Simple sugars are the smallest sugar molecules and are easily digested and absorbed. By contrast, starches (or ‘complex carbohydrates’) are combinations of simple sugars formed into long chains that have to be broken down by additional digestion before they can be used. Dietary fibers are carbohydrates that are not completely digestible.

Basically, the carbohydrates in dog food are supplied by cereal grains which are broken down into simple sugars. While there aren’t specific minimum requirements for carbohydrates for dogs, they are useful as a ready source of energy.

When animals consume diets containing more carbohydrates than are needed, the excess energy is stored in the form of glycogen in the liver and muscles and is converted to fat. During periods of fasting, stress, or exercise, glycogen is broken down to glucose and delivered to the bloodstream to provide needed energy.

Carbohydrates may make up 40 to 55% of dry dog food. Most of the carbohydrates in pet foods come from cereal grains, such as corn, wheat, rice and barley.

One carbohydrate-related issue to watch for is diabetes mellitus, a disease of carbohydrate metabolism resulting from too little insulin secretion from the pancreas. Symptoms may include increased thirst, increased appetite, weakness, weight loss and increased urination. Low levels of insulin prevent sugar in the blood from entering muscle and fat cells where it would be used for energy. This leads to an increased use of fat for energy while sugar accumulates in the blood.

Although the use of fat for energy may seem advantageous, it generates by-products the body can't use or easily get rid of. Owners who have a diabetic animal or suspect their pet may be suffering from diabetes should consult their veterinarian.

Carbohydrates that are not completely digested cause (or are associated with) persistent gastrointestinal upset, including gas and/or diarrhea. The most common carbohydrate malabsorption problem is an inability to break down lactose, the sugar found in milk.