Nutrition and aging in dogs and cats: assessment and dietary strategies

Abstract

Importance

Aging of the body is continuous and progressive, but the signs of aging become more evident with time. Nutrition is one of the primary biological functions. Eating is a daily necessity for maintaining life and health. The expression of an animal’s maximum health potential can be achieved by nutrition tailored to its needs.

Observations

This article presents current knowledge on the physiological consequences of aging. The main factors contributing to increased life expectancy and well-being are the maintenance of optimal weight, body condition, and lean mass. The nutrition response includes an adequate energy and protein intake, with protein and amino acids being the main contributors to the maintenance of lean mass, phosphorus limitation (but not restriction), a calcium to phosphorus ratio above 1 in the diet, and the intake of all essential nutrients. The intake of long-chain omega-3 fatty acids and a related amount of antioxidants may contribute to a better cognitive function in older pets. In practice, nutritional assessment is a part of the clinical examination to diagnose the need to adapt the diet. Age-related nutritional constraints must be considered, even in cases of illness, and applied when they are not contradictory to constraints linked to pathology. Eventually, nutrition can be adapted to suit the pet's age and requirements, regardless of the type of food.

Conclusions and Relevance

Nutrition is one of the keys to health. Adapting the diet on a case-by-case basis requires choosing the most suitable balanced food, whether the diet is commercial, home-prepared, or mixed, preferably formulated by a board-certified vet nutritionist.

INTRODUCTION

Pet dogs and cats are living longer [1], and the population of older pets is growing. The latest data shows that the % of dogs and cats over 7 years old represents 52.4% of dogs and 51.7% of cats in 2022, compared to 41.6% and 46.1%, respectively, in 2012 [2]. This poses a scientific and medical challenge, as well as an economic one, because all these dogs and cats eat every day. Although syntheses and proposals are regularly proposed, there is currently no independent consensus publication on the specific nutritional needs of older dogs and cats [3], leaving room for a vast variety of food proposals.

Furthermore, it is necessary to distinguish between aging animals and animals with illnesses [4,5], whether chronic or not. Although the risk of certain diseases increases with age [6], and aging is associated with a progressive decline in all functions, animals shall not be fed for a pathological situation from which they do not sufferyet.

METHODS

This narrative review integrates a comprehensive synthesis of the peer-reviewed literature with the authors’ clinical and academic expertise in veterinary nutrition, gerontology, and internal medicine. The relevant studies were identified through PubMed, Web of Science, and Scopus using combinations of keywords such as “nutrition,” “aging,” “senior dog/cat,” “nutritional assessment,” “dietary requirements,” and “age-related diseases.” The selection prioritized recent systematic reviews, original research, consensus guidelines, and studies providing quantitative nutrient targets or clinically validated assessment methods, including the World Small Animal Veterinary Association (WSAVA) Nutritional Assessment Guidelines and National Research Council (NRC) 2006 [7] nutrient requirements. The evidence was organized into four domains: 1) relationship physiological changes with age, lifespan, and nutrition; 2) ageing modification of nutrition requirements; 3) nutritional assessment and monitoring in clinical practice; 4) nutritional adaptation in common age-associated diseases.

The recommendations are based on current evidence and the authors’ collective experience in applying nutritional assessment, prescription, and monitoring protocols in clinical practice, to translate scientific evidence into effective veterinary care for aging dogs and cats.

OBSERVATIONS

Section 1. Relationship between the physiological changes with age, lifespan, and nutrition

What is being old? What does life expectancy mean? When is a cat/dog old?

Ideally, dietary adjustments should be recommended from a certain age or time to address the nutritional problem in older animals. Aging is a progressive, inexorable process that begins very early in life. Therefore, it is difficult to definitively propose an age at which a particular nutritional intake should be adjusted, particularly because the life expectancy of dogs and cats has increased steadily over the past several decades in countries where animals are medicalized. This is well reflected in studies considering life expectancy at birth [1].

Therefore, the age in years is not the only possible criterion. Some definitions relate to a proportion of life expectancy. Others include a "senior" age and a "geriatric" age, as in humans. The AAHA proposal [8] even extends to five life stages in dogs, including puppy, young adult growth, mature adult, senior (last 25% of estimated life expectancy), and end of life (terminal stage). Extreme longevity refers to some populations with particularly long-life expectancies [9].

