Written by Dr. Jill Cline, Ph.D.
SOURCES OF PROTEIN IN DRY DOG FOOD
The type and quality of protein is an important consideration when selecting the best food for your dog. Protein in dog foods can be supplied by meat and poultry sources, plant sources (grains), or by a combination of the two. Several factors should be considered when evaluating protein quality and selecting the best food for your dog.
Your Dog’s Requirement for Protein
Dietary protein is necessary for the growth and maintenance of almost all tissues of the body. Protein is a major structural component of hair, skin, tendons, ligaments and cartilage as well as the enzymes that catalyze all essential metabolic reactions. Hormones that act as the body’s chemical messengers and antibodies that comprise the immune system are also composed of protein. The body’s protein stores are not static, but rather are in a constant state of flux, as cells and tissues wear out. The diet provides a regular supply of new protein to replace these normal losses.
Changing Needs: An optimal level of protein is needed in your dog’s diet throughout life. Growing puppies have higher protein requirements than adult dogs, and adult dogs who are working hard have slightly higher needs than those who lead a less active lifestyle. Dietary protein should contribute a minimum of 25 % of food energy for growing puppies and at least 20 % of food energy for adult dogs.
Amino Acids: Amino acids are the basic units of proteins and are linked together by peptide bonds to form long protein chains. Protein consumed in the diet is digested in the small intestine into its component amino acids, which are then absorbed into the body. Like other animals, the dog has a metabolic requirement for 22 amino acids. However, 12 of these, the nonessential amino acids, can be synthesized by the dog in adequate amounts to meet needs. The remaining 10 amino acids, called the essential amino acids, cannot be made by the body in sufficient amounts and must be supplied by food protein (Box 1).
ESSENTIAL AMINO ACIDS FOR DOGS
Histidine Methionine Phenylalanine
Isoleucine Leucine Tryptophan
Arginine Valine Lysine
Protein Ingredients in Dog Foods
A variety of protein-containing ingredients are included in dog foods. Many of the ingredient terms that are found on pet food labels are defined by the American Association of Feed Control Officials (AAFCO). The following protein sources are common ingredients in dry dog foods:
Chicken: When “chicken” is included on the label as a singular ingredient with no other qualifying term, it signifies that only the flesh and skin (with or without bone), from parts or whole chicken is included. AFFCO requires that to use this ingredient label, other parts of the chicken, such as feathers, heads, feet and entrails cannot be included. Maybe include protein range for ingredients?
Lamb/Beef/Pork: Similarly, a label of a meat product with no other qualifying term must include only the clean flesh of the identified mammal. The flesh may include striated skeletal muscle, tongue, diaphragm, heart, esophagus, overlying fat and skin, sinew, nerves and the blood vessels normally found with the flesh. Hooves, blood, hair, horn, hide trimmings, and stomach and rumen contents must be excluded.
Chicken or meat meal: The term “meal” refers to any ingredient that has been ground or otherwise reduced in particle size. For example, “chicken meal” is rendered, dry, ground, whole chicken exclusive of heads, feet, viscera, or feathers. Rendering refers to heat processing the protein source to remove water and fat. The end product is ground into small particles to create a protein-rich meal.
Meat or poultry by-products: When the term “by product” is added to an ingredient name, this signifies that secondary products have been included with that ingredient. For example, chicken by product refers to the clean parts of carcasses of slaughtered chicken that may also contain bone, heads, feet, and viscera, but no feathers. Similarly a meat meal may include lungs, spleen, kidneys, brain, livers, blood, bone, some fatty tissue, and stomachs and intestines free of contents, but no hair, horns, teeth or hooves.
Corn gluten meal: Corn gluten meal is one of the most commonly used plant-based protein sources. It is the portion of the corn kernel that remains after most of the starch and germ containing portions of the grain have been removed. Corn gluten meal should not be confused with ground corn, which is included as a carbohydrate source rather than a protein source in many foods.
Soybean meal: Several forms of soybean products are included in pet foods. The most common soy protein sources are defatted soybean meal and soybean flour. Soybean meal is produced by grinding the flakes or cakes that remain following processing to extract oil from the beans (defatting). Soybean hulls or mill run consist primarily of the outer covering of the soybean. When these ingredients are reported on a label, they are not present as a protein source, but rather to contribute fiber to the diet.
