Cats and dogs are both members of the order Carnivora. While the name implies that both are specialised meat-eaters, each species originated from different branches, with the domestic cat (Felis catus) being part of the Felidae family and the domestic dog (Canis familiaris) being part of the Canidae family. However, the nutritional requirements, feeding behaviour and food selection choices vary considerably between the two species.

Dogs were likely the first animals to be domesticated [11]. They share a long history of co-existence with humans, with the dog’s direct wolf ancestor (Canis lupus), thought to have been utilised as guards and hunters alongside human hunter-gathers [12]. It is believed that divergence from their carnivorous wolf ancestors took place between 13,000 and 17,000 years ago, when the increased availability of human food waste associated with the move to an agricultural existence created a new ecological niche [13]. Wolves took advantage of this new niche and became more accustomed to human contact. Over time, humans became experienced at selecting for specific tameness traits in dogs and established control over proto-dog mating, ultimately resulting in the evolution of the domestic dog [12].

Compared to their carnivorous wolf ancestors, domestic dogs can consume foods of both animal and non-animal origin and are therefore classified as facultative carnivores [13,14]. They are often described as opportunistic eaters that spend a short period of time consuming large amounts of food [6,15]. As a result, food is normally eaten in a gluttonous manner with minimal chewing taking place, with food being regurgitated later when they have removed themselves from the presence of other pack members [6]. In the wild, food sources for dogs vary greatly, ranging from insects, berries, and grass through to animal faeces and carrion [6]. In addition to consuming these foods, gnawing on bones, and other animal parts is also seen in the wild dog [6]. The ability to adapt to a vast selection of foods likely resulted from needing to survive through sustained periods of feast and famine and to cope with variable nutrient availability, which in turn allowed the change from a predominantly carnivorous to a more omnivorous diet [13].

In contrast to dogs, cats were domesticated approximately 9000–10,000 years ago from the African wildcat (Felis silvestris), making them one of the most recently domesticated mammal species [12,16]. Rather than being actively sought as household pets by humans, cats likely became associated with people to take advantage of food scraps and vermin found in their settlements and are believed to have naturally diverged from their wildcat ancestors [12,17].

Cats are solitary hunters that will often kill much smaller prey than their body mass, which results in them needing to have multiple kills a day to meet their energy requirements [6]. This prey is normally eaten immediately, and cats show a preference for food at body temperature and will often consume carrion that has cooled to ambient temperatures [18,19]. Smaller prey are often consumed in one portion, but with larger prey, the flesh will be ripped off and the organs will be consumed [6]. For example, when Eurasian lynx were followed over a three-month period in spring, they preferentially ate muscle tissue, body fat, and internal organs, except the digestive tract, of 359 prey species in the wild (made up of predominately roe deer at 69%) [20]. Of these kills, the meat (muscle) and organ meats (lung, heart, kidney, liver, and spleen) were completely consumed in 90% of analysed cases [20].

Cats are also classified as intermittent feeders and consume multiple, small meals throughout a 24-h period [21,22,23]. Cats are also much more selective eaters compared to dogs and can detect small differences in the composition of the food they are offered [6,21]. Cats are defined as obligate carnivores in their methods of ingesting, digesting, and metabolising meat-based diets [21]. Without animal-derived protein, severe nutritional deficiencies can occur in cats.

Both cats and dogs tend to display neophilic behaviour, defined as liking towards a foods that is new, as opposed to neophobia, which is described as the avoidance of new food [24,25]. In extreme cases, some cats may also exhibit metaphilia, which is defined as a clear preference for change or variation from a familiar food [25]. Age also has an influence on whether an animal displays neophilia or neophobia. For example, puppies show a higher level of neophilia than adult dogs because they are constantly exploring their environment and learning (by trial and error or social learning) to eat new foods. In contrast, neophobia is an adaptive behaviour to prevent possible intoxications and displayed more in adults [24]. Overtime, the feeding experiences become less variable for cats as a dynamic equilibrium between the purchasing habits of owners and the food preferences of the cat is reached [15].