In cats, for which the size is less heterogeneous, four stages have been proposed by a panel of experts [10]: growth up to one year, young adult (1–6 years), mature adult (7–10 years), and senior (> 10 years). Considering the variety of sizes and life expectancies in dogs, these ages may vary according to the format, with the smallest showing the longest life expectancy.

These proposals and the assessment of risk at each stage are being considered to enable the development of a preventive medical strategy throughout the life course and improve the chances of older animals to be adopted [11]. The associated nutritional recommendations are in both cases linked to a good assessment of the body condition and muscle condition, possibly considering the concomitant pathology [9,12].

Physiological changes with aging in relation to the nutritional function

All biological functions degrade progressively with age. Even if macroscopic signs constitute diagnostic stigmata [13], an assessment of this degradation is complex, with the interaction of various factors such as cognition, activity, and body condition [6,14,15]. Few studies, recent or not, are available. They can even give the impression of contradictory results depending on the conditions of the studies.

1) Body composition

With age, the fat mass tends to increase. For example, an update of several studies showed that it increases from 15%–20% at one year old to 25%–30% at 8–10 years old [16]. On the other hand, the distribution of fat mass changes with an increase mainly in visceral fat, compared to subcutaneous fat [17], which would promote a state of insulin resistance [18].

At the same time, aging is accompanied by sarcopenia, with a decrease in lean mass compared to fat mass with age [19]. The distribution of this amyotrophy would be uneven, with a more marked degradation of specific muscles, comprised mainly of type II (glycolytic) fibers [20]. In addition, the presence of frailty was linked with decreased muscle strength in up to 42% of healthy-appearing older medium-sized (> 20 kg) dogs [21]. Indeed, sarcopenia is also associated with a decreased lifespan in dogs and cats [22]. This phenomenon would be due to autophagy, which is the destruction of myocytes by their lysosomes that contributes to the reduction of the "muscle reserves" of amino acids available in the event of a sudden demand for them [23].

2) Digestive tract

From feet to mouth, the changes that accompany aging are harmful to the animal. First, the oral function is impaired, with decreased salivation [24], tooth wear, tartar, and a high frequency of periodontal diseases [25], which contribute to the emergence of oral flora unfavorable to the animal. Unlike in humans, where this is a major concern, few swallowing problems have been reported in dogs. Nevertheless, a decrease in neurons in the sympathetic ganglia and in the myenteric and Auerbach plexuses has been identified in dogs with age [24]. On the other hand, no work has been carried out to show an abnormality of gastric motility, with delayed gastric emptying, in old dogs. Similarly, there does not appear to be any achlorhydria related to age in this species. Age also affects the small and large intestinal morphology with smaller villi in the jejunum, and greater colonic crypt depth, in senior dogs, compared to young dogs, which can have consequences on intestinal absorption and colonic fermentations [26]. The histological changes are correlated with age in dogs [27].

3) Digestion

Very few studies have been conducted to assess the digestibility in dogs and its evolution with age. Generally, they concluded that there is no difference between young and old dogs in terms of apparent digestibility of proteins, lipids, and dry matter [28,29,30], and even for calcium, phosphorus, and zinc [31]. A study where dogs were supplemented with either soluble fiber, in the form of beet pulp, or insoluble fiber, revealed a decrease in protein and fat digestibility in older dogs when soluble fiber was present, compared to younger dogs [32]. In cats, the data are contradictory, and the recent data tend to show no real decrease in digestibility [29].

4) Ancillary organs

With age, the functional capacity of the liver in dogs likely decreases because, histologically, fatty infiltration of hepatocytes and nodular hyperplasia lesions have been observed [33,34]. Nodular hyperplasia of the exocrine pancreas also increases with age in dogs, without any associated clinical manifestation [35].

5) Five sensory organs

On another level, a decrease in the sense of smell has been highlighted in dogs [36,37]. This reduction in the senses, smell, and potentially, taste, could harm the dog's appetite, suggesting that it is necessary to increase the palatability of a ration in older dogs [38]. Although not proven, it is often suggested that older cats [6] and dogs have a reduced thirst, as in humans, which would increase the risk of dehydration [38], and encourage action on water consumption.

