FAQs about cat nutrition

Introduction

Cats were first domesticated by humans some 10,000 years ago, and since then they have become one of our most popular companion animals. This has, not unnaturally, meant that we want to offer them the best possible care, and to this end the last few decades have seen much research effort focused on the feline species, and especially towards their nutritional requirements. It has gradually become apparent that the cat is an animal full of peculiarities, not least from the dietary point of view, and this article will explore the practical implications of some of these.

Q: Can a cat be fed a diet free from animal products?

A: Cats are often referred to as being strict or obligatory carnivores; in the wild they consume food that is almost entirely from animal sources, they are anatomically designed to hunt and eat prey, and their metabolism is adapted for their natural diet 1 (Figure 1). Cats have various nutritional requirements, of which some must be supplied via their diet as they cannot be produced in sufficient quantities by their intrinsic metabolic pathways. Research shows that the cat has at least 45 nutrients which are essential for health 2, and because of their metabolic peculiarities they have higher requirements for protein, arginine, methionine, cysteine, taurine, arachidonic acid, vitamin A, vitamin D, niacin and pyridoxine when compared to omnivores 1,2.

Some essential dietary nutrients are primarily sourced from animal-derived ingredients; these include several amino acids (lysine, methionine, cysteine and taurine), some vitamins (A, D and B₁₂), and some fatty acids (arachidonic, eicosapentaenoic and docosahexaenoic acids). It can be challenging to obtain these from sources other than animals, so to formulate a feline diet that is free from animal products these nutrients must be provided by alternative means, either from chemically synthesized products or from specific non-animal sources. However, such options are not necessarily straightforward, and their use can raise other challenges. Firstly, it is not enough to simply know a given product contains the desired nutrient; it is essential to know the actual level of the required nutrient it contains. This is because the product may have a lower concentration of the substance than an equivalent animal-based product, or have a lower activity (e.g., it may contain vitamin D₂ instead of D₃), so the bioavailability (i.e., the amount of an ingested nutrient that is available for metabolism or storage within the body) needs to be quantified.

Secondly, some ingredients that are vegetable-based or derived from non-animal sources can also negatively impact a diet; they may alter the overall digestibility and bioavailability of its nutrients – for example, by adversely affecting the content and structure of the carbohydrate – and they can reduce the overall palatability.

And thirdly, the possible risks posed by such diets in terms of their potential impact on common diseases must also be evaluated – such as the effects on urinary pH, with the possibility that this can predispose to the formation of bladder calculi. In short, there are considerable challenges in producing an animal-free diet for cats that is not only complete and balanced when subject to chemical analysis, but proven to be adequate as a long-term maintenance food. As far as can be ascertained, these data are lacking for any of the commercially available “vegetarian” or “vegan” foods presently marketed for cats, and research to date shows that they often do not even meet the recommended essential nutrient levels 3,4,5. One study has suggested that in some cases the health of cats fed an animal-free diet was unaffected 3, but it was unclear if these cats had outdoor access where they could have hunted – and in addition the evaluation period may not have been long enough for individual animals to display any signs of deficiency.

The author is unaware of any major multinational pet food company that currently produces a feline diet that is free from animal ingredients. This in itself is significant; since these companies have the in-depth knowledge and the financial resources required to research and produce this type of diet, and given that this product would find a ready market with some pet owners, it can be argued that such foods pose for now at least, an unnecessarily high risk for cats. Decades of research has revealed a great deal of unexpected information on feline nutrition – and we are still learning – so on this basis any formulation for cat food should be supported by good scientific evidence in order to be as safe as possible. The bottom line must be that, for now at least, cats should be fed a meat-based diet, as the alternatives raise the real possibility for sub-optimal long-term nutrition. 

Q: Is feline heart disease linked to diet?