Overall, cats and dogs continue to be the most used animal models in assessing pet food palatability. While dogs show greater acceptance of a wide variety of foods, their opportunistic feeding behaviour and tendency to consume the first food chosen may prove challenging when looking to identify the fundamental components that drive food intake. In comparison, cats show greater selectivity and can detect small changes in food composition.

Despite being classified as carnivores, cats and dogs have specific dietary nutrient requirements, with cats notably having more specialised nutrient needs than dogs. For example, cats have a higher minimum requirement for dietary protein than dogs, at 26% versus 18% on a dry matter (DM) basis, with protein requirements increasing to 30% for growing stages and 22.5% for lactation [26]. Within these protein requirements, essential amino acids must also be present at specified levels to deliver a complete and balanced diet [26,27]. This is likely due to their ability to regulate the enzymes that catalyse amino acid metabolism being impaired [28].

Taurine is the only amino acid able to conjugate bile acids in cats. Unlike other mammals that can use glycine as an alternative to taurine, cats do not possess this ability [7]. Additionally, renal function and structure are maintained by cats with the help of taurine, and it also has roles in cardiac function, sight, and reproduction. However, cats and kittens are unable to synthesise or recycle enough taurine to meet their needs, and so it must be provided in their diet.

Arginine is another essential amino acid required for growth and in the detoxification and excretion of ammonia as urea [30]. It is of great importance to cats, as ammonia toxicity can result if they are fed diets lacking in arginine [31]. Unlike other mammals that can synthesise arginine from ornithine and citrulline in the intestine, cats have a lack of the enzymes pyrroline-5-carboxylate synthase and ornithine aminotransferase, which are required to produce arginine [32].

The macro-minerals that are essential for cats are calcium, phosphorus, magnesium, sodium, potassium, and chloride, and the microminerals that are essential for cats are iron, copper, zinc, manganese, selenium, and iodine [26,27]. In this discussion, focus will be placed on vitamin A and niacin, as these two follow unique synthesis pathways in cats. Calcium and phosphorus will also be examined as the key microminerals required in the highest amounts for the adult cat.

While the main source of dietary vitamin A for most species is in the non-toxic plant pigment form of β-carotene, cats lack the dioxygenase enzyme required to start the conversion of carotenoids to retinal, and therefore a dietary source of preformed vitamin A is required [3,7,33,34]. In contrast to β-carotene, preformed vitamin A can be toxic if consumed in large amounts [33]. As a fat-soluble vitamin, excess vitamin A is not excreted through urine when consumed in excess. Instead, appreciable amounts are stored in the liver as well as fatty tissues throughout the body [33,35]. Whilst important for vision, bone and tooth growth, as well as reproduction and maintenance of skin and mucous membranes, vitamin A toxicity in cats can result in muscle soreness, tenderness of joints, and hyperesthesia, particularly along the neck and forelimbs of cats due to the development of bony exostoses [33,35,36]. As carnivores, most of the vitamin A is consumed as preformed retinyl palmitate stored in tissue and is particularly abundant in the liver of their prey [33]. In commercial diets, a safe upper limit for vitamin A levels in cats has been set at 333,300 IU/kg, which is equivalent to 99.99 µg/g of retinol [27]. Since beef liver has been shown to have vitamin A levels of 283.19 µg/g [37], a cat would only be able to consume 35% of their total daily intake as beef liver, assuming no other ingredients supply any vitamin A.