6) Microbiome

Studies on the evolution of microbiota and fermentation products with aging in dogs are inconsistent, which is likely related to the diet chosen, sampling techniques, and analysis methods. Some authors have reported an increase in Bacteroides [39,40] or in Fusobacteria [41] with age, while others have observed the opposite for Bacteroïdes [42] and for Fusobacteria [43], respectively. Nevertheless, if a change was observed in an old dog regarding lactobacilli, the authors agreed to describe a decrease with age [42,44] and in cats [45]. One of these studies further showed a link between increased Actinobacteria and poorer performance on a memory test [43].

Only fecal butyrate, valerate, and isovalerate appear to be modified with aging, but again in a different direction depending on the authors, with an increase [26,40] or a decrease [39] of these three molecules, in old dogs. The other authors showed that fecal acetate and propionate do not appear to be altered by age in dogs [26,46] or that acetate increased with age [39].

7) Glomerular filtration rate

Even in the absence of chronic kidney disease (CKD), glomerular filtration rate (GFR), which is a key indicator of the kidney function, declines slightly in a relatively linear fashion in dogs [47]. If GFR is higher in 2-month-old Beagle puppies than in 6–9-year-old Beagle dogs, with 6.2 ± 0.7 vs. 4.1 ± 0.5 mL/min/kg, respectively [48]. In adults, however, aging appeared to have a limited effect, or, in any case, the effect was obscured by other influences, as shown in longitudinal [49] and cross-sectional [50] studies. Indeed, the GFR value and serum creatinine values, while in the normal range, must be interpreted considering the breed size and format, including the muscle mass, which correlates with higher physiological values [51], and more than the age, particularly in small breed dogs [50]. Avoiding overdiagnoses of deficiencies in cases of suspected early stages of renal disease is of particular interest. Physiological changes associated with aging can impact the nutritional function, potentially affecting the nutritional needs and feeding patterns, but nutritional factors may also affect life expectancy.

Nutritional factors affecting life expectancy

Several theories have been proposed, and numerous studies on the difference in life expectancy between species and the maximum life expectancy for a given species have been published since the beginning of the 20^th century [52,53]. Few nutritional factors have led to longitudinal studies. The most commonly included variables are body condition and sometimes muscle condition. Finally, regarding dogs and cats, an animal's body condition throughout its lifetime significantly affects its life expectancy.

Thus, overweight and obesity reduce the life expectancy of dogs by approximately 2 years, whether under experimental conditions [54,55] or in companion dogs [1,9,56,57], e.g., increasing the risk of numerous diseases and shortening the age of clinical signs of chronic diseases such as osteoarthritis (OA).

Overweight and obesity increase the risk of many diseases in cats [58,59,60]. In other studies, however, a body condition considered overweight still appears to be associated with a longer life expectancy [1,58,61]. The interpretation of these seemingly contradictory data raises questions. Does excess weight protect surviving cats? Currently, no study provides a more precise picture. Energy malnutrition can lead to a suboptimal body condition. The origin of this energy malnutrition varies and can be associated with a loss of fat mass, a loss of lean mass, or both. Schematically, it could be a lack of energy or protein, or a diversion of energy to the benefit of a parasite or a metabolic or pathogenic process.

In cats, being too thin or too fat has long been known to increase the risk of diseases [59]. A specific parameter in cats is a tendency to lose weight with age. This can be a direct consequence of a disease, but it can also be the precursor to a later pathology. Indeed, some authors have reported that cats with certain diseases begin to lose weight 2.25 or even 3.75 years before their death, depending on the disease in question [62]. Furthermore, older cats have a high risk of sarcopenia, a loss of lean body mass [62].

One study [58] observing the maximum body condition score in a population of neutered cats reported that cats with a maximum body condition score of 4 or 9 had a more limited life expectancy. Nevertheless, the interpretation of these data must be made considering the biases of this study. Moreover, the authors present this information, considering all previous data and the associated risks of overweight and obesity in young animals.

Concept of frailty

The concept of frailty emerged in humans in a general context of advanced age, coupled with pathologies or circumstances that weaken the body. The first studies in dogs are beginning to produce results [9,12,21,63,64]. The objective is to identify risk factors and offer clinical assistance to detect a risky situation. In other words, a geriatric assessment can lead to practical recommendations and support for management.

The interpretation can vary according to the population studied, the size and representativeness of the sample, whether the population studied was already elderly or not at the beginning of the study, and the duration of observation, ranging from just a few months to a lifetime. Nevertheless, regarding the nutritional status, assessed by body condition and muscle condition, available studies show concordance and suggest that the optimal body condition and muscle condition are the preservation factors for the dog [9,12,21,64].

