NUTRIENTS & DIET
We have discussed in previous articles the essentials of water and carbohydrates in your pet’s diet. This month we will look at the merits of protein.
Protein is composed of amino acids, which are the building blocks of the body. Proteins are used for energy and in the production of enzymes, hormones, antibodies, and in making muscle and other structural tissues. While people often mistakenly are concerned about the protein content of food, in reality it’s the amino acids that are important. The protein sources used in formulating the diet must contain the proper amounts of the essential amino acids needed, or your pet will suffer from an amino acid deficiency despite an adequate protein intake.
While plants can produce all of the 24 amino acids, they can’t really produce enough of each or in the right amounts. With few exceptions an all-plant (vegetarian) diet would not provide enough of the right amounts and right balance of amino acids required for our pets. Keep in mind that cats are true carnivores requiring meat in their diets; and even though dogs are more omnivorous than cats, they too are carnivores and must have meat in their diet.
Pets cannot make all the amino acids they need; therefore, their diets must provide those they cannot make in their bodies. These required amino acids are called essential amino acids. The pet’s body, on the other hand, can make nonessential amino acids, from other amino acids that they get through their diet. A high-quality protein diet contains all the essential amino acids required in the right amounts and right rations. Protein is supplied in a pet’s diet through animal (preferably) or plant material; in cheaper diets the animal source might be “animal meal” or “animal by-products” rather than whole, dressed animal. These less wholesome ingredients are not desired in the diets of pets.
The biological value of protein is based on the protein’s unique combination of its building blocks, the amino acids. People and animals use the amino acids obtained by digestion of protein for growth and building tissues and organs. When discussing proteins, the biological value of the protein is important. Eggs are ranked at 100, being considered the best food for providing high-quality protein. Digestibility is also important; the protein must be easily digested and assimilated into the body before it can be of any value to the pet. While it would be most helpful to know the amino acid content of the protein in commercial pet foods, the label only needs to list the crude protein. It’s important to read the label to see the quality of ingredients, as crude protein can include items such as feather meal, hair, hooves, tendons, and ligaments. While these animal by-products certainly provide protein, they are essentially non-digestible and provide no biologic value (amino acids) to the pet. Cheaper, generic foods are more likely to contain these by-products as sources of protein for the diet.
Protein deficiency is not common but can occur during illness, when the animal eats less or refuses to eat. In this situation, the pet still requires amino acids. If he isn’t eating, the pet will literally break down his own muscle tissue to supply his body with protein. This happens quite readily when the pet becomes ill, and is an important reason why sick pets don’t recover from illness quickly. Providing food for a sick pet is so essential that hospitalized pets are force-fed to maintain a positive energy and protein balance to help them recover quickly. You can see how important it is to make sure that sick pets continue to eat.
There are some unique requirements for amino acids in our pets. For example, cats, being true carnivores, require high levels of the amino acid taurine. Taurine is only found in meat; cats cannot be fed diets based on soy, dairy, or plant material without added taurine. Taurine deficiency has been associated with blindness and dilated cardiomyopathy in cats.
Taurine is not made into proteins synthesized by the body but instead remains as a free amino acid. It exists as a free amino acid in many body tissues, including the brain, nervous tissue, heart muscle, skeletal muscle, retina of the eye, liver, red and white blood cells, as well as in milk and as a complex with bile salts excreted by the gallbladder.
Since taurine is a free amino acid found in meat, it is important when preparing diets at home that if the meat is cooked in water, the water be added back to the diet to preserve taurine. That’s because the longer the meat is cooked, the more taurine will leach into the water. Owners choosing to feed raw meat to their cats will not have this problem.
Cats also require citrulline, as they cannot synthesize it. They can convert arginine to citrulline as long as the diet contains adequate amounts of arginine. Diets deficient in arginine, citrulline, and the amino acid ornithine predispose the cat to developing hyperammonemia (too much ammonia in the blood), which can be fatal. Since most protein sources contain adequate arginine, additional supplementation is usually not needed. Cats with hepatic lipidosis are often supplemented with arginine to help detoxify ammonia and prevent worsening of clinical signs. Diets designed for pets with cancer contain additional arginine as this amino acid decreases tumor growth and spreading (metastasis). Arginine is also involved in the formation of nitric acid. Nitric acid is used by the body to help regulate blood flow through blood vessels and regulate blood pressure. Nitrates (isosorbide dinitrate, nitroglycerin) may be used by doctors to help pets with heat disease. It may be possible that additional arginine could be helpful in controlling blood flow and blood pressure in dogs and cats with heart disease, but this has not been investigated.
Glutamine is present in high-quality proteins and is not often added to pet foods. Additionally, because heating and cooking destroy L-glutamine, supplementation with L-glutamine is best accomplished by adding the supplement (often in a powdered or capsule form) to the food immediately before feeding. (Capsule glutamine supplements can be given directly to the pet as directed by the veterinarian.) Dogs and cats experiencing stress-illness, cancer chemotherapy, immunosuppressive illnesses (such as feline leukemia virus or feline immunodeficiency virus infection), or surgery—may not be able to maintain glutamine stores in their bodies and might benefit from additional supplementation. Glutamine may also retard the cachexia (wasting) seen in many pets with cancer. Glutamine, being the preferred energy source for cells of the intestinal tract, is useful as a supplement for pets with acute and chronic gastroenteritis (especially when diarrhea occurs) and from gastroenteritis that results from cancer chemotherapy.
Excess dietary protein has been linked to fear-related territorial aggression in some dogs. It is theorized that increased protein interferes with the transport of the amino acid tryptophan across the blood-brain barrier, which can decrease serotonin formation. Serotonin is an important neurotransmitter, and low levels have been linked to various behavioral problems. Often feeding these dogs a reduced-protein diet can cure this behavioral disorder.
Next month we will discuss fat as an essential nutrient for your pet.
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