In contrast to the synthesis pathways of taurine and arginine, which are characterised by low enzymatic activity at different points, cats possess all the enzymes and pathways required for niacin synthesis [38]. Niacin is a water-soluble vitamin essential for energy metabolism and can be metabolised in one of two ways using tryptophan; one results in the production of acetyl CoA and CO₂, and the other to nicotinamide adenine dinucleotide (NAD) [38]. In cats, the activity of picolinic carboxylase, the enzyme catalyzing the first step of the degradative pathway to acetyl CoA and CO₂, is upregulated, resulting in niacin being broken down faster than it is produced [39]. This upregulated catabolic pathway allows the carnivorous cat to consume a high protein tryptophan-rich diet while also preventing free tryptophan and its intermediates from accumulating to toxic levels, which can have undesirable metabolic side effects in the cat [40]. Additionally, cats are supplied with enough NAD and nicotinamide adenine dinucleotide phosphate (NADP) coenzymes through the dietary consumption of meat, meaning they have no need to produce niacin from tryptophan [38].

Calcium and phosphorus are the two most abundant minerals in the body. They are necessary for the growth and maintenance of bones and teeth, with calcium also being involved in blood clotting and nerve impulse transmission, and phosphorus playing an important role in energy metabolism as a component of adenosine triphosphate [41]. Bone material and fish provide a good amount of calcium for pet foods. Phosphorus is also provided by meat and vegetables, particularly cereal; however, in grains, phosphorus is presented in a less bioavailable form known as phytate [41,42]. In addition to these organic forms of calcium and phosphorus, inorganic sources (additives) are also used in the industry but have a higher bioavailability [43]. For example, phosphate salts are highly soluble compared to bone ingredients, resulting in increased absorption and postprandial serum levels. This can have a negative impact on phosphorus homeostasis and contribute to renal damage [41].

In addition to bioavailability, dietary levels of phosphorus and calcium can also have varying adverse effects on feline health. Low dietary levels of phosphorus are associated with an increased risk of hypercalcemia [41]. High dietary phosphorus levels greater than 3.0 or 3.6 g/1000 kcal may lead to kidney damage or dysfunction and chronic kidney disease (CKD) in healthy cats, particularly when the highly available soluble inorganic salts are provided [41,43]. High levels of phosphorus also severely disrupt the hormonal regulation of phosphate, calcium, and vitamin D [43,44]. In terms of calcium, plasma calcium levels are generally well-regulated; however, low levels can have immediate detrimental effects, including cardiac arrhythmias [41]. A sudden increase in dietary calcium may increase the risk of calcium oxalate stone formation as well as lead to calcification and possibly kidney injury and impaired function [41].

The availability of calcium and phosphorus is also impacted by their relative proportion to each other [41]. Many cases of hyperparathyroidism have resulted from a Ca:P imbalance, which can result from feeding high-protein meat products that are sufficient in phosphorus but low in calcium [35,41,42]. Conversely, increased calcium absorption may take place when presented with a high ratio of calcium to phosphorus [41]. A Ca:P ratio of 1:1 to 2:1 is considered to reduce the likelihood of calcium- or phosphorous-related issues [35].

Cat foods must also contain a minimum of 9% crude fat on a dry matter basis for maintenance, compared to 5.5% in dog foods [26]. Within the fat requirements, arachidonic acid is essential for cats and must be present at 0.2% on a dry matter basis in food [27]. While there are no additional fatty acid requirements for adult cats and dogs, kittens and puppies have the additional requirements for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). EPA is important for supporting the body’s natural anti-inflammatory response, and DHA plays a vital role in neurological and retinal development [45,46,47].

While the specific minimum nutrient requirements of cats and dogs have been established, research has shown that cats and dogs are able to select a ‘target intake’ of protein, fat, and carbohydrates to achieve nutritional adequacy when given the choice between diets differing in macronutrient composition [48,49].

An extensive study by Hewson-Hughes et al. [49] used geometric analysis to assess macronutrient selection in dogs when presented with six dry-format (extruded) diets and six wet-format (retorted) diets for five different dog breeds. It was found that after initially selecting a diet significantly lower in fat, dogs were able to regulate their dietary macronutrient level based on the metabolisable energy compositions of 30% protein, 63% fat, and 7% carbohydrate, with values showing similarities across the different breeds.

