Starch is the form of stored energy for PLANTS, whereas animals store this energy as fat. Corn, barley, and rice are common sources of starch. Digestion of starch begins in the mouth, with the enzyme salivary amylase. Cats lack this amylase.
This is why feeding cats starch sources is generally frowned upon. However, studies have shown that cats can digest most of the starch from these sources. When the starch sources go through the cooking phases to get made into cat food, it improves the digestibility of these starch sources. Extrusion is one of the phases. This is how the food gets processed and made into what you see as dry cat food. Extrusion actually heats up the starch and gelatinizes it, making it more digestible.
With that being said, it is advisable to stay away from starches.
Cellulose is a source of dietary fiber; however it is an indigestible source. Cellulase is essentially the enzyme responsible for the breakdown of cellulose. Cats don’t have this cellulase and cannot extract any nutritional value from the cellulose. Foods that contain a high level of cellulose are usually intended for hairball control and weight control. The reason it is high in weight control diets is because of the fact that it is indigestible. The food is allowed to be called ‘light’ or ‘low calorie’ because the cellulose is not an actual metabolizable energy source. Another example of trickery.
The most common form in cat food is powdered cellulose. It usually comes from trees like pine and bamboo. When paper is made at the pulping mills, this is where this powered cellulose comes from. This is one ingredient that should be avoidable if possible.
Glycogen is a form of rapid energy for cats. The simplest form of carbohydrates is glucose, among others. Glucose is used as an energy source for the body. When the carbohydrates are broken down, glucose is created. Glycogen is formed when the body stores this glucose. Muscles and the liver store this glycogen. Now, for cats it is a little different. Cats normally do not store the excess carbohydrates as glycogen. Excess carbohydrates are stored as fat. In humans, excess carbohydrates are first glycogen, and when there is still too much, then it is stored as fat.When a cat has no quick source of fuel, it uses up the stored fat and protein to create chemical reactions and use the stored fat as an energy source.
While cats can digest and absorb certain carbohydrates, they do not have a specific dietary requirement for carbohydrates. Cats are able to maintain blood glucose levels when fed diets that are high in protein, and low in carbohydrates. Cats have low activities of the enzymes responsible for carbohydrate breakdown; therefore this limits their ability to metabolize large amounts of carbohydrates. Many of the lower priced/lower quality cat foods are filled with cheaper carbohydrate sources, rather than real protein. This is where the idea that no carbohydrates at all is the best thing for a cat.