Digestibility is the reason for all this information about types of food and ratios. A dog food may contain the exact balance required, but if the ingredients used for that food are indigestible, the dog will starve. On the other hand, when a dog’s food is made well, from ingredients approaching full digestibility, s/he may produce a stool only two to three times a week. When the digestive system can break down a food completely, and absorb all its nutritive content, there is little to no waste.
In general, animal food sources are more digestible than plants. However, most carbohydrates come from plants. And cellulose, which makes up a large portion of plant carbohydrate, cannot be digested.
Dogs’ digestive systems do not contain the right enzymes for the job. Thus alfalfa, which contains virtually every nutrient a dog needs, is useless because the dog cannot digest it. Yet gelatin is equally unsuitable because, while it is highly digestible, it lacks two essential amino acids.