Prior to the work in dogs, Hewson-Hughes et al. [48] conducted the same study with cats and found that they selected dietary macronutrients based on the metabolisable energy compositions of 52% protein, 36% fat, and 12% carbohydrate. As well as the optimal levels, the study revealed that a carbohydrate ceiling effect of approximately 300 kJ/day (72 kcal/day) is displayed by cats. This low intake of carbohydrates is likely associated with many sensory and metabolic adaptations, such as their inability to detect sweetness due to their lack of sweet taste receptors [48].

When wet diets of varying protein and carbohydrate contents were fed to cats, the cats were able to regulate their macronutrient intake to obtain 53% of metabolisable energy from protein and 11% from carbohydrate [50]. The results obtained from a geometric assessment of cats’ intake of macronutrients reflect similar levels of protein, fat, and carbohydrate in prey consumed by free-roaming cats at 52:46:2% [16]. This illustrates that, in terms of macronutrient selection, cats are driven to foods with a high protein and fat content and avoid carbohydrate-rich foods.

The study by Salaun et al. [50] also assessed the effect of adding a palatability enhancer to diets and found that cats consumed more food, but no difference was observed in their protein or carbohydrate intake patterns. A follow-up study by Hewson-Hughes et al. [51] also examined the effect of adding positive (fish), neutral (rabbit), and negative (orange) flavors to diets varying in protein:fat energy ratios of 10:90, 40:60, and 70:30. Cats were able to distinguish between flavors added to the foods, with fish preferred over rabbit and no addition or flavor, and orange flavor being the least preferred in the short term. However, in the long term, cats selected similar protein and fat intake regardless of flavor combination, suggesting that macronutrient balancing is a key driver for longer-term food selection and intake in the domestic cat [51].

Today, domestic cats and dogs receive most, if not all, of their nutritional requirements from commercially prepared pet foods. Although there are a variety of foods available, pet foods typically fall under one of three broad categories: dry, wet, or semi-moist foods, depending on their processing method, methods of preservation, and moisture content [52]. Along with these three main diet types, foods can also be formulated to be complete, balanced, or complementary. Complete and balanced foods deliver all nutrients at the correct levels to pets when fed as a single food source. In contrast, complementary foods, such as pet treats and mixers, generally lack some essential nutrients, so they can only form 10% of the daily intake and must be fed alongside another type of food to ensure the animals nutrient requirements are met [53].

Dry pet foods have a typical moisture content between 10 and 12% and rely on this low moisture content for preservation. Dry pet foods often include cereal grains and by-products, soybean products, animal by-products, fats, and oils, as well as the inclusion of vitamins and minerals, which are generally mixed to form a dough, which then undergoes further processing to extrude and dry [26,54]. There are many forms of dry pet food, including baked, air- or freeze-dried, and extruded products, with the latter accounting for most dry pet foods available on the market [26,52].

Extrusion can produce a range of products with different shapes, ss, and colours. This often has little to do with nutritional adequacy for pets but provides visual variety to pet owners [26,54].

Baked kibble and biscuits are the least common types of dry pet food. To form a dough suitable for biscuits, a formulation with a high proportion of wheat is traditionally used. For biscuits, the dough is cut into shape before baking in an oven, whereas for kibble, a large sheet is baked and then broken up to form a kibble [52,54].

Air-dried and freeze-dried pet foods are also becoming increasingly popular types of dry food on the market. Compared to traditional dry food cooking methods, air-dried pet food typically uses low drying temperatures (usually below 100 °C) with gentle airflow for a long drying time [55,56,57]. Freeze-drying is beneficial in retaining the properties of the raw material better than the air-dried product [58]. However, both options provide end products that are minimally processed to help maintain the nutritional value of the raw material, which can be lost in traditional manufacturing processes.

The ingredients included in dry pet food are much the same for cats and dogs, although more emphasis is placed on the inclusion of proteins and fats of animal origin in dry cat foods [59]. Dry pet foods have the benefit of being a relatively cheap and useful source of energy compared to wet and semi-moist pet foods. Dry foods are also very easy to store and dispense; however, they are often less palatable than the other food formats, particularly to cats [59].

