What is Protein?

Proteins are complex molecules present in all living organisms and provide many essential functions in the body. These molecules are formed through the ‘building blocks’ of 20 amino acids, which can be deemed essential or non-essential – those deemed essential cannot be synthesised in the body and must be obtained through dietary intake, whereas non-essential can be synthesised by the body through the metabolism of other amino acids. The number of essential amino acids depends on the species in question, for example, humans require nine essential amino acids (EAAs), whereas dogs require ten, and cats require eleven (Table 1).

1. Table, 1: Essential Amino Acids for Humans, Dogs and Cats.

Amino acids 1-9 Essential for humans, 1-10 essential for dogs and 1-11 essential for cats.

N.B. Although taurine is a type of amino acids (B-amino-sulfuric acid) it is not incorporated into protein and not considered as one of the 20 protein building blocks.

The structure and function of a protein/peptide can vary greatly depending on the amino acid sequence (Fig 1) Although there is no definitive rule, shorter chains of amino acids (e.g. 2-50) may be described as peptides, whereas proteins are considered to comprise of 50 or more amino acids. A key role for many proteins is providing support and structure in the body, including formation of muscle tissue, bone, connective tissue, skin and hair (including aspects such as pigmentation). All enzymes within the body are proteins, carrying out various vital chemical reactions from breaking down starch in food (e.g. amylase) to repairing DNA (e.g. DNA ligase). Peptides and proteins are also responsible for the production of hormones, neurotransmitters and other chemical messengers – which are all responsible for the transfer of information around the body to ensure correct biological processes and functions are carried out.

Figure 1. Some of the roles of protein in the body.

Within the body, proteins are in a constant state of breakdown and resynthesis (known as protein turnover) (Fig, 2. Protein molecules are broken down to amino acids, which can then be re-used to build more protein. These endogenous amino acids are not however one hundred percent available for the synthesis of new protein as some may also be converted to other compounds such as glucose and dopamine etc. Therefore, an exogenous supply of amino acids -provided by dietary protein intake – is required to complement the endogenous amino acids, and meet the body’s demand to synthesise new protein.

On occasions where the dietary intake of protein is insufficient to meet these demands the process can become unbalanced resulting in excessive breakdown of protein relative to protein synthesis within the body, which in turn can be detrimental to health and performance. In contrast, ensuring an adequate supply of dietary protein can allow for optimal performance of the body – for example maintaining skin and coat integrity and supporting optimal immune functioning.

Figure 2. Overview of protein synthesis and degradation.

As protein is essential for cell and tissue growth, this is reflected in a higher protein requirement during periods of rapid growth or increased demand such as in young growing animals and during gestation/lactation. This higher requirement makes allowance for the additional demands of milk production, growth and development. FEDIAF guidelines provide separate guidance on the protein levels recommended for early and late growth phases of puppies, and for kittens. Early growth in puppies has a minimum requirement of 25g/100g dry matter (DM) in the first 14 weeks (when growth rate is rapid), dropping to 20g/100g DM in the late growth period1. Whereas kittens as obligate carnivores require a minimum protein inclusion of 28g/100g DM throughout the 9-12 months of growth¹.

A higher protein requirement for cats compared to dogs is reflected throughout the life stages in FEDIAF guidelines due to cats having a heavier reliance on protein as an energy source, in comparison to dogs who can utilise alternate resources such as carbohydrates². In cats a minimum protein requirement of 25g/100g DM is advised for active adult cats, with a higher recommended requirement provided for indoor/neutered individuals¹. FEDIAF guidelines for dogs state a minimum recommendation of 18g/100g DM for an active adult dog¹, or 21g/100g DM for less active dogs.