A: The short answer here is – in some situations, possibly. In the late 1980’s taurine was identified as the key nutrient which could reduce the prevalence of dilated cardiomyopathy in cats 6. The mechanism by which low levels of taurine within cardiac muscle result in dilated cardiomyopathy and heart failure is not yet fully understood, although it is postulated to be due to disturbances in the calcium and energy metabolism of the myocardial muscle 7,8. Taurine is a non-protein β-amino sulfonic acid which is abundant in the cat’s natural prey. In contrast to most mammals, cats have low concentrations of cysteine dioxygenase and cysteine sulfinic acid decarboxylase, key enzymes in the metabolic pathway of taurine synthesis 9. This idiosyncrasy means that cats are unable to efficiently synthesize taurine using methionine and cysteine, and therefore are dependent on their diet to provide the taurine they need as a metabolic substrate.

Once taurine was recognized as an essential nutrient, manufactures started to incorporate it in commercial petfood diets, and what was once a frequent cause of cardiomyopathy in cats became a rare occurrence. Most cases are nowadays identified in cats fed homemade diets, but it should be stressed that it is not just this species that may be susceptible. A recent Food and Drug Administration review in the USA has highlighted a possible link between certain diets and the development of dilated cardiomyopathy, with most of the recently reported cases being in dogs, although a small number of cats were also affected 10. Various factors are still unclear, but the investigation is centered around the diets that were fed to affected animals and, more specifically, a suspicion that the diets had low bioavailability levels of taurine. It should be stressed that there are diverse factors involved in the etiology of dilated cardiomyopathy, and that more robust data collection will be needed before any conclusions can be drawn from this review.

If dietary taurine deficiency is identified as the likely cause in a case of feline dilated cardiomyopathy, with tests demonstrating low levels of the amino acid in plasma and whole blood (Figure 2), supplementing the diet with taurine usually results in an immediate improvement of heart function and – assuming that the cat survives the immediate critical period – the condition should resolve within 6 months. So whilst it is true that certain diets can cause heart disease in cats, a well-formulated commercial diet is unlikely to predispose to cardiomyopathy.

Q: Should a cat be given milk to drink?

A: Most cats enjoy drinking milk, but this does not necessarily mean it is healthy for them (Figure 3). Perhaps a better question is “how much milk is appropriate to give a cat – and what problems can be linked to milk consumption?” Certainly cats, like all other mammals, naturally survive exclusively on milk in their first weeks of life, and even after they start to ingest solid food they will partially rely on their mother’s milk until they are fully weaned (Figure 4). The lactase activity in a cat’s intestine declines with age, and with it the capacity to digest lactose, the main sugar present in milk. If the amount of lactose ingested by a cat surpasses its capacity to digest it, the remaining lactose will ferment in the gastrointestinal tract, leading to clinical signs such as vomiting and diarrhea. An adult cat’s capacity to digest lactose is therefore less than that of a kitten, but research has confirmed that an adult cat can cope with (at least) 1.3 g lactose per kg bodyweight on a daily basis 11. It is notable that the lactose concentration in milk is remarkably constant, both between different species (e.g., cow, sheep or goat’s milk) and across different types of milk (i.e., skimmed, low-fat or whole milk), with an average maximum of 5% 12. This means that a cat should be able to cope with ingesting up to 25 mL per kg bodyweight per day; so if a 4kg cat was to drink less than 100 mL of milk each day, it is unlikely to develop any clinical signs of lactose intolerance. This does not exclude the fact that a few individuals can have a particularly low digestive capacity for lactose, so in some cases clinical signs can develop even if a cat consumes a small quantity of milk.

However, quite apart from the lactose intolerance aspect, if a cat is given milk regularly it is important that the overall nutritional profile of the diet and the amount of energy ingested is also taken into account. A queen’s milk is a very complete food for her kittens, containing all the essential nutrients, but it does not constitute a balanced diet for a cat after weaning. Milk is also energy dense, so if offered to a cat on a regular basis it should either be regarded as an integral component of a complete and balanced diet, or considered to be an extra treat; with this scenario the amount offered should not exceed 10% of the energy content of the basic diet. Given that whole bovine milk has an energy density of 69 kcal/100 mL (or around half of this for skimmed milk) 12 and that a castrated 4kg cat has an average daily energy requirement of 130-190 kcal/day (52-75 kcal/kg^0.67), the daily amount of milk to be offered as a treat would be 20-30 mL, or around double this amount if skimmed milk is used. At this level, the lactose intake is well below the maximum.

