Written by Dr. Jill Cline, Ph.D.
Because protein is such an important essential nutrient, many pet owners are interested in both the amount and the quality of protein that is included in their pet’s diet. Information regarding dietary requirements and the sources of protein that are found in pet foods can help owners to select foods that best meet their dog or cat’s needs.
Functions of Protein
Protein in the body: Dietary protein is necessary for the growth and maintenance of almost all tissues of the body. It is the major structural component of hair, skin, tendons, ligaments, blood cells and cartilage. Protein comprises the enzymes that catalyze all metabolic reactions, the hormones that act as the body’s chemical messengers and the antibodies that comprise the immune system. The body’s protein is not static, but rather is in a constant state of flux as cells and tissues wear out and are replaced. The diet provides a regular supply of new protein to replace the losses that occur during normal protein turnover. Any dietary protein that is not used for structural or metabolic functions can also be used as an energy source.
Amino acids: Amino acids are the basic units of all proteins and are linked together by peptide bonds to form long protein chains. Protein that is consumed in the diet is digested by the body into its component amino acids, which are then absorbed in the small intestine. Like other animals, dogs and cats have a metabolic requirement for 22 amino acids, but 12 of these, the nonessential amino acids, can be synthesized by the body in adequate amounts to meet daily 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 dietary protein (Box 1). In addition to these 10 essential amino acids, the cat also has a dietary requirement for the amino sulfonic acid, taurine.
ESSENTIAL AMINO ACIDS FOR DOGS AND CATS
Histidine Methionine Phenylalanine Isoleucine
Leucine Tryptophan Arginine Valine
Lysine Threonine Taurine (cats only)
Factors Affecting Protein Requirements
Although protein is the nutrient that is of interest, animals do not have a dietary requirement for protein per se. Rather, the body requires a proper balance of the 10 essential amino acids (plus taurine for cats). These are provided by the protein that is consumed in the diet. Following digestion in the small intestine, dietary protein’s component amino acids are absorbed into the body where they can be “reassembled” by cells into the structural and metabolic proteins of the body. For this synthesis to take place, all of the amino acids that are needed for a particular protein must be available in sufficient amounts to the body’s cells. A deficiency of just one essential amino acid directly influences the ability of the animal to use other amino acids and to properly replace protein losses and build new tissues. Therefore, it is not only important that enough protein is provided in the diet, but that the protein that is present contributes the correct proportions of all of the essential amino acids that are needed by the dog or cat. The body does not readily store amino acids that are not utilized, so the diet must provide a regular, daily supply of essential amino acids.
Although it is amino acids that the body needs (as opposed to entire protein molecules), we still refer to dogs and cats as having a “protein requirement” because amino acids are most typically supplied in the diet in the form of intact protein. An individual dog or cat’s daily protein requirement is influenced by several factors that are associated with the diet and with the animal:
• Protein digestibility: 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. More digestible proteins provide a more readily available source of amino acids than less digestible protein sources (Box 2).
• Amino acid balance and bioavailability: The amino acid balance and bioavailability of a protein source is the relative amount of the essential amino acids that the protein can provide for use by the animal’s tissues and cells. A protein that provides all of the essential amino acids is considered to be of very high quality. The higher the quality of a protein, the less the amount of that protein that is needed to meet the animal’s essential amino acid needs. Therefore, just like digestibility – as amino acid balance and bioavailability improve the percent total protein that must be included in the diet to meet needs decreases.
EFFECT OF PROTEIN DIGESTIBILTY ON
The pet food label provides an estimate of a food’s crude protein content on the 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. As a result, the percent of protein needed to meet dogs’ protein requirements would be higher in Dog Food B than in Dog Food A.
*Digestibility information can be obtained by contacting the pet food manufacturer
• Energy (caloric) density: Provided that adequate energy-providing nutrients are included in the food (i.e. carbohydrate and fat), as a pet food’s energy density increases, a higher proportion of protein must be included in the food to meet requirements. Although this relationship may seem counter-intuitive, it due to the need to reduce the volume that is fed of higher energy-density foods. Dogs and cats typically eat (and are fed by their owners) to satisfy their energy needs. This is illustrated most directly by the observation that pet owners adjust the volume of food that they provide in response to the pet’s body weight and/or growth rate. Foods with higher energy density (i.e. more calories per cup) are fed in lower quantities. Because of this, energy dense pet foods must also be more nutrient dense, with protein contributing a higher proportion of the food to still meet the animal’s daily protein requirement (Box 3)
• Age, stage of life, and activity level: A dog or cat’s protein requirement is affected by age, physiological life stage and frequency and intensity of exercise. Protein requirements are highest during periods of rapid growth and development in puppies and kittens for the formation of new tissues. In reproducing females, protein needs are increased during pregnancy and lactation to provide for the growth of fetuses and milk production. Although most levels of exercise do not result in an increased protein need above that obtained through increased energy intake, endurance training in dogs and physiological stress in both dogs and cats can result in increased protein requirements. Finally, contrary to common belief, protein should not be reduced in the diets of older pets. Healthy seniors require an optimal level of high quality protein that is equal to or slightly higher than levels included in adult maintenance diets. This occurs because older animals become less efficient at utilizing protein and maintaining their protein reserves. Sufficient dietary protein is essential for older pets to support healthy body conditions, prevent loss of muscle mass, and to maintain healthy immune systems.
