The percentages of nutrients shown in the guaranteed analysis section on a pet food label (protein, fat, fiber, moisture) are expressed “as fed” – meaning, as the food is delivered in its package. Some percentage of the food is comprised of moisture (water), which of course contains no protein, fat, fiber, or any other nutrient. Kibble generally contains about 10% moisture; canned foods usually contain about 78% moisture.
So, when a canned food label says that a food contains (for example) 9% fat, in order to reallyunderstand how much fat you are thinking about feeding to your dog, you have to remove the moisture from the equation; you want to know how much fat (in this example) is in the food part of the food – the “dry matter.” Any serious discussion of nutrition, or comparison of dry and wet diets, requires the conversion of the nutrient values from “as fed” to “dry matter.”
To calculate the nutrient levels in a food as dry matter (DM) percentages, first determine the amount of dry matter in the product. You do this by subtracting the percentage of moisture from 100. Then, divide the “as fed” percentage of the nutrient you are curious about by the amount of dry matter; that will give you the dry matter percentage.
For example, if a canned food has 78% moisture and 9% fat as fed (as seen on the guaranteed analysis):
100 – 78 = 22% dry matter (DM)
9 ÷ 22 = .40 = 40% fat DM (on a dry matter basis)
To compare that product to a dry food, do the same calculation for the dry food you may have been feeding. We’ll use the numbers from a bag of food our dogs are currently eating; the guaranteed analysis says it contains 10% moisture and 13% fat.
100 – 10 = 90% DM
13 ÷ 90 = 14% fat DM
So the canned food contains a little more than three times the amount of fat than the dry food does, on a dry matter basis. Yowsa! We’d be careful if we were considering adding some of this food to our dog’s diet – we’d add only small amounts – and we certainly wouldn’t switch quickly from the dry food to this canned food. Doing so would just be asking for digestive upset, and could provoke a pancreatitis attack in susceptible dogs.
Source: Whole Dog Journal