If you need to change pet food brands, changing them gradually is the best way to avoid digestion problems. It's recommended for all pets, when changing foods, to mix them over a period of five to nine days to allow the pet's system to adapt to the new formula. This can be done by gradually decreasing the amount of the old brand food while simultaneously increasing the amount of the new brand food fed at each meal. Each diet has different ingredients and nutrient composition. When the level of some nutrients like protein or fat changes, the cat's digestive system must adapt the levels of enzymes produced to the new diet. Sudden changes can result in diarrhea, cramping, bloating, or gas in your animal as its system adapts.