Nutrition of aging cats

Introduction

It is only relatively recently that life stages have been defined for cats, although these vary slightly between different organizations (Table 1). According to the American Animal Hospital Association (AAHA) and the American Association of Feline Practitioners (AAFP) 2021 guidelines, feline life stages are divided into five categories: kitten (from birth up to one year of age), young adult (1-6 years), mature adult (7-10 years), and senior (>10 years), with an additional “end-of-life” stage that can be at any age 1. The International Society of Feline Medicine (ISFM) designates the stages slightly differently, as follows: kitten (birth-6 months), junior (7 months-2 years), adult (3-6 years), mature (7-10 years), senior (11-14 years), and super senior (15+ years) 2. These differences aside, the increasing understanding about different life stages in our companion animals means that there is now more interest in appreciating the nutritional differences between these stages, including during aging. While organizations such as the American Association of Feed Control Officials (AAFCO) and the European Pet Food Industry Federation (FEDIAF) have nutritional recommendations and guidelines for growth (which FEDIAF further divides into early and late-stage growth phases), reproduction, and adult, there are no guidelines for senior pets. This proves to be a challenge, as understanding how to care for pets in their older age is an issue that is becoming more important as the population ages; approximately 20-40% of cats in the United States could be classified as “senior” and “super senior” 3 (Figure 1).

Table 1. Life stage descriptions by pre-eminent veterinary feline health organizations.

While there are some studies on nutrition for older cats, there is also a dearth of studies that look at specific nutrients, both for essential nutrients and for those that we think may be of benefit outside of these key requirements. This article will review nutrition-by-nutrient and discuss what we do know, as well as what we believe based on current evidence, to be important considerations for our senior feline companions. 

Overall diet digestibility/calories

When it comes to digestibility, we have long known that mature and senior cats have decreased digestibility, with up to 33% of cats having reduced fat digestibility, and approximately 20% having reduced protein capabilities 4. In addition, more recent information has shown that a decline in the weight of older cats, both in those considered “healthy” and those which are “non-healthy”, may at least in part be attributable to a decreased digestibility of nutrients. It is also common to see loss of lean body mass and a drop in muscle condition score in older cats, and this may be partly attributed to decreased protein digestibility resulting in a negative nitrogen balance 5. Furthermore, questions have been raised about whether protein amounts currently recommended for maintaining nitrogen balance in cats are enough to also maintain lean body mass, and if higher requirements should be considered 6.

There are other reasons besides a reduced diet digestibility which can lead to weight and lean body mass loss in older animals. In elderly humans, the senses of smell and taste diminish, and it is likely that this is also the case with other animals 7. Strategies to help with this include offering diets with varying aromas and flavors, and also providing food at the optimal temperature of 37°C/98.6°F 8 (Figure 2). Disease can also lead to reduced appetite and interest in food due to pain (such as with dental disease), metabolic derangement (such as uremic toxins in chronic kidney disease (CKD), or increased cytokine production in inflammatory diseases), and/or medications (such as chemotherapeutic agents that can directly affect the taste of foods). Diseases that occur commonly in older cats also can directly impact weight and muscle loss, such as seen with hyperthyroidism, chronic enteropathy, lymphoma, and diabetes mellitus. Finally, sarcopenia, or the loss of lean body mass associated with aging and unrelated to disease, has been described in both dogs and cats 9.

Therefore, senior, even healthy, cats may require a higher caloric intake to maintain weight, and a diet that has higher digestibility might result in improved nutrient absorption. Feeding guidelines are provided by manufacturers to give an average estimate of requirements, but individuals may vary up to ±50% from average 10. Assessment of body weight, body condition score and muscle condition score, and appropriate caloric, and potentially diet, adjustments should be done regularly for all cats that are considered senior or super senior (Figure 3).

Dietary moisture

Dehydration can occur in aging animals due to a decreased thirst drive, decreased mobility and/or a disease process 11, but it can be difficult to assess the hydration status of a patient, as clinical signs of dehydration are not consistent despite significant water loss 12. Therefore, it is important to encourage water intake in senior cats, even if they do not appear overtly dehydrated (Figure 4). Ensuring clean, fresh water is a central tenet in this endeavor, but providing dietary moisture by feeding a wet diet can also improve intake. Dental disease is common, with one study reporting it to be present in over 50% of senior cats 13, so if there is evidence of oral discomfort or a reduced ability to chew kibble, feeding a wet diet may be of some benefit (alongside other appropriate measures for the underlying problem) (Figure 5). Other conditions where dietary moisture is important for management include CKD and all forms of urolithiasis. However, it is essential to remember that increasing dietary moisture also decreases the caloric density of a diet. Therefore, it is important to ensure feeding amounts are appropriate for the cat’s daily needs and that intake levels remain sufficient. If the cat has difficulty consuming the daily volumes required, mixed feeding or a dry diet may help to meet energy needs.