Finally, another aspect to consider is the possibility that some cats can be allergic to casein. Although the literature does not report this as being prevalent in cats 13, obviously such individuals should not be fed milk at all.

In summary, it can be said that milk in moderation is acceptable for most cats, but excessive amounts can be detrimental in the long term.

Q: Can a cat digest and metabolize carbohydrates?

A: The metabolism and physiology of the feline species has evolved on a diet based on small prey (such as mice and birds) which provide very limited amounts of carbohydrate 14, and when given the choice domestic cats will still select diets that have a low-carbohydrate content 15. These facts lead to the assumption that a cat’s digestive system and metabolism cannot cope with carbohydrates. In fact, glucose (one of the simplest carbohydrates) is just as essential for the feline carnivore as for omnivores or herbivores. Glucose is the main or only source of energy for brain and red blood cells, leukocytes and some specific cells within the renal medulla, testes and eye 16. Glucose is also required for the synthesis of some non-essential amino acids, vitamin C and nucleic acids, and (in lactating queens) to manufacture lactose 16. Nevertheless, carbohydrates are not mandatory in the cat’s diet, as they can synthesize the glucose that they require from other sources.

Blood glucose concentrations in the cat will rapidly return to normal baseline levels after intravenous glucose administration, and the fasting glucose blood levels in this species are very similar to that of other mammals who have different dietary needs 16, so it is clear that cats can indeed metabolize carbohydrates. Although they have evolved to have a more limited digestive capacity for complex carbohydrates than other domesticated species (e.g., dogs or pigs 17), cats can digest and absorb carbohydrates efficiently as long as they are properly processed (i.e., ground and/or cooked) and are not ingested in excessive amounts 18. So the question is not so much “should cats be fed carbohydrates?” but rather “what needs to be considered when including carbohydrates in a cat’s diet?” – which is why cats should not be fed carbohydrates that are raw or in amounts that exceed their digestive capacity.

Q: Can a cat become diabetic if fed a high-carbohydrate diet?

A: Diabetes mellitus is a relatively common disease in cats, with some studies indicating a prevalence as high as 1.25% 16, and owners of affected cats may ask if the carbohydrate component in their pet’s diet has contributed to development of the disease (Figure 5). Firstly, it is appropriate to consider why, since a cat’s natural diet typically has very low levels of carbohydrates (at around 2% on a metabolizable energy (ME) basis 14), most commercial cat foods contain relatively high levels of carbohydrates. This is not simply because this class of nutrient is cheaper or more sustainable than protein or fat, or that it is a technological requirement for making kibble. It is also because carbohydrates have various beneficial properties; they are extensively digested and absorbed in the gastrointestinal tract, they can partially replace fat and protein in the diet as an energy source, and they also have a metabolic-sparing effect on amino acids.

Since feline diabetes is an endocrine disorder that is caused mainly by an intolerance to glucose due to insulin resistance, it can be difficult not to assume that the carbohydrate content of a cat’s diet is the cause of the disease, but the evidence to support this hypothesis is very weak 16. There are some data that suggest offering affected cats lower carbohydrate diets can result in better glycemic control and higher clinical remission scores 19, and diabetic cats can certainly benefit from such diets, but this does not necessarily mean that carbohydrates are the cause of the condition.

