Evaluating diets for healthy pets (Proceedings)
Characteristics of an adequate diet
Pets require a satisfactory diet to maintain normal structure and function at all stages of life. I define a satisfactory diet as one that is complete, balanced, palatable, digestible, and safe. Complete means that the diet provides adequate amounts of all required nutrients; balanced means that the nutrients are present in the proper proportions. Balance is crucial because excesses of some nutrients may cause deficiencies of others. The completeness and balance of a diet can be evaluated by comparing the chemical composition of the diet to the pet's nutrient requirements. Regardless of the chemical composition, the diet also must be sufficiently palatable to be eaten in quantities great enough to support normal function, and the nutrients must be digestible enough to become available to the animal. Nutrient digestibility is measured by subtracting the amount of the nutrient found in the feces from that present in the food. Even if a diet is complete, balanced, palatable and digestible, it also must be safe – containing no harmful substances. What is an appropriate diet for one life stage, however, may not be for another. Nutrient needs vary with the stage of the pet's life, with the needs for lactation and early growth being as much as four times greater than for maintenance of older, sedentary adults. Deficiencies in the nutrient content of diets often appear only during the periods of greatest nutritional stress, so it is during these times that one must be most careful to recommend the best quality diet available to the client.
Pet foodsPet food is available in three physical forms: dry, semi-moist, and canned. Each form represents a standard method of food preservation, and each has advantages and disadvantages. Quality, however, does not depend only on physical form; good and bad examples of all forms are sold. Making direct comparisons of nutrient content among these different forms of food may be quite misleading because of the different amounts of water and energy in the three forms. For example, the amount of protein guaranteed on the label of dry cat foods is about 35%, and in canned foods about 10%. Canned foods, however, contain more water than dry foods do. For products of differing moisture content but similar energy density, a more appropriate way to compare the nutrient content is on a "dry matter" basis. To compare nutrient contents on a dry matter basis, divide the nutrient of interest by the total dry matter. Because labels provide % moisture in the food, the % dry matter is calculated by subtracting the % moisture from 100% (% dry matter = 100% - % moisture). In the above example, dry foods contain about 10% moisture, or 100% -10% moisture = 90% dry matter. Dividing 35% protein by 90% dry matter (and multiplying the result by 100) equals 38% protein on a dry matter basis. Canned foods usually contain 78% moisture, which equals 22% dry matter, so 10% protein divided by 22% dry matter equals 45% protein on a dry matter basis. Calculated on a dry matter basis, the canned food in this example contains 18% more protein than the dry food does.
Another way to compare dietary constituents is on an energy basis. This method is most useful for foods differing in fat content, because the metabolizable energy (ME) density of fat in pet foods is approximately 8.5 kcal per gram, versus 3.5 kcal/gram in carbohydrate and protein. Many commercial diets contain an overall ME density of 350 kcal/ 100 gram for dry and 100 kcal/ 100 gram for canned food as fed. To compare the foods above on an energy basis, 35 grams protein/100 grams diet divided by 350 kcal ME/100 grams diet equals 1 gram protein per 10 kcal ME for the dry diet, and 10 grams protein/100 grams diet divided by 100 kcal ME/100 grams diet equals 1 gram protein per 10 kcal ME for the canned diet as well. If we were comparing two diets of differing fat content, however, the result might have been quite different, as examination of Figure 3 will reveal. Because of differences in energy and moisture content among diets, nutrient comparisons between foods are most meaningful when made on an energy basis. Recent changes in Association of American Feed Control Officials (AAFCO) regulations permit ME estimates being added to pet food labels, but to date manufacturers have been slow to provide this information on labels. It is, however, available by calling the manufacturer or visiting their website.
Dry pet foods are predominantly extruded, expanded products. Extrusion is a process whereby dry ingredients are cooked with steam and pressure to kill microorganisms, and to increase digestibility of the ingredients. For example, starch in the mixture is partially degraded, "gelatinized", in the process, which enhances its digestibility. The heated food is then forced (extruded) through a small opening in a plate at the end of the pressure chamber. The sudden change in pressure when the food exits the chamber causes the gelatinized carbohydrate to expand. The shape of the opening in the plate can be varied to produce different shapes of the food.