Despite a common misbelief amongst pet owner³, a reduction in protein intake for senior pets is not advised. As the aging process progresses, it becomes increasingly challenging for the body to maintain lean body mass (LBM), and subsequently a change in total bodyweight amongst aging pets is a commonly seen trend⁴. FEDIAF recommend that protein intake is to remain the same as adult recommendations⁵, however Laflamme et al (2005) recommend that in many cases an increase in dietary protein could be beneficial for aging pets, especially if the intake level of food has decreased⁶. As pets age factors such as underlying disease impact on the efficiency of protein metabolism, and if the dietary supply of protein is not sufficient muscle protein may be broken down and utilised. Over a prolonged period of time this can lead to sarcopenia in senior cats and dogs, and subsequently impact on morbidity and mortality.

As with all dietary components, ensuring the minimum requirements are met within the diet aids the health and wellbeing of pets. Historically there has been a belief amongst many pet owners that feeding high protein diets could in fact be detrimental to health – with an association to increasing the risk of kidney problems. For healthy individuals there is no evidence to support this. The exception to this would be dogs and cats with impaired renal function, and such individuals with pre-existing conditions may be advised to consume a tailored diet with a reduced level of protein to provide nutritional support for the management of a veterinary diagnosed condition⁷.

So, what are the advantages of feeding above these minimum requirements?

• Fulfilling the energy requirements of an individual through a high level of protein in the diet can limit the need for the inclusion of other components such as fat. Therefore, feeding of diets high in protein has been shown to be beneficial in facilitating weight loss in overweight dogs and cats.
• High protein diets can be of benefit to aging pets to provide an easily digestible source of energy and reduce the reliance on components such as fat in order to maintain LBM and aid longevity.
• Diets formulated with high levels of animal protein are likely to correlate to high levels of dietary collagen being present. Collagen is key to bone health and crucial to joint health, allowing for mobility and flexibility throughout the life stages. Collagen also plays a role in the maintenance of skin and coat health.
• A higher level of protein can aid muscle growth and repair which may be of particular interest to owners of working or sporting dogs who have a high physical demand on a daily basis.
• Providing the body with an abundance of protein, and therefore amino acids allows for the replacement and replenishment of cells across the body, thus aiding general health.

Although the total crude protein level of a pet food serves as an indicator for adequate AA levels, it does not necessarily guarantee all requirements will be met. As each AA plays a different role within the body (see Table, 1) FEDIAF nutritional guidelines provide detailed guidance on the AA breakdown required to reduce the risk of deficiency. Diet formulation must ensure a complete AA profile is provided to the pet, taking in to account species and life stage requirements, which may require the use of additional synthetic amino acids being added to a formulation.

All pet food manufacturers are legally required to declare the crude protein level under the analytical constituent section of product packaging. It is important when comparing products on the basis of protein level to take in to account not only the animal’s requirement, but also the moisture content of the food to ensure a direct comparison is (this is of particular importance when comparison between kibble and raw feed is being made).

The sources of protein used in pet food are becoming increasingly diverse, offering a wide variety of choice to the consumer. Traditional animal sources such as chicken, beef and salmon can be readily found on retail shelves, whilst more novel proteins such as buffalo, rabbit and kangaroo have all been included within modern pet food formulations. Often these proteins are selected for their unique nutritional properties and the subsequent marketing opportunities they provide.

Plant based proteins such as soya, pea protein, and corn gluten are increasing in prominence in modern formulations. Despite an initial misconception that plant-based proteins are a low-quality protein and provide an inadequate supply of EAAs, it has been shown digestibility in both cats and dogs is not negatively impacted on by the use of plant proteins⁸. If correctly formulated, plant proteins in conjunction with animal proteins, or as the sole protein source, can meet the protein requirements of both dogs and cats. Brown et al (2009)⁹ fed a meat free diet to working sled dogs with no negative impact on health or performance. The study demonstrates that providing the correct level of protein with a complete AA profile should remain a priority in diet formulation over the source of protein. However although acceptance of plant based proteins has increased amongst consumers, Dodd et al (2019)¹⁰ highlighted that owner diets are a major factor in the motivation to solely feed plant based diets to pets. Although many owners surveyed would consider feeding a plant-based diet if commercially available, of surveyed pet owners only 1.6% of dogs and 0.7% of cats were fed solely on plant-based diets.

