Ever noticed that box full of phrases and numbers called a guaranteed analysis? Every pet food has one, but I for one never knew what it meant until I started doing this research.

AAFCO asks pet food manufacturers to list guaranteed amounts for certain nutrients. Under their model regulations (which are actually just recommendations), pet food manufacturers are expected to list guaranteed minimum amounts of protein and fat, and maximum amounts of fiber and moisture. The idea is to give consumers a way to compare pet foods and know that their pet's food contains plenty of the core nutrients they need.

But here's the big problem: AAFCO's nutrient profiles don't guarantee the source or digestibility of ingredients. Just because a pet food contains a minimum of 8% protein doesn't mean a cat can actually digest it or access its important amino acids. In fact, in a documentary I watched a couple of weeks ago, one woman made a "pet food" out of sawdust, motor oil and old shoe leather. And that slop would meet AAFCO's requirements! Gross.

On top of that, AAFCO doesn't ask or expect products to meet their nutrient profiles. All manufacturers have to do is list how much of each nutrient their product contains.

Manufacturers can choose to include other guarantees based on AAFCO's nutrient profiles. For example, cat foods often list a guaranteed maximum for ash. Ash is left on meat after it's been cooked at high temperatures, and it's made up of important minerals, but for years scientists thought it was bad for feline urinary tracts. The link between ash and urinary crystals isn't as strong as the scientists used to think, but some people still choose low-ash food. Manufacturers are also instructed to include guarantees for vitamins or minerals in any pet food that's marketed as a vitamin or mineral supplement.

Sometimes pet foods contain nutrients that AAFCO hasn't investigated, and they may want to put these in the guaranteed analysis. For example, omega-3 fatty acids might be very good for pet health, but they aren't recognized by AAFCO. When manufacturers put omega-3s on the guaranteed analysis, they have to include a note saying that the nutrient is "Not recognized as an essential nutrient by the AAFCO Dog (or Cat) Food Nutrient Profiles."

So, while it's good to know what percentage of your pet food is fat and protein and how much is filler, it's just as important to know what the source of those nutrients is, and that's why you have to look at (wait for it. . .) the ingredients.