Wet foods typically have a moisture content of 74–78% and exist in a variety of forms, with canned and pouch products being the most common [26,52,54]. Many of the same ingredients used in dry pet foods are also included in canned food at differing levels [26]. In canned foods, there is a much higher inclusion of fresh or frozen meat, poultry, or fish products, and animal by-products, usually at levels between 25–75%, and cereal flour is used as gelling agents [26].

There are three general types of wet food: loaf, chunks or chunks in gravy, and a chunk in loaf combination. All three are preserved via heat treatment, where cans are filled with the wet slurry of ingredients, sealed with a double seam lid, and retorted at a defined temperature and time profile that kills food-borne pathogens [60,61,62,63]. This produces food-safe products that have a long shelf life and no special storage considerations [26,54,59].

There can be considerable damage or loss of nutrients during heat processing and storage. During canning, ascorbic acid is unstable in the high-moisture environment of wet pet food. Heat- and moisture-liable vitamins such as thiamine, folic acid, and β-carotene also show losses. Vitamins that are usually stable, such as riboflavin, niacin, pantothenic acid, choline, vitamin B12, and biotin, have good processing resistance, except for biotin in wet dog food. Vitamin losses during storage were minimal compared to the losses during processing, due to the protective environment of the can. However, thiamine and vitamin B12 were the two main vitamins lost during storage. To combat these losses, manufacturers add compensatory amounts to formulations to ensure adequate levels are retained following heat treatment [59].

In addition to the loss of nutrients, Maillard products, formed via a chemical reaction between amino acids and reducing sugars in wet food during heating, result in the production of different flavours and a brown colour, which are associated with decreased protein digestibility but increased palatability [7,52,54,59,64]. Compared to dry foods, canned foods are generally more desired by cats as they reflect similar properties to meat and also contain little or no cereal or carbohydrate.

Semi-moist products are relatively uncommon and exhibit a moisture content which can range from 25 to 35% and are stable at room temperature [26,54]. To achieve its shelf-life stability, the water activity, defined as the water that is available for bacterial and fungal growth in or on the surface of food, needs to be controlled [59]. Manufacturers will include ingredients classified as humectants, such as salts, simple sugars, glycerol, and corn syrup, in formulations that control the water activity [26,59]. To prevent the growth of yeasts and moulds, preservatives such as potassium sorbate may also be added [52].

Semi-moist foods use similar ingredients to dry and wet foods. They are prepared in a similar manner to dry foods, with the addition of meat or meat by-products prior to extrusion. The ratio of dry to wet ingredients can range from 4:1 to 1:1 in this type of food. Semi-moist products often come out in patties or roll-like form for dogs, or in single-serve packages of small bite-sized pieces for both cats and dogs [26]. This type of food has a softer texture than dry food, which has a positive influence on food acceptance, the amount of food required to meet a pet’s caloric needs, and palatability, defined as their preference or choice for a particular food over another [52].

While the three main types of pet foods have different processing methods, preservation techniques, and moisture contents, products can be compared nutritionally on a dry matter basis.

Vegetarianism and veganism have become increasingly popular dietary choices among the global human population [65]. Vegetarians are defined as those who do not consume meat, poultry, or fish, with vegans being seen as a smaller group of vegetarians who do not consume any animal products whatsoever [66]. A study by Leahy and colleagues [67] estimated that there are one and a half billion vegetarians globally. Of these, 75 million are vegetarians by choice, with this figure predicted to rise with increasing affluence and education. The remaining are vegetarians by necessity, such as those in the developing world with a lack of choices in foods that they can consume. Adoption of a vegetarian lifestyle by individuals is largely due to ethical, ecological, religious, empathy for animals, and health reasons [8,65,68,69].

In terms of pet food, ethical concerns about commercial pet food appear to be the primary motive for owners feeding cats vegetarian diets [65]. However, there have been several reports of the nutritional inadequacy of vegetarian and vegan diets for dogs and, more commonly, cats due to their obligate carnivore status, which has implications not only for health but also the welfare of both species [8,14,65,68,69,70,71].