Mirroring the trends of the human food chain, customer perception of protein quality is increasingly influenced by the sourcing and procurement of raw materials. For animal produce this has an increasing focus on welfare claims, provenance and certification processes (for example organic). This humanisation has resulted in a premium market surrounding the marketing claims associated to protein sources. Nutritionally in recent years the pet food market has seen an increased consumer interest in high protein products, with a further interest in fresh meat inclusion¹¹. Many of these high protein products are marketed with a link to the ancestral roots of our canine and feline companions – but is important to consider that the quality and bioavailability of the protein is just as important to its utilisation as the total level within a product.

Protein is a vital component of any diet, regardless of the species or life stage of the animal. With industry guidelines ensuring that minimum requirements are met within the products on offer to pet owners, pet food manufacturers look to exceed these requirements and aid bioavailability in pursuit of optimising the benefits provided to the animal. With a broad range of products on offer to owners, the crossover of trends from the human market provides unique selling points and novel attributes to aid product differentiation. Further to this, research findings from human nutrition often have transferable links to dogs and cats, and this cross-species knowledge allows for the implementation of technological and processing advances to be implemented by the pet food industry – and ultimately provide high quality products to pet owners, aiding the health and wellbeing of pets.

1. FEDIAF. Nutritional Guidelines for Complete and Complementary Guidelines. 1–98 (2021).
2. Buff, P. R., Carter, R. A., Kersey, J. H. & Bauer, J. E. Natural Pet Food: A Review of Natural Diets and Their Impact on Canine and Feline Physiology. J. Anim. Sci. 92, 3781–3791 (2014).
3. Hutchinson, D., Freeman, L. M., Schreiner, K. E. & Terkla, D. G. Survey of Opinions About Nutritional Requirements of Senior Dogs and Analysis of Nutrient Profiles of Commercially Available Diets for Senior Dogs. Int. J. Appl. Res. Vet. Med. 9, 68–79 (2009).
4. Pérez-Camargo, G. Cat Nutrition: What Is New in the Old? Compend. Contin. Educ. Pract. Vet. 26, 5–10 (2004).
5. FEDIAF. FEDIAF Scientific Advisory Board Statement Nutrition of Senior Dogs. https://fediaf.org/press-releases/2151-how-to-feed-a-senior-dog.html (2017).
6. Laflamme, D. P. Nutrition for Aging Cats and Dogs and the Importance of Body Condition. Vet. Clin. North Am. – Small Anim. Pract. 35, 713–742 (2005).
7. Elliott, D. A. Nutritional Management of Chronic Renal Disease in Dogs and Cats. Vet. Clin. North Am. – Small Anim. Pract. 36, 1377–1384 (2006).
8. Golder, C., Weemhoff, J. L. & Jewell, D. E. Cats Have Increased Protein Digestibility as Compared to Dogs and Improve Their Ability to Absorb Protein as Dietary Protein Intake Shifts From Animal to Plant Sources. Animals 10, 1–11 (2020).
9. Brown, W. Y., Vanselow, B. A., Redman, A. J. & Pluske, J. R. An Experimental Meat-Free Diet Maintained Haematological Characteristics in Sprint-Racing Sled Dogs. Br. J. Nutr. 102, 1318–1323 (2009).
10. Dodd, S. A. S., Cave, N. J., Adolphe, J. L., Shoveller, A. K. & Verbrugghe, A. Plant-Based (Vegan) Diets for Pets: A Survey of Pet Owner Attitudes and Feeding Practices. PLoS One 14, 1–19 (2019).
11. Vinassa, M. et al. Profiling Italian Cat and Dog Owners’ Perceptions of Pet Food Quality Traits. BMC Vet. Res. 16, 1–10 (2020).