As I’ve been updating this book, I’ve taken a hard look at the numbers involved. I’ve also developed spreadsheets to calculate some of the more complicated formulas. I’ve learned a few things about the numbers that I’d like to share.
Let’s first look at the Guaranteed Analysis. Want to know why every pet food company tries to steer you away from using these numbers? The reality is that there’s a huge “fudge factor” built in. The pet food manufacturing process isn’t very precise or consistent, even using the latest computer manufacturing technology (which not every company uses). The biggest variations come in the ingredients. Mother Nature isn’t consistent, so ingredients like corn can have different nutrient content, and different moisture content. Suffice it to say that there are plenty of variables, and they all come into play to make each batch of food slightly different than the last one.
Because of these differences, there has to be enough room in the Guaranteed Analysis so the manufacturer doesn’t bust out beyond the limit he’s guaranteeing and violate the law. In other words, if the protein level in the final product ranges from 26.8% to 27.8%, the manufacturer will give himself some breathing room and put 26% on the label as the guaranteed minimum. The same goes for the minimum level of fat, and the maximum levels of fiber and moisture.
Here are some examples of the Guaranteed Analysis and the reality, which is the Average Analysis – several samples analyzed and averaged:
Those discrepancies affect other numbers, too. The best way to determine the Metabolizable Energy (kilocalories) for example, is to feed several dogs the food, collect urine and feces, and calculate what goes in versus what comes out. But that’s an expensive proposition, and generally only the larger companies will run this test. It’s much easier to use a slightly-complicated formula, called the Modified Atwater Formula, which is based on some basic knowledge of how much energy protein, fat, and carbohydrates contain.
To show you how these figures can vary, I plugged the two sets of numbers for Iams Chunks/MiniChunks into the Modified Atwater Formula. I used the ash content from the Average Anal-ysis, and the 3.3 oz. weight of a cup of the food that Iams uses.
Based on the Guaranteed Analysis, ME is 327.5 Kilocalories/cup.
Based on the Average Analysis, ME is 347.8 Kilocalories/cup.
Based on Animal Tests by Iams, ME is really 381 Kilocalories/cup.
Just to add another layer of confusion, two formulas can be used. The Atwater Formula is the one used to calculate the ME of foods for people. It assumes a high level of digestibility. The Modified Atwater Formula includes adjustments for lower quality ingredients, such as those traditionally included in pet food. The problem is that some foods (especially raw meat diets, high meat content commercial foods, most canned petfoods and baked foods) are more digestible than ever before. Should we use the Atwater Formula or the Modified Atwater Formula? Here’s what the Atwater numbers look like. You can compare them to the Modified Atwater numbers above.
Based on the Guaranteed Analysis, ME is 364.3 Kilocalories/cup.
Based on the Average Analysis, ME is 386.6 Kilocalories/cup.
As you can see, the Atwater Formula using the Actual Analy-sis figures is almost exactly what the Animal Tests showed the real ME is. To me, that says that this food is made with the equivalent of human-grade food, at least when it comes to providing ME.
The moral of this story is that the Guaranteed Analysis has a wide margin of error built in, and isn’t a great set of numbers to use for comparison. The Average Analysis is much more accurate, but much harder to extract from the petfood companies. Iams has their numbers and plenty of other data on their website (www. iams.com), but you may have to call the other companies to get the Average Analysis for other brands.
The other moral of the story is that the ME should also be considered a guideline, and another piece of the puzzle. In the example above, there was a 14% difference between the standard calculation and the real, Average Analysis-based ME.
In our reviews, the ME shown is either the number supplied by the manufacturer (which could be by calculation or by feeding trial), or one that we calculated using Modified Atwater. We ran into some cases where the manufacturer didn’t have a clue how to calculate the ME, and others where our calculations were much more accurate than theirs. We provide the numbers in which we have the most confidence. For raw food diets, we provide both Modified Atwater and plain-old Atwater numbers.
Ross Becker – 1999