the carbohydrates and the way in which they are prepared are important factors in their digestibility and utilization.

Carbohydrates are often classified into two groups:

1. digestible (sugars and starches)
2. indigestible (fiber)

1. Digestible: Sugars and Starches (Soluble)

Most types of sugars are digested easily by dogs or cats and are almost always quickly utilized for energy. Starches, on the other hand, must first be broken down into sugars in order to be usable. The primary sources of carbohydrates in most diets are cereal grains, such as: barley, brown rice, oats and rye. Proper grinding and cooking of cereal grains is necessary for the animal to efficiently digest cereal starches.

Recent work comparing the glycemic index (the ability of the grain to raise blood sugar) of various grains in dogs has generated much interest. While likely of minimal importance for most healthy dogs and cats, the use of specific grains and the avoidance of others could prove useful. It is likely important in pet food formulation and in an owners ability to manage the diets of their diabetic animals, young and old.

Sugars and starches supply calories and make up a high percentage of most dry and semi-moist pet foods as well as many canned pet foods. The extrusion process used to make most dry pet foods and the baking process used to make biscuits requires some carbohydrates. Because of the calories they provide, sugars and starches help limit the use of protein for energy purposes. This allows the protein to be used to meet the animal's amino acid requirements rather than its energy needs. Carbohydrates exceeding the amount needed to meet the animal's energy requirements are stored in the body as fat, which may lead to obesity.

2. Fiber 

Fiber is well known (even in the human diet) for its role in regulating bowel function and transit time in diarrhea and constipation. Many apparently contradictory statements about fiber stem from the fact that there are different types of fiber all of which exert different effects. Unfortunately, pet foods are still required by law to list crude fiber, a method for measuring fiber that was developed in the 1800's, which is not very useful. In human dietetics, newer methods that measure dietary, insoluble and soluble fiber types are used and hopefully these will eventually find a place in pet food.

Soluble fibers (hemicelluloses and gums) often help nourish the cells in the large intestine and encourage the growth of beneficial microorganisms. In contrast, insoluble fibers (cellulose and lignins) move rapidly through the gastrointestinal tract and provide bulk but no calories. For this reason, high fiber diets are useful for reducing the energy content of the diet and have been recommended for overweight animals. In contrast, diets high in fiber are not recommended for dogs and cats with high energy requirements (growth, late gestation, lactation, stress, and work).

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