How to Calculate the Carbohydrate and Protein Content of Cat Foods

After reading your last 2 posts on how to feed cats with diabetes, I realize that I don’t really understand how to determine the carbohydrate or protein content of cat (or dog) foods. I have looked on the packaging of a few diets and can only find listing for crude protein (listed as a minimum), without any listing for carbohydrates. Is fiber the same as the carbohydrate content?

For example, when I go onto the Fancy Feast website for the nutritional information for the Classic Chopped Grill Feast, the nutritional information is listed like this:

• Crude Protein (Min) 10.0%
• Crude Fat (Min) 6.0%
• Crude Fiber (Max) 1.5%
• Moisture (Max) 78.0%
• Ash (Max) 3.3%
• Taurine (Min) 0.05%

So how in the world do I calculate the percent protein content (as the % of calories ingested) from this information? Again, carbohydrates aren't even listed so how do I calculate that figure? Obviously, I need a need a crash course on how to determine the the protein and carbohydrate content of fed foods so I can recommend the best diets to feed.

My Response:

That is actually a great question, as it is not intuitive. The pet food companies are only required to provide a "Guaranteed Analysis" on the pet food label, as well as a list of ingredients (1,2). They never list the carbohydrate content on the label. Carbohydrate must be calculated by subtracting all of the other ingredients from the diet (see below); it is not determined directly in pet food.

The other confusing thing about pet foods is that, although the label lists the breakdown as Guaranteed Analysis, this is not a very accurate way to express the protein or carb content of a diet. It is much more accurate to express the composition of the food on a dry matter basis (DMB) or even better, as the metabolizable energy (ME) or calorie basis, but that information will never be found on the label.

What does Guaranteed Analysis tell us?
As you pointed out in your question, the guaranteed analysis lists the minimum levels of crude protein and fat and the maximum levels of crude fiber and moisture. The term “crude” signifies that all sources of protein, fat, and fiber are included—not just digestible sources. Guaranteed analysis should always be evaluated in conjunction with the ingredient list to help us determine the adequacy of the diet.

Looking at these percentage values by themselves is of little value in evaluating or comparing the carbohydrate, protein, or fat content of different cat foods. Also, the information on guaranteed analyses listed on pet food labels can never be totally accurate since the label only requires minimums and maximums, not exact measurements.

Converting the Guaranteed Analysis to percentage values on a dry matter or calorie basis
If we are going to use the guaranteed analysis at all, it is best to convert the protein, fat, and carb listing to % values on a dry matter basis (DMB) or metabolizable energy (ME)/calorie basis.

To convert the guaranteed analysis into a dry matter profile, this is what we can do. Begin at 100%, subtract the values for % moisture as listed on the label. The remaining percentage gives us the actual amount of dry matter in the pet food product. Next, take the listed percentage of one of the main nutrients (protein or fat), and divide that number by the percentage of dry matter. This will give us the actual dry matter percentage of the nutrient in the food.

So for your example Fancy Feast food (above), the water content is listed on the label as 78% (max moisture), and the crude protein content analysis is guaranteed to be a minimum of 10%. Subtracting the 78% water content from 100% leaves us with 22% dry matter in the cat food product. Then dividing that 10% protein value by the 22% dry matter calculates out to give us a protein content of 45.5% (DMB).  For the 6% fat figure, this calculates out to give a a fat content of 27.3% (DMB). As you can see, the percentages aren't even close to those we started with on the label.

But what about the carbs? As you stated in your question, they aren't listed on the guaranteed analysis! To estimate the carbohydrate content from the percentage figures on the label, we must first add up everything else listed within guaranteed analysis and subtract that total amount from 100%. So again, we need to add up the protein, fat, fiber, moisture, and ash percentages, and then subtract from 100 to get a % carbs portion on a dry matter basis. So in this example food, the total adds up to 98.8%, leaving on 1.2% carbs.

Next step: Converting the percentage from dry matter basis to metabolizable energy/calorie basis
When analyzing a diet, I like to examine or calculate the amount of the calories each nutrient (i.e., protein, fat or carbohydrates) provides — this is called the “metabolizable energy,” abbreviated ME. This measure disregards any part of the food that does not provide any energy (kcal) such as water, ash, or fiber. It only considers the 3 nutrients that provide the needed calories and nothing else.

It's these ME percent values I am referring to when I recommend that diabetic cats be fed a diet containing less than 12% carbs and more than 40% protein.

To convert the nutrient composition from a DMB to a ME basis, we must remember that protein and carbohydrate both provide approximately 3.5 kcal/g of food, whereas fat provides much more — approximately 8.5 kcal/g of food. Thus, in this diet, the energy provided by protein is 45.5% times 3.5 kcal/g, or 159 kcal, and the energy provided by fat and carbohydrate are 232 kcal and 4 kcal, respectively, for a total of 395 kcal. The percentage of metabolizable energy that is provided from protein, fat, and carb is then calculated (by dividing the 159, 232, and 4 kcal each by 395 kcal and multiplying by 100), which gives us a protein, fat, and carb content (ME) of approximately 40.3%. 58.7%, and 1.0%, respectively.

Too Complicated? Here's 2 Simpler Ways to Get the Same Information

Instead of going through all of these calculations, one should remember that some pet food companies list the composition of their diets on a DMB or as a percent of calorie (ME basis) on their website. If you call any reputable pet food company, they can certainly provided that information for us. But for those who don't have time to call pet food companies, here are two easy ways to get the information.

First way: Go to the BalanceIT website and use the Guaranteed Analysis Converter (3). Once on this web page, simply enter the values for crude protein, crude fat, moisture, fiber, and ash (see Figure below). This converter will calculate the percent of calories that food provides as protein, fat, and carbs (ME basis). This does all of the calculations for you!

Guaranteed Analysis Converter available on the BalanceIT website.

Again, we must remember that the information provided with the guaranteed analysis will never be 100% accurate since the pet food label only requires minimums and maximums, not exact measurements. But these calculations still should provide a pretty good estimate of the diet composition.

Second way: Go to CatInfo.org website and open the link to the Protein/Fat/Carbs chart on the right panel of the home page (4). This gives a list of many different kinds and brands of canned cat foods, providing the composition of each diet (protein, fats, and carbs) both as a percentage of calorie provided (ME basis) as well as on a dry matter percentage.

This is the most up-to-date list of canned cat food available.  Unfortunately, some pet food companies change their products and formulations often, making at least some of the information on any compiled list rapidly go out-of-date. As Dr. Pierson has told me, it can be very difficult to pry these numbers from some companies (they act as if this is top secret information); therefore, keeping any list completely current would be a monumental task.  That all said, I find this information to be an extremely useful guide when selecting the best cat food diet(s) to feed my diabetic patients.

References:

1. AAFCO web site. Nutritional Labeling. 
2. U.S. Food & Drug Administration Center for Veterinary Medicine web site. Pet food labels— general. 
3. BalanceIT website
4. CatInfo.org website