Protein, while being an energy producing nutrient, has a more important role in providing amino acids. Amino acids are the building blocks of tissue. As such, amino acids are necessary for the proper formation of skin, hair, muscles and the cells that make up many other organ systems. In addition, amino acids are used to make enzymes, which along with vitamins and minerals, are necessary for proper metabolism.

Proteins are made up of amino acids, much like a pearl necklace is made up of individual pearls. Proteins vary in length from just a few amino acids to those containing many thousands of amino acids. While there are many different types of amino acids, most proteins are composed of about 20 different amino acids. Just as distinct words can be made from different groupings of the 26 letters of the alphabet, distinct proteins can be made by grouping the individual amino acids in a specific and unique order. When a dog or cat consumes a diet containing protein, the protein is broken into its component amino acids and these are absorbed in the small intestine. The amino acids are then reassembled into a different order, thereby making the specific proteins the body needs. Extra amino acids are broken down and used for energy.

Although protein does provide energy, its primary purpose is to build body tissue. This is why diets must also contain plenty of other sources of energy such as fats and carbohydrates. In this way, protein can be saved for its primary purpose. It was once thought by some that using protein for energy led to kidney disease because the excretion of protein by-products would over-work the kidney, however, recent research in dogs suggests that this is not true.

In most animals, including dogs, liver enzymes adapt to the amount of protein consumed. This allows the animal to conserve nitrogen (a component of protein) when fed a low-protein diet and excrete nitrogen when fed a high-protein diet. However, cats do not have this ability. This makes sense when one considers that many animals, including dogs and humans, are omnivores (eat other animals or plants) and eat a wide variety of foods, some of which are high protein and others that are not. In contrast, cats are obligate carnivores (eat meat only) by nature and always consume a high protein diet. Thus, the protein requirements of cats are much higher than that for dogs or humans.

Of the 20 or so different amino acids found in of the body of dogs and cats, both species are known to be unable to synthesize 10 of these. Thus, these 10 amino acids need to be supplemented in the diet. Cats also appear to have a requirement for an eleventh amino acid, taurine. While not a "true" amino acid, (taurine is actually an amino-sulfonic acid) it is typically included do to its similarities to the group. These "needed" amino acids are more commonly referred to as "essential amino acids". If a growing puppy or kitten does not obtain these essential amino acids through their diets, growth will stop. The essential amino acids are listed below and we will explore each in more depth in the following paragraphs.

The amino acids cysteine and tyrosine are considered only partially essential, since cysteine can be made from methionine and tyrosine can be made from phenylalanine. Animal source proteins usually contain a better proportion of these essential amino acids than do plant proteins. Oftentimes, the proper blend of animal and plant proteins can provide the essential amino acids more efficiently than either source alone, a process known as complementation.

High quality proteins are those that provide the right balance of amino acids when compared to the animal's needs. There are two things to consider when determining protein quality. First, the amino acids in the protein must be similar to those required by the dog or cat and is availability. The perfect balance of amino acids does not do an animal any good if those amino acids are not digested, absorbed, and utilized.

As mentioned earlier, cats have a special need for the essential amino acid, taurine. They are unable to manufacture this themselves, and so it must be in their diet. Dogs and most other animals can usually make taurine from the essential amino acid methionine. Animal, poultry and fish products contain taurine, however plant based ingredients do not. This is one reason why cat food should have some animal source ingredients. While it has been known since the 1960's that cats required taurine, it was not until the late 1980's that pet food manufacturers realized the effect that processing had on the availability of taurine. One of the leading causes of death in cats at the time (dilated cardiomyopathy, a weakening of the heart muscle) could have been prevented by increasing the amount of taurine in the diet. Amazingly, cats with advanced stages of cardiomyopathy could be completely cured in as little as 2 to 4 weeks when given a taurine supplement. Since the late 1980's when cat foods were supplemented with additional taurine, the incidence of dilated cardiomyopathy in cats has declined dramatically. The other sign of taurine deficiency in cats is blindness, which cannot be reversed by supplemental taurine.

Recent evidence now suggests that dogs consuming a diet that has limiting amounts of methionine and cysteine and is low in taurine may result in low blood concentrations of taurine. Some, but not all dogs with low blood taurine, show some of the same deficiency signs (dilated cardiomyopathy) that taurine deficient cats exhibit. This may be related to breed size with large and giant breed dogs being more susceptible. For this reason, most manufacturers are now supplementing canine diets with taurine.

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