Every nutrient in your cat's food has a purpose. Water is the most important nutrient and accounts for between 60 to 70 percent of an adult pet’s body weight. Proteins are the basic building blocks for cells, tissues, organs, enzymes, hormones and antibodies, and are essential for growth, maintenance, reproduction and repair. Fats are the most concentrated form of food energy, providing your pet with more than twice the energy of proteins or carbohydrates. Carbohydrates provide energy for the body’s tissues, and vitamins and minerals are necessary for muscle contraction and nerve conduction.