A healthy diet is a balanced diet containing all the nutrients your pet requires. Dietary requirements change with age and may also be affected if your pet is suffering from some diseases. As pets get older they may lose muscle mass and gain weight more easily. Overweight animals have a higher risk of diseases such as arthritis, heart disease and diabetes. Check your pet's weight every 6 months to make sure it is not overweight and prevent excessive weight gain by cutting back on calories if your pet's weight increases.