Protein

While we know that some senior cats have reduced protein digestibility, it is less clear what level of protein should be recommended for older individuals that are otherwise considered healthy. Given that CKD is a fairly common diagnosis in older cats, with one estimate suggesting 30-40% of animals over 10 years of age being affected 14, and because decreased dietary protein is generally recommended for cats in the later stages of the disease, it is sometimes believed that an early reduction in dietary protein for all older cats might be of benefit. However, there is insufficient evidence to confirm that reducing dietary protein as cats age will reduce the occurrence of CKD. In addition, as mentioned above, sarcopenia is documented in cats as early as 7 years of age, so given the compromised protein digestive capacity in older cats, a reduced protein diet may increase the risk of developing sarcopenia. Therefore, as cats are obligate carnivores and have a high dietary protein requirement, these factors suggest that feeding lower levels of protein should be reserved for cats with active renal disease and is not recommended prophylactically for healthy older cats.

Fat

As previously noted, senior cats can have reduced fat digestibility which can contribute directly to decreased caloric absorption, as fat has more calories than either carbohydrate or protein per gram. Feeding foods with a higher fat content, particularly diets which also have higher protein and carbohydrate digestibility, can be good options for cats that might be otherwise healthy but that are losing weight and body condition over time. Conversely, if a cat is overweight, using a diet that is lower in fat and thus lower in calories would be preferred. The Association for Pet Obesity Prevention in the United States estimated in 2022 that 61% of pet cats are overweight or obese 15, and with many cats living longer, it is likely that this statistic includes those in the senior category. Cats with obesity are more likely to have respiratory, dermatological, musculoskeletal and/or dental problems, as well as urinary tract disease and diabetes, therefore while higher levels of fat might be recommended for cats that are unintentionally losing weight, there may be situations where weight loss is the goal, requiring lower levels of fat intake.

Carbohydrates

While carbohydrates are not a required nutrient, they are important in that they help provide energy needs and balance protein and fat content to the desired proportions in the overall diet. Keeping in mind that for healthy aging cats (rather than where digestible carbohydrates might need to be evaluated more closely, such as with a cat with diabetes mellitus – Figure 6), consideration of carbohydrates on their own is often only done after protein and fat content have been addressed. However, cooked carbohydrates, which are readily digested and absorbed by cats, can have a protein-sparing effect by providing a source of glucose instead of relying on glucogenesis using protein. There is no evidence that digestible carbohydrates induce either obesity or diabetes in cats, and thus there is no reason to avoid this nutrient. In fact, higher fat diets are more likely to lead to obesity, and the amount of fat in a given diet can be decreased by increasing the percentage of protein and/or carbohydrate content.

Undigestible carbohydrates are a form of dietary fiber which can promote a healthy intestinal tract. Fermentable fibers (beet pulp, chicory pulp, fructo-oligosaccharides) are utilized by the intestinal microbiota, and fermentation end-products such as short-chain fatty acids may directly benefit animals during aging. In senior dogs, feeding a diet with a higher concentration of total dietary fiber has been shown to decrease intestinal ammonia 16, an important factor in cases of CKD and hepatic encephalopathy. However, dietary fiber does decrease the caloric density of a diet, and can affect palatability, so the right balance for the individual is key.

Vitamins and minerals

Phosphorus and calcium:phosphorus

While there is insufficient evidence that higher protein diets can cause CKD in senior cats, there is evidence that diets with higher amounts of phosphorus may do so. In two recent studies, diets with differing phosphorus content and calcium:phosphorus ratios were tested in a cohort of healthy cats. The diets with the highest phosphorus level (the majority of which was from inorganic sources) and lowest calcium:phosphorus ratio induced renal changes in the cats 17. Further testing of different diets with varying levels of total phosphorus, inorganic phosphorus sources and calcium:phosphorus ratios indicated that a high level of inorganic phosphorus leads to a dose-dependent peak in plasma phosphorus concentrations which is higher than what occurs with organic phosphorus 18. However, not all forms of inorganic phosphorus cause the same postprandial rise in plasma phosphorus concentrations. While more work in this area is needed, avoiding diets that have high levels of soluble, inorganic phosphates and those with a calcium:phophorus ratio of less than 1:1 is advised 19.

