Protein Overview - Nutrition

1. What is Protein?

Protein is a macronutrient comprised of amino acids chains folded into primary, secondary, tertiary, and quaternary structures to form the physical components of plants and animals.

2. Why is it Important?

Protein is required to provide both nitrogen and essential amino acids. There are no protein stores in the body and dietary protein intake replenishes nitrogen and amino acids that are lost during normal cellular turnover in the body. Dietary nitrogen is used to synthesise non-essential amino acids and other nitrogen containing compounds. Essential (indispensable) amino acids are those that cannot be synthesized by the animal and must be consumed each day; non-essential (dispensable) amino acids can be made in the liver if adequate levels of nitrogen and carbon are available. Amino acids are the building blocks of structural proteins, and other proteins that can have enzymatic, transport, regulatory, contractile or defensive functions. Protein requirements increase during growth and reproduction to meet the additional demands for tissue deposition and milk production. Cats and dogs have a relatively high protein requirement which is thought to be due to diet induced evolutionary adaptions;1 both species have evolved on high protein meat based diets and this may explain why they have a relatively high metabolic set point for protein catabolism and higher urinary nitrogen loss compared with herbivores, or other omnivores.2 Cats have a particularly high protein requirement, because unlike the dog and most other animals, they are unable to down regulate urea cycle enzymes in response to low protein diets.3

3. Roles in the Body

Both essential and non-essential amino acids are required to form the structural components of the body, such as muscle, connective tissue, and blood cells. Adequate amounts of protein are necessary for proper growth in puppies and kittens.4 Protein is also required for maintenance of lean body mass in adult dogs and cats.4 The fate of metabolised protein is to serve as a source of nitrogen for incorporation into other compounds in the body, such as heme, catecholamines, and neurotransmitters while the carbon is used in gluconeogenesis and energy production.

4. Consequences of Protein Deficiency

1. Growth: Inadequate protein intake as well as deficiencies in any single essential amino acid will result in poor growth in puppies and kittens or failure to gain weight.4
2. Maintenance: Inadequate protein intake will result in weight loss in adults due to an inability to maintain a positive nitrogen balance and replace amino acids lost during whole body protein turnover.5,6 Dogs are able to down-regulate hepatic enzymes used in amino acids metabolism, gluconeogenesis, and urea production and can adapt to low protein intakes. Cats are unable to adapt to low protein or protein-free diets and can develop hepatic lipidosis during periods of starvation.5

5. Toxicity

There are no reports of toxicity associated with high intake of protein in healthy dogs and cats. Animals with compromised hepatic function (i.e., liver failure) may have impaired metabolism of proteins; compromised renal function may result in decreased tubular clearance of urea.

6. Dietary Sources

Protein is found in animal and plant protein sources, such as muscle and organ meats, eggs, dairy protein (e.g., casein), cereal grains, and pulses (i.e.,legumes). Animal sourced proteins are generally more highly digestible than plant sourced proteins for both dogs and cats.7,8 Processing of proteins can affect their digestibility; heat processing can increase digestibility for some proteins, while rendering and drying can decrease digestibility.9-11 Presence of soluble fibres and carbohydrates within a diet can also decrease protein digestibility.12-14 In dogs, digestibility varies with size, breed, and age, with older dogs experiencing a decrease in digestive capacity.6,15 Cats have a shorter small intestinal length to body size ratio which may limit their ability to digest protein, and may explain why protein digestibility is lower in cats compared to dogs.4 Crude protein as listed on a pet food label is a measure of dietary nitrogen and does not reflect the source or quality of protein in a commercial diet.

7. Diagnosing Protein Deficiency

No changes may be seen with short-term (<2 weeks) protein deficiency in dogs. Cats fed protein deficient but adequate arginine containing diets may show no clinical changes over the short-term; longer-term total protein deficiency (5-6 weeks) may result in hyperbilirubimemia and clinical signs of hepatic lipidosis. In both dogs and cats low serum albumin and loss of lean body mass are seen with longer-term protein deficiency.