Blood glucose levels rise after a meal, followed by a physiological insulin release from the pancreas to counteract this effect. If there was a connection between higher levels of dietary carbohydrates and diabetes, it would be expected that this mechanism would be altered in some way, resulting in lower glucose tolerance and/or lower insulin sensitivity. Some studies have found changes in glucose tolerance when feeding cats high-carbohydrate/low-protein diets when compared to high-protein/low-carbohydrate diets, but other studies have not confirmed this 20. At least one study has failed to establish any relation between the carbohydrate content of the diet and insulin sensitivity 21, and it is also true that even diets with high levels of starch do not usually cause hyperglycemia and glycosuria in cats.

In addition, the idea that raised blood glucose levels secondary to high-carbohydrate diets in cats are responsible for excessive insulin secretion from pancreatic β-cells (which could then lead to destruction of the cells and subsequently diabetes mellitus), has been refuted by a study that showed that feline pancreatic β-cells are more responsive to amino acids and less responsive to glucose than the β-cells of omnivorous species 22. This suggests that there is more to the etiology of feline diabetes than the carbohydrate content of a diet. Nonetheless, studies have shown that the feline species has a lower glucose clearance rate than in dogs or humans, and that a chronic hyperglycemia state (30 mmol/L over a 10-day period, achieved by glucose infusion) can cause dysfunction and loss of β-cells which impairs insulin secretion 16. It is, however, important to note that this scenario does not mimic a cat’s physiological response to food intake, so in conclusion there is no solid evidence that currently supports a hypothesis that the carbohydrate content of a diet can cause diabetes in the cat. Obesity, due to inactive lifestyle and excessive caloric intake, and advancing age remain the greatest risk factors for diabetes mellitus 16.

Q: Can a cat be healthy without drinking water?

A: Cats cannot survive without water, but they will satisfy their fluid requirements both by exogenous means (from drinking water and the water content of their food) (Figure 6), and by endogenous processes (from water produced from oxidation of carbohydrates, fats, and protein). They will lose water in their urine, faces and via evaporation processes, and whilst various factors (including disease, and the ambient temperature and humidity) can increase these losses, the typical average daily water requirement of a cat is about 50 mL per kg body weight 23.

When necessary, cats have an incredible ability to concentrate their urine by reabsorbing water within the kidneys. Although this is probably a valuable evolutionary adaptation to aid survival in arid environments, it has been suggested that this ability, along with a low water intake, may contribute to the development of some of the urinary tract disorders which are so prevalent in this species 24. To reduce this risk, any measure or strategy that encourages a cat to ingest more water is to be encouraged – so for example, by providing water fountains or extra water bowls around the house (Figure 7). However, at the same time it is important to avoid anything that may upset the cat, as stress can be a major contributor to some feline health issues. The factors that help maximize a cat’s water intake whilst minimizing any element of stress need to be tailored for each individual animal 24, and the best approach is simply to allow the cat to decide how, where, and when to drink.

But drinking water is not the only option available; the water contained within its food is also a good option to fulfil a cat’s fluid requirements. Food is a good source of water for cats in nature, since the prey they catch to survive in the wild (small rodents and birds) will have a water content of around 70% 14. Offering a diet with a high-water content is obviously both very effective and stress-free, as long as the cat enjoys this type of food, and many years ago it was shown that the cats could meet their water requirements solely from a diet of fish or meat 25. More recent research has shown that a cat’s daily water intake and the volume of urine it produces is significantly higher when wet food (which contains around 75-80% water) is offered rather than dry food (which is about 8% water) 26. Therefore, while any stress-free method that encourages a cat to drink is to be welcomed, offering a high-moisture content food may, for many cats, be the easiest way to ensure sufficient water intake for good homeostasis.

Conclusion

The cat is unique in many ways, and its dietary requirements and peculiarities underline the adage that “a cat is not a small dog” – the nutritional needs of the feline species can vary quite markedly from the canine requirements, and it is always worthwhile devoting a significant period of any consultation to discussing a cat’s dietary needs. Good pet health starts with good communication in the consult room, and it is essential for the small animal clinician to have excellent nutritional knowledge in order to deliver the best advice to owners.