Protein Nutrition
September, 1997
The pet food industry has expanded considerably in the past
5-10 years. There is now a seemingly endless choice of economy
brands, premium brands, prescription brands and “natural”
dog foods. Finding a brand that is reliable, consistent from
batch to batch, relatively inexpensive and provides good
nutrition can be difficult. One of the most common nutrition
questions I encounter is regarding protein content of foods.
Proteins are an essential part of all foods and they provide a
number of functions in the body. They serve as components of
enzymes, hormones, a variety of body secretions and structural
tissues such as muscle, hair and nails. Proteins of any source
are long chains of smaller units called amino acids. Different
proteins have different amino acids in their structure. There are
22 different amino acids and all animals need all 22, but many
can be manufactured by the body if simple raw materials are
supplied. Dogs cannot synthesize 10 of them. They are therefore
called “essential” amino acids and must be supplied in
the diet.
Two measures of the quality of the protein contained in any food
are digestibility and biological value. Digestibility is the
measure of how much of the protein that is eaten is actually
absorbed through the intestines. The undigested protein is
eliminated in feces. Two factors that affect the digestibility
are the source and the processing. Most processing involves
cooking, which may make a food more acceptable but usually
decreases the protein digestibility. Foods that contain a large
amount of hide or hoof will have poor digestibility without
excessive processing, due to the source of the protein. Such a
food could be reported to contain an “adequate” level
of protein, but if it’s largely indigestible, it is of no
use to the dog.
The concept of biological value is a little more difficuly to
explain and to understand than digestibility. Generall,
biological value is a measure of how much of the digested protein
is acutally used by the body. More specifically, it is a measure
of how well the amino acid content of the protein in the food
matches the animal’s requirements. Egg protein usually has
the highest biological value. Other animal proteins (fish, milk,
beef, liver, chicken) usually have higher biological values than
vegetable proteins (soybean, wheat, corn).
However, the right combination of vegetable sources can contain
protein with a higher biological value than either of its
components. This is possible when on source contains extra
amounts of those amino acids that the other source is deficient
in. Corn and soy protein is an example of this.
When feeding protein of low biological value, one could simply
increase the total amount fed, in order to supply adequate
amounts of the few amino acids it is deficient in. However, this
would mean the animals would be receiving excessive amounts of
the other amino acids. While no toxicities are common from
ingesting amino acids above the required amount, the extra
protein has to be dealt with. The nitrogen-containing portions of
those amino acids are eliminated in urine and the remainder is
handled the same as extra energy in the diet. If not needed, the
animal deposits it as fat.
Feeding extra amounts of protein or protein of low biological
value therefore leads to an extra workload on the kidneys.
Feeding high protein diets over an entire lifetime has been shown
to cause kidney disease in rabbits and rats. Studies in dogs and
people are not as conclusive. However, it is a well established
fact that dogs already in kidney failure do best if they eat a
diet with just a marginal amount of high quality protein. This
decreases the amount of nitrogenous waste for the kidneys to
eliminate. For those animals that survive a bout of kidney
disease or those that are caught early in the course of chronic
renal failure, it is best to feed a prescription diet for life. A
homemade diet that is also beneficial contains appropriate
amounts of egg, meat, rice and bread.
Dog food labels rarely contain more information regarding protein
content than a minimum percent of crude protein. “Crude”
protein measurement is aptly named. It is calculated by measuring
the nitrogen content of the food and then assuming that its
protein sources all contain the average 16 percent nitrogen. This
is not always the case, but measuring the nitrogen content is a
whole lot easier and cheaper than measuring the actual protein
content.
Most maintenance dog foods contain 18-20 percent crude protein
while performance diets contain 25-30 percent. An active hunting
dog would benefit from being fed a performance, or high protein
diet, but the label contains no information regarding the most
important factor - the quality of the protein (both digestbility
and biological value). How can a concerned owner choose a good
quality dog food by reading the label? One good indication is
finding the letters AAFCO on the label. Dogfoods may claim to be
“complete and balanced” by one of two different
methods:
-By passing feeding trials approved by the Association of
American Feed Control Officers (AAFCO).
-By containing at least the minimum amount of each nutrient as
receommended by the National Research Council (NRC).
Meeting NRC requirements is easier, but does not take into
account such factors as acceptability, digestibility or
biological value of a nutrient. While AAFCO approval is not a
guarantee of highest quality, finding it on the label is one
quick way to choose a pet food that has proven itself in a
real-life situtation.