Consumption of carbohydrates is not actually essential to life but very helpful in providing non-fat calories in this time when 60% of pets are overweight or obese. Carbohydrates are a very broad group of nutrients broken down into three large sub-categories: readily digested simple sugars (glucose), delayed digested complex sugars (starch) and indigestible more complex forms better known as fiber.
Carbohydrates (sugar and starch) may be burned for energy or incorporated into body tissues. If consumed in excess, carbohydrates will be converted and stored as body fat.
Dietary fibers are the provided by plant-derived ingredients which are not fully broken down by the animal's digestive enzymes. They are further described as soluble (dissolves in water) or insoluble (does not dissolve in water). Soluble fibers may be utilized by bacteria in the intestinal tract and confer health benefits to the pet, while also providing a sense of fullness with a meal. Insoluble fibers are resistant to enzymes and provide bulk to the stool which may absorb water and ease passage and prevent constipation.
Some pet owners refer to the fiber ingredients as "fillers" in pet foods, while others are referring to grain ingredients. There is confusion because it is an invented marketing term. There is no regulatory definition or common understanding of the term "fillers".