Updated 12/3/22. Everyone is familiar with the 3 basic nutrient categories: protein, fat and carbohydrate. But there's a 4th nutrient that's just as important (if not more!), yet it is usually overlooked by most guardians and even veterinarians—water! You may know that two-thirds of an animal's body is water, but in molecular terms, the body actually comprises more than 99% water molecules! The kind of water your cat drinks can have a major impact on her health, because all water is not created equal. The basic types are: municipal tap ...