All Protein Sources are Not Created Equally
The quality of the protein in dog food is influenced by its digestibility and amino acid balance and bioavailability. Digestibility refers to the proportion of a protein that can be broken down into its component amino acids in the small intestine for absorption into the body. Amino acid balance is the relative amount of essential amino acids that a protein source contains. Bioavailability refers to the proportion of the absorbed amino acids that are available to the body’s tissues for use. These indicators of protein quality are measured by pet food manufacturers in the following ways:
• Protein digestibility: Pet food companies measure protein digestibility using various types of feeding trials. The basic format involves feeding a food containing the test protein to a group of dogs for a selected period of time. The amount of undigested protein excreted in the feces is measured and used to calculate protein digestibility. In general, high quality protein sources are more digestible than lower quality sources (Box 2).
• Amino acid balance: A protein’s amino acid composition is determined through laboratory analysis of the protein. the essential amino acid balance of the test protein is then assessed by comparing the amino acid profile to that of a high quality reference protein, such as egg protein. This procedure functions to identify the essential amino acid that is in greatest deficit in the test protein (called the “limiting amino acid”). A second measure evaluates the contribution that the protein makes to all of the essential amino acids needed by dogs, and a third calculates the proportion of the protein that is made up of essential amino acids. All three of these tests provide information about the quality of the protein in terms of its ability to provide dogs with needed essential amino acids.
• Amino acid bioavailability: Assessment of amino acid bioavailability from a protein source once again involves feeding studies. Growth and tissue development are measured to reflect a protein’s ability to supply essential amino acids to target tissues. One measure that provides comprehensive information is called net protein utilization (NPU). A protein’s NPU value reflects both the protein’s digestibility and the bioavailability of its component amino acids to the body and target tissues.
The pet food label provides an estimate of a food’s crude protein content on its guaranteed analysis panel. This measure reflects only the total amount of protein and does not indicate differences in protein digestibility between high and low quality protein sources. For example:
Dog Food A contains 21 percent crude protein and is 86.0 % digestible.
Dog Food B contains 23 percent crude protein and is 76.0 % digestible.
Food A: 21g protein/100g diet x 0.86 = 18.1g protein absorbed
Food B: 23g protein/100g diet x 0.76 = 17.5g protein absorbed
Although the crude protein value reported for Dog Food A is lower than that for Dog Food B, Dog Food A’s higher digestibility results in more protein being available to the dog, in a given volume of food.
Food Selection Tips
Proteins that are highly digestible and contain a balance of all 10 essential amino acids are considered high quality proteins. In contrast, those that are less digestible or limiting in one or more of the essential amino acids are of lower quality. A protein’s digestibility and amino acid bioavailability is affected both by the source of the protein and by the processing methods that are used to produce the food. Here are a few tips to help you to distinguish between the protein sources included in dog foods:
• Processing can affect protein quality: Proper processing techniques and cooking temperatures support optimal protein digestibility and bioavailability. Conversely, a protein’s digestibility can be significantly reduced by over-processing or excessive heat treatment. The rendering process used to produce meat and poultry meals can alter the structure of some amino acids. Although these modified amino acids are still absorbed into the body, they are not available to tissues for use. Because rendering can negatively affect protein quality, including whole poultry or meat in a dog food generally increases the food’s protein quality when compared with foods that contain only meals or by-product meals.
• Animal-source vs. plant-source proteins: There is controversy among dog enthusiasts about the use of plant-source proteins in pet foods. Some believe that because the dog evolved from a predatory species, they cannot digest or utilize plant products. In actuality, the dog is quite omnivorous in nature, and can thrive on diets that contain a variety of animal and plant products. However, just like other ingredients, the quality of these sources can vary:
• Order of ingredients: Pet food manufacturers must list ingredients on the label in order of their preponderance by weight. Therefore, the first three to four protein-containing ingredients on the dog food’s label will indicate the food’s principle protein sources. Use this list to identify the type of protein source that you prefer for your dog.
• Bottom line: Quality costs more: Just as in human foods, products that are of higher quality are of greater nutritional value to your dog and tend to be more costly. In pet foods, the highest quality (and most expensive) protein ingredients such as whole chicken, beef, egg, and lamb are used in premium and super-premium products. Conversely, by-product meals are of lower quality, less expensive to manufacturers, and are found in the more economical brands of food.
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