The terms crude protein, crude fat, and crude fiber all refer to specific analytical procedures used to estimate these nutrients in foodstuffs. On the average, protein contains 16% nitrogen. Crude protein is the estimate of total protein in a foodstuff that is obtained by multiplying its analyzed level of nitrogen by a constant. Slight inaccuracies in this estimate are caused by variations in nitrogen content between proteins and by the presence of nonprotein nitrogen compounds in the foodstuff. Crude fat is an estimate of the lipid content of a food that is obtained through extraction of the food with ether. In addition to lipids, this procedure also isolates certain organic acids, oils, pigments, alcohols, and fat-soluble vitamins. On the other hand, some complex lipids, such as phospholipids, may not be isolated with this method. Crude fiber represents the organic residue that remains after plant material has been treated with dilute acid and alkali solvents and after the mineral component has been extracted. Although crude fiber is used to report the fiber content of many commercial products, it usually underestimates the level of true dietary fiber in a product. It has been determined that the crude fiber method recovers only 50% to 80% of the cellulose, 10% to 50% of the lignin, and less than 20% of the hemicellulose in a given sample. ¹ Consequently, crude fiber may be a measurement of most of the cellulose in a sample, but it underestimates all of the other dietary fiber components. Consumers can use the guaranteed analysis panel to provide a rough estimate of protein, fat, and fiber content in a particular pet food. However, these numbers should only be considered as a starting point when comparing different products or brands, and they should not be assumed to represent the actual levels of these nutrients in the food.

When examining the guaranteed analysis panel of a pet food, consumers must always take into account the moisture (water) content of the product. The amount of water in a food significantly affects the values listed in the guaranteed analysis table, because most pet foods display nutrient levels on an “as-fed” (AF) basis, rather than a dry-matter basis (DMB). As-fed means that the percentages of nutrients were calculated directly, without accounting for the proportion of water in the product. Pet foods can vary greatly in the amount of water they contain. For example, dry cat and dog foods usually contain between 6% and 10% water, but canned foods contain up to 78% water. ² In order to make valid comparisons of nutrients in foods with different amounts of moisture, it is necessary to first convert nutrients to a DMB. Similarly, the caloric content of a pet food also affects the interpretation of the guaranteed analysis panel. Caloric density must always be considered when comparing levels of protein, fat, carbohydrate, and other nutrients in different pet foods (see pp. 145-147).

The ingredient list is another important item that consumers examine when choosing a pet food. The terms used for ingredients in pet foods are limited to those assigned by AAFCO through AAFCO’s Ingredients Definition process. This process includes oversight by the Food and Drug Administration (FDA), when applicable. The new FDA Amendments Act (FDAAA), passed in late 2007, requires increased involvement of the FDA in ingredient definition and standards (see Chapter 14, pp. 124-125). Cumulatively, these regulations mean that pet foods contain only those ingredients that are defined and accepted by these agencies. New ingredients can only be introduced following submission of data to AAFCO and the FDA and completion of the approval process. In addition to formally approved ingredient definitions, terms for ingredients that are considered to be “common and usual” are also allowed. Examples of these include ingredient designations that are also used in human foods, such as beef, wheat, oats, and water.

The pet food ingredient list must include all ingredients in the food, and the list is arranged in decreasing order by predominance by weight. The list may be quite simple, containing just one or two ingredients, such as in certain types of treats (i.e., freeze-dried liver or salmon). More commonly, pet foods contain multiple ingredients to provide a diet that is complete and balanced. In no case can any single ingredient be given undue emphasis in the list, nor can designations of the quality of ingredients be included. The ingredient list indicates where the principal components of a pet food come from—animal products or plant products. In general, if an ingredient from an animal source is listed first or second in a canned pet food or within the first three ingredients of a dry pet food, the food can be assumed to contain animal products as its principal protein source.

Most popular and generic brands of pet food are formulated as “variable-formula diets.” This means that the ingredients used in the food will vary from batch to batch, depending on the availability and market prices of ingredients. In contrast, many premium foods are produced using fixed formulas. In this case, the company will not change the formulation in response to normal fluctuations in market prices. Checking the ingredient list of several bags of a particular pet food over a period of time can indicate whether the company is using fixed or variable formulation. Although the pet owner may pay slightly more for a fixed formulation diet, the consistency between batches of food is a distinct advantage to the dog or cat that is consuming the food.

Although the ingredient list can provide general information about the type of ingredients included in a food, it does not provide information about the quality of its components. Ingredients used in pet foods vary significantly in digestibility, amino acid content and availability, mineral availability, and the amount of indigestible material they contain. Unfortunately, there is usually no way of determining the quality of the ingredients by using the ingredient list. In fact, some premium foods with high-quality, highly available ingredients may have an ingredient list that is almost identical to that of a generic food that contains poor-quality ingredients with low digestibility and nutrient availability. Therefore the ingredient list alone should never be used to compare two pet foods because the differences in the qualities of ingredients are impossible to know from this information.

Like the guaranteed analysis, the list of ingredients can be deceptive, in some cases, because manufacturers are not required to list the ingredients on a DMB. This is usually not a problem in dry pet foods because most of the ingredients included in these diets have a relatively low moisture content. However, canned products may contain ingredients with vastly different amounts of moisture. As a result, an ingredient that actually contributes a low proportion of nutrients to the food may be listed first if it has a high water content, while an ingredient that contributes a large proportion of the nutrients to the food may be lower on the ingredient list if it has a low moisture content. A common example involves the use of texturized vegetable protein (TVP) in canned pet foods. TVP is composed of extruded soy flour that is dyed and shaped to resemble meat products. The actual meat ingredients in a product that contains TVP are usually listed high on the ingredient list because they have a very high moisture content. Conversely, TVP has very low moisture and so its total weight is much lower, and it is found much later in the ingredient list. This can be misleading because most of the protein in such a food comes from the TVP and not from the animal-source ingredients that are listed first.