Protein Requirements of Dogs and Cats
Dogs: Dogs require an optimal level of dietary protein to support normal daily turnover, for the growth of new tissues, and to maintain a healthy immune system. Dietary protein should contribute a minimum of 20 to 25% of the calories in foods formulated for adult dogs and at least 25 % of the calories in foods formulated for growing puppies. Higher percentages are needed when foods containing lower quality proteins or foods that are very energy dense are fed. This occurs because the dog must consume higher amounts of lower quality proteins to obtain all of their needed amino acids. Conversely, when an energy-dense food is fed, the proportion of protein in the diet must be slightly higher to compensate for the lower volume of food that the dog eats to meet his or her energy needs.
Cats: One of the most distinctive differences between the dietary needs of dogs and cats is that the protein requirement of cats is substantially higher than that of dogs. Dietary protein should contribute a minimum of 34 % of the calories in a diet formulated for adult cats and at least 40 % of the calories in a diet formulated for growing kittens. The same rules for protein quality and energy density apply to cat foods. While it may be tempting for some owners to feed their dog’s food to their cat, many commercial dog foods do not contain adequate protein for cats. Commercial cat foods on the other hand are specifically formulated to meet the cat’s higher protein needs as well as the cat’s requirement for taurine.
Protein deficiency: Although absolute protein deficiency is uncommon in dogs and cats, it can occur if an inadequate diet is fed. Signs of protein deficiency include depressed appetite, impaired growth, weight loss and the development of a rough and dull hair coat. Because sufficient dietary protein is necessary for a healthy immune system, pets who are fed diets that are marginal or low in protein may have compromised immune system function, leading to enhanced susceptibility to infection and disease. This type of immune compromise can arise from very marginal protein deficiencies that may take weeks or months to manifest.
Protein excess: Many commercial pet foods contain slightly more protein than the minimum requirements of dogs and cats. This can be due to the quality of protein in the diet (higher absolute amounts of lower quality proteins are needed to meet requirements) or due to the energy density of the food (higher absolute amounts of protein are needed in more energy dense foods). Regardless, no protein source is perfect and so there will always be some excess of amino acids provided by the diet. Amino acids that are not used by the body for protein synthesis can be used as an energy source, providing the same amount of calories per gram as digestible carbohydrate. Alternatively, if too many calories are being consumed, the surplus amino acids are converted to fat for energy storage in the body. This means that protein is not “stored” in the body, as excess amino acids are converted to fat or used as an energy source. Although there is concern among some pet owners that excess protein in the diet can damage the kidneys, numerous studies have shown that protein intake does not contribute to the development of kidney dysfunction in healthy animals. In addition, because it is important for healthy older dogs and cats to receive optimal amounts of high quality protein to support immune function, lean body tissue, and normal daily protein turnover, the protein in the diets of geriatric dogs and cats should not be restricted simply because of advancing age.
Food Selection Tips
No one source of protein is perfect. Each is low or deficient in one or more amino acids, relative to the needs of the animal consuming it. Consequently, amino acid deficiencies and excesses can be balanced by feeding a combination of protein sources. For example, soybean meal and corn gluten meal compliment each other perfectly. The amino acids which are deficient in one are present in the other. Here are some additional tips for selecting an optimal protein source for your dog or cat:
• Select foods according to life-stage nutrition: Growing puppies and kittens have slightly higher protein requirements than mature adult pets. Similarly, a hard working dog who is competing in Agility trials every weekend or a dog who herds sheep every day may need a more energy dense food, which will contain a higher total protein content. Finally, single protein source foods can be select for pets who have intolerances or allergies to one or more specific proteins. Consider your pet’s age, activity level and health needs when evaluating foods.
• Consider the processing that is used: 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 pet food generally increases the food’s protein quality when compared with foods that contain only meals or by-product meals.
• Examine the 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 to feed your dog or cat.
• Bottom line: Quality costs more: Just as in human foods, pet foods that are of higher quality are of greater nutritional value and tend to be more costly. In dog and cat 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.