Although a wide variety of pet foods exist, most utilise significant quantities of animal by-products, and pet food production is tightly related to both livestock production and the human food system [72]. By making use of by-product streams, the pet food industry does not directly compete with the human food industry. Instead, it reduces the environmental load of the human food system by utilising inedible meat, poultry, and fish by-products and co-products that would otherwise go to waste [73]. As a result, the transformation of low-value animal by-products into value-added pet food has played a major role in the growth and expansion of the pet food industry [74], and they are a nutritious animal-sourced ingredient for cats to meet their obligate carnivore status [52]. However, little has been reported on the palatability of individual by-product ingredients of animal origin.

Meat is defined as the flesh derived from any species of slaughtered mammal and is made up of muscle tissue but may also include intramuscular fat, connective tissue of the muscle sheaths and tendons, as well as blood vessels [52,59]. Lean meats lacking fat tend to have similar proportions of water and protein (75% and 25%, respectively), whether from different parts of the same carcass or even from different animals such as cattle, lamb, pigs, or poultry ([59]).

Meats are a good source of amino acids, fat, iron, and some B vitamins such as niacin, thiamine, riboflavin, and vitamin B12 [59]. Compared to that for human consumption, meat for pet food is obtained by mechanically separating excess muscle meat from bones using a machine to deliver a final product that is finely ground and paste-like in texture [27].

By-products are classified as “a protein source consisting of organ meats, scrap meat, bone, blood, and fatty tissue from mammals, but do not include hair/hide, horns, hooves or teeth, or intestinal contents” [75]. Animal-sourced proteins are generally regarded as being of higher quality and superior in amino acid balance compared to other ingredients in pet food [52,76]. Additionally, the consumption of offal often reflects the feeding behaviour of the larger wild cats, which preferentially consume organ tissues [75,77]. As ingredients, offal meats are a rich source of trace elements, with levels being much higher than those in muscular tissue [78] and animal by-products, and are beneficial in providing essential nutrients.

Large differences in nutrient content are generally exhibited between different offal types, particularly in terms of the fat and vitamin contents [59,79,80].

However, both muscle and offal meats have very low calcium contents and have unfavorable calcium to phosphorus ratios that can range from 1:15 to 1:26. Most meat and offal are also deficient in vitamins A and D. Liver and kidney are the exceptions and provide a good source of these vitamins, although vitamin A toxicity can be a problem with the liver [59].

Although ingredients are primarily used in a blend to provide specific nutrients in diets, examining the compositional variation of individual meat by-products may help determine what drives food selection and preference on a fundamental level in companion animals.

Many canned and pouched pet foods contain considerable amounts of textured vegetable protein (TVP), an extruded soybean product typically made from defatted soy grits or flour used to form meat-like chunks [81,82]. While the aim of TVP is for it to look like meat, it usually has a similar nutrient profile as soy flours [83]. Plant-based proteins used in pet food manufacturing have less complete amino acid profiles than animal-based proteins [76]. Soy is the best of the plant-based sources of protein; however, in terms of amino acids, it is rich in lysine and limited in sulfur amino acids, namely methionine and cysteine [84,85].

Although carbohydrates are not considered essential for cats, as their natural diet contains little carbohydrate, commercial cat foods, particularly dry diets, can contain as much as 40% carbohydrates [77,86]. This is particularly evident in economy brands, in which the first ingredient is likely to be a named grain or cereal. Despite their obligate carnivore status, carbohydrates do provide a fiber source in the diet, which is important for gut health, but too much can lead to obesity [77]. Additionally, cats can utilise starch as a glucose source to provide cellular energy [77]. This does provide a cheaper source of energy for pet food manufacturers; however, proper processing or cooking is necessary to make starches digestible to cats and dogs [75]. Typical sources of carbohydrate in pet foods include various grains, such as brown rice, oats, sorghum, potatoes, and legumes [77,87,88].