Dear Christopher Cat
My veterinarian recommended that I feed my cat a diet lower in protein and that I add some dry cat food to her diet. But I just read the label, and it says the dry food is higher in protein than the canned food. Now what?
Actually, the dry food might really be lower in protein than the canned food. Labels can be confusing; here’s how to figure it out.
The percentages in the Guaranteed Analysis section of the food label pertain to the actual food as it is formulated and fed to your pet. These numbers include both the solid ingredients and the water in the food.
To compare two foods, it is necessary to convert the “as fed” percentages on the label to “dry matter.” This calculation subtracts the water, leaving only the solid food ingredients which can then be directly compared.
You'll need to do the dry matter calculation because canned food is mostly moisture (about 70-80% of what's in the can is water), while dry food is only about 6-10% moisture.
To calculate dry matter, subtract the moisture, and then divide the ingredient's percentage by the percent dry matter.
For example: A canned food label states the food is 10% protein and 80% moisture (i.e., 20% dry matter). Thus, 10% / 20% = 50% protein on a dry matter basis.
And another example: A dry food label states the food is 20% protein and 10% moisture (i.e., 90% dry matter). Thus, 20% / 90% = 22% protein on a dry matter basis.
Bear in mind that the percentages in the Guaranteed Analysis section of the food label are minimum and maximum percentages. Contact the food manufacturer for exact figures.
If all these numbers drive you to distraction, give this math challenge to your child -- or ask your veterinarian to recommend a food.