The beauty of the guaranteed analysis is that it gives you a lot of information about what is inside the bag of pet food. Once you understand how to read it, you will be better equipped to compare different varieties of pet food.
By AAFCO regulations, the guaranteed analysis is only required to list four nutrients: crude protein, crude fat, crude fiber and moisture. However, many pet food companies list additional guarantees not only to provide you with more information about the food but also as a mark of quality. The more things that are guaranteed, the more things that regulatory agencies can test for and fault a company if they do not meet the level on the label. Because of variances between different types of laboratory equipment, sometimes foods can be faulted even if they truly are not deficient in one of their guarantees. Added guarantees mean that the company is working hard to manufacture a precise formulation each and every time, and you can be assured that bag to bag, that product is likely to be more consistent than a product that only guarantees the four required nutrients.
The first nutrient listed is crude protein. This is a measurement of the guaranteed minimum level of protein in the food. If the food guarantees a minimum of 21% protein, it is not going to contain 32% protein. By AAFCO regulations, a diet that states a guarantee of 21% protein may have no less than 20.4% protein. There is not a specified maximum, but the protein is typically within 2% of the target. So a 21% protein formula would range from 21% to 23%, but would most often be right at 21% or slightly higher. Your dog will benefit from a food that has protein from animal protein sources. After you check the level of protein, look at the ingredient listing to see where that protein is coming from.
The next listing is crude fat. This is also a minimum guarantee, with a 10% allowed variance. So, if the guaranteed minimum fat content is 15%, the minimum allowed by AAFCO would be 13.5%. Most foods very closely target the fat level, so expect very little variance in this nutrient.
Next comes crude fiber. This is typically pretty low, 2–3%, and is a maximum level. In hairball formulas for cats and weight loss formulas, you will usually see a higher level of fiber, usually 6–8%. Higher fiber formulas will result in larger stools than low fiber formulas, but this is to be expected.
Finally, you will see the moisture guarantee. In dry formulas, this is typically 8–12% maximum, and in canned formulas it is typically 75–85%.
Protein and fat will show the widest variance among different types of pet foods. Cat foods have higher protein content than most dog foods. Formulas specifically designed for athletes, puppies, and low carbohydrate formulas for dogs will have high protein content and often high fat content as well.