We’ve spoken many times about the importance of including adequate amounts of protein in a cat’s diet, but all protein is not created equal. Unfortunately, it can be difficult for owners to determine whether the percent protein that appears on a food’s guaranteed analysis will actually meet their cat’s needs.
The crude protein percentage of a food is measured through a type of chemical analysis that determines its nitrogen content, which is based on the fact that proteins are made up of amino acids and amino acids contain nitrogen. This is the reason why unscrupulous producers have added melamine to food products (e.g., Chinese infant milk formulas and protein supplements added to pet foods). Melamine contains a lot of nitrogen (its chemical formula is C3H6N6 for anyone who’s interested) and can therefore “trick” the analysis into reporting a falsely elevated protein percentage.
But I digress. What I really wanted to talk about is the quality of the protein included in a cat’s food. Absent adulterants like melamine, the protein percentage listed in the guaranteed analysis contains proteins from both animal and plant sources. The chemical analysis used to come up with that number can’t differentiate between the two.
Cats are obligate carnivores. Since their natural diets consist almost entirely of other animals, they have a unique dietary need for specific types of amino acids (called essential amino acids) that they can’t make on their own. The system makes sense. Why waste your body’s precious resources manufacturing these amino acids from scratch when they are readily available in your food?
But some commercially available cat foods rely heavily on plant-based sources of protein. Plants can be a good source of protein in general but not the specific mix of amino acids that cats need to thrive. Therefore, these cat foods appear to be high protein based on the guaranteed analysis, but they cannot deliver an optimal amino acid profile without supplementation. Look at cat food labels and pick foods with meat or meat meals high up on the ingredient list.
Another limitation of the guaranteed analysis is that it does not provide any information about the digestibility of the protein in the food. For example, shoe leather contains a lot of protein, but while including it in a cat food might raise the percent protein on the guaranteed analysis, it won’t provide much protein to the cat. I suspect most would simply pass right through the gastrointestinal tract.
Labels aren’t a big help in determining how digestible protein sources are, so I use a cat’s response to the diet as a gauge of digestibility. If after a few weeks of eating the food, a cat is producing a “normal” amount of firm stool (not large quantities of loose feces that may contain mucus) and he or she has that unmistakable glow of good health, you’ve probably found a food that contains a good amount of digestible protein.
Dr. Jennifer Coates