Proteins are the primary components of many body tissues and also aid in the transportation of substances in the blood. Proteins serve as the nitrogen source for animals, which, unlike some plants, cannot fix atmospheric nitrogen.

Proteins are large, complex molecules composed of hundreds to thousands of amino acids. Dogs and cats do not have a protein requirement per se, but have known daily requirements for each essential amino acid.

Therefore, amino acids must be in the diet, but the source of these amino acids (chicken, beef, soybean) is irrelevant. Since pets have requirements for specific amino acids the essential amino acid concentrations are more important than the crude protein number listed on the pet food label.

Animal source proteins (chicken, beef) contain more essential amino acids than some plant proteins (soybean), although combinations of the two are often complimentary. In fact, when both are combined in proper proportions, they enhance the overall biologic value of the protein in the diet.

Amino acids are classified as either essential or non-essential. Essential amino acids cannot be synthesized by the body in sufficient quantities and therefore must be supplied in the diet.

Non-essential amino acids can be synthesized by the body from existing carbon and nitrogen and need not be present in food as long as certain physiological or pathologic conditions do not exist.

Egg and liver are of the highest protein quality because of their amino acid profiles. In fact, egg protein is typically used as a reference for comparing all other proteins and is given a score of 100 based on its biological value.

The quality of the protein affects the amount that must be provided in the diet to meet a pet's protein requirement. As protein quality increases, the amount of protein required in the pet food decreases.

Some pet owners wanting to save a buck or two often reach for the cheapest pet food on the shelf. However, in the long run they probably really aren't saving much money because they have to feed more of the lower quality diet to meet their pet's nutritional needs.

Cats metabolize protein differently and require at least twice as much protein as dogs. (Which explains why cat food is more expensive than dog food.) Young animals also require more protein than adults.

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