Pet foods are formulated based on the criteria provided. For example, using a dry adult cat food the nutritionist would use a variety of ingredients typically used in dry cat food products. Because the product is a cat food, one would set nutritional targets appropriate for an adult cat. Using a computer program that uses a technique known as linear programming, the nutritionist can then formulate the diet. Linear programming allows the nutritionist to simultaneously balance as many nutrients as needed in order to meet the animal's nutritional requirements without providing excesses. While all nutrients are important, the experienced nutritionist knows which ones require special attention. The vitamins and trace minerals are all provided by vitamin and mineral premixes, so as long as they are added to the formula these nutrients are present at the necessary levels.

For cats, particular attention needs to be paid to protein, fat, the essential fatty acids linoleic and arachidonic, the minerals calcium, phosphorus, magnesium, and potassium, and the amino acids lysine, methionine, tryptophan and taurine. When using common pet food ingredients most of the other nutrients will be present in the appropriate amounts. After a number of attempts, the nutritionist would have a formula to be tested.

Once a final formula is achieved, the next step is to produce the product and begin a series of feeding tests. Three animal feeding tests and a series of chemical analysis are usually performed on a prototype pet food.

1. Palatability Test

The first animal test is a palatability test in which the animal's preference for the food relative to another is measured. This is typically done by providing the dog or cat with two bowls of food, one containing the new product and the other containing the old product or a product comparable to the new product. This is done for 2 to 4 days using 10 to 20 animals. Since some animals show a side preference, the bowls are rotated on a daily basis so that diet A is on the left side on day 1 but on the right side on day 2. The amount of food consumed for each diet is measured daily for each animal and a preference is determined.

2. Digestibility Test

In this test, 6 to 8 animals are fed the diet as the sole source of nutrients for 14 days. During the first 9 days, the animals are acclimating to the diet and so no feces are collected. For the last 5 days, the food consumed and the feces produced are collected, weighed and analyzed for dry matter, protein, fat and energy. By calculating how much the animal eats and how much is lost in the feces one can determine the digestibility of the nutrient. For example, if an animal eats 425 grams of protein and excretes 55 grams then the protein digestibility is 87% ((425-55)/425). Sometimes people refer to total digestibility. This is a measure of the amount of dry matter that is digested, not what is actually absorbed.

3. Life Stage Test

The last test is life stage testing. The minimum requirements are specified by the Association of American Feed Control Officials (AAFCO). There are three different life stages which can be tested: gestation/lactation, growth and maintenance. If a food passes both gestation/lactation and growth testing, the food can be labeled as complete and balanced for all life stages. Each test involves the use of at least eight animals and a variety of measurements are required. These include body weight, food consumption, a variety of blood parameters and a complete physical exam performed by a licensed veterinarian. The length of the test ranges from 10 weeks for a growth test to 26 weeks for a maintenance test.

Other testing is often done on products depending on the type of product and the claims being made. However, for cat foods, reputable manufacturers would also determine the effect of the diet on urine pH in order to minimize the risk of FLUTD.

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