However, it must be acknowledged that in the manufacturing of pet food, the levels of protein and organic phosphorus tend to run together; that is, the higher the level of one of these nutrients is, the higher the likelihood that the other one will be as well. We are seeing newer diets on the market (particularly therapeutic diets targeted for early kidney disease) where this is less likely, due to careful ingredient selection, but for many maintenance diets it may not. Thus, maintaining high dietary protein levels whilst avoiding the highest phosphorus levels might be challenging when evaluating regular, maintenance-type diets for older cats.

Other minerals and vitamins

While it is a given that cats of all ages require complete and balanced diets with appropriate vitamin and mineral content, there may be some further considerations for aging cats. Because of the decreased fat digestibility capacities of these animals, vitamins that are fat-soluble may have compromised absorption. To date, we have no data on deficiencies of these vitamins in older cats due to poor absorptive capacity. However, should a cat be eating a diet that is not being properly absorbed, or is not complete and balanced, and is losing body and muscle condition, deficiencies of these key nutrients may be a consideration.

Antioxidants in vitamin form, such as vitamins C, E, and the provitamin beta-carotene, may be of benefit in older animals. While vitamin C is not an essential nutrient for cats (as they can synthesize it endogenously), there is some evidence that higher levels of vitamin E and beta-carotene in feline diets can contribute to increased longevity 20. While not necessarily an issue for healthy older cats, water-soluble vitamins such as vitamin B₁₂ can be lost if there is increased water loss through polyuria or through malabsorption, and supplementation may be considered for those animals.

EPA & DHA

One of the most common conditions in senior cats is degenerative joint disease (DJD), or osteoarthritis; one study found that 92% of cats over the age of 14 had radiographic evidence of DJD 21. While there are numerous types of dietary supplements suggested for individuals with this condition, the studies using eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are the strongest in reporting a benefit from these additives. One study showed that cats with osteoarthritis receiving supplemental fish oil had higher activity levels, less stiffness, could jump higher, and showed more owner interaction 22 (Figure 7).

Nutrients for cognitive decline

As there are increasing numbers of cats falling into the “senior” and “super senior” categories, cognitive decline and cognitive dysfunction are becoming more common and easily recognized, even in cats that are otherwise deemed healthy. In fact, this is often considered part of the normal aging process. One study showed 36% of owners of cats aged 7-10 years (mature adult and senior categories), and 88% of owners with cats between 16-19 years (super senior category) reported that their pets had developed age-related behavioral problems 23. These changes can lead to a decreased quality of life for the cat and the owner, and as such there is a great deal of interest in finding diets and/or supplements that can slow this process.

A number of additives have been studied, with mixed results. Some evidence has been seen for supplementary S-adenosyl-l-methionine (SAMe), and there are anecdotal reports on other additives such as melatonin, L-theanine, milk protein hydrolysates, and pheromones. However, most of these supplements have been used to address general anxiety issues in cats and not cognitive decline specifically. Further clinical trials examining the effects of additives on cognitive decline are warranted. Medium chain triglycerides (MCTs) are another nutrient that has been used in dogs to address cognitive dysfunction, but further work needs to be done in cats. Previous studies using MCTs in this species had indicated there may be palatability challenges, although newer reports suggest this may not be the case 24,25. Recent research is targeting the gut-brain axis with the idea of improving brain function during aging. In addition to studies involving MCTs (which focus on the action of ketones and direct effects of the medium-chain fatty acids), prebiotics and probiotics are other considerations for future work that may address the cognitive needs of aging animals.

Conclusion

Many companies now offer diets or supplements labeled for senior cats based on their own considerations and beliefs without having the research or understanding about what is needed. While we are beginning to understand the nuances of some nutrients, the nutritional requirements for aging cats is still largely undefined. It is a broad category that encompasses consideration of several nutritional needs, even for seemingly healthy, older cats, so it is difficult to standardize recommendations for all; instead, dietary advice should be done on an individual basis. As our population of cats gets older, we continually look to better understand their needs and provide evidence-based recommendations to support them into their senior and super senior years.