Section 2. Ageing modification of nutrition requirements

The identified aging factors should guide the optimization of an animal's diet throughout its life. Nevertheless, once the animal reaches advanced age, even if the diet has been optimized beforehand, one may wonder how to modify nutrition, how needs change, and how to adapt to meet them, considering the changes related to advanced age. Feeding an animal is a daily task that should be reasonably simple, but the function of nutrition is highly complex and involves several levels.

·Nutrition includes the quantity and composition of food consumed and its distribution, which must ensure that appetite is satisfied.

·Nutrition includes also the body’s use of this food, including digestion, absorption, metabolic transformations, and finally the body’s use of nutrients down to the cellular level.

Optimally fulfilling the function of nutrition helps limit the consequences of malnutrition (deficiencies, excesses, and imbalances) and prevent nutritional diseases. Hence, nutrition allows a given individual to express their maximum nutritional health potential. This involves understanding the animal's needs and its digestive and metabolic capacities, and having sufficient information on the food (composition, digestibility, and palatability). In practice, a simplified overview is necessary to answer the only questions of practical interest: which feed is most appropriate for which animal? How much should be given? How? In how many meals? Without going into metabolic details, these simple questions imply the following nutritional goals and the means to achieve them: maintain (or reach) optimal body weight (opt BW) and condition, and maintain health and optimal body condition.

Energy requirement (ER)

Maintaining or reaching an optimal and stable weight requires adequate coverage of an animal’s energy needs. Evaluating energy needs is essential to recommend the correct amount of food each day. ER includes basal metabolic plus contingent expenditures related to feed utilization, thermoregulation, physical activity, and production (growth and reproduction).

Many individual characteristics (e.g., breed, sterilization, activity, average or sedentary lifestyle, climate temperature) influence basal metabolic rate, contingent expenditures, and therefore maintenance ER (MER; Table 1). Maintenance ER (MER) seems to decline by 20%–30% in senior dogs, compared to young dogs, at least in Labrador retrievers, Siberian huskies, Great Danes, Papillon, and Beagles [52,65,66]. A study of 319 healthy adult pet dogs at their ideal body weight (BW) showed that MER regularly declines with age [67]. On the other hand, a closer look at ER shows that lean body mass is one of the most influential factors in basal metabolism [68,69,70,71]. Age may not alter a dog's overall ER [72], but a dog's ERs may be altered when its lean body mass is altered [73]. Few studies have included dogs of all ages, or followed them throughout their lives, recording body condition, lean body mass, and life expectancy. In a longitudinal study, the factor explaining the best life expectancy among Labradors is the preservation of lean mass and the control of fat mass gain [74]. Thus, individual characteristics and optimal weight for optimal body condition and muscle condition must be considered when assessing the individual energy needs of dogs and cats, as reported elsewhere [3,75].

Table 1. Summary of the factors affecting the energy needs of healthy dogs and cats in optimal BW and condition, compared to the maintenance energy requirement of 130 BW^0.75 (NRC 2006^a) for dogs and of 70 kcal/kg BW for active intact cats.

Species	Dog	Cat
Breed	(+10%–20%) Greyhounds, Great Danes	(+20%) Sphynx, nude cats
	(−20%) Nordic dogs (Husky, Retrievers, Newfoundlands…), Border collie, Cocker, Beagle
Intact/neutered	(−20%) Neutered	(−20%) Neutered
Mode de vie	(−20%) Sedentarity (indoor + 1 h walk/d)	(−20%) Indoor
Activity	(+5%–300%) exercise, sport or working activity, sled dog races	ND
Environmental temperature (main living temperature)	(−20%) above 25°C	ND
	(+20%) below 5°C

BW, body weight; NRC, National Research Council; ND, not documented.

^aFrom reference [7].

Nutrients requirements

Meeting nutritional needs helps preserve the major functions, immunity, maintenance, and cell renewal of the body. This involves avoiding nutritional diseases, whether caused by deficiency, excess, or imbalance. In other words, it allows the body to express its maximum health potential and body composition by optimally meeting its basic individual nutritional needs. This involves providing all the necessary nutrients (Table 2) in the right amounts for each animal and in the amount of food defined by the ER. A feed with a certain concentration of each of the essential nutrients shall be formulated for each animal category, considering the characteristics which increase or decrease ER, but not only its age.

Table 2. Essential nutrients for dogs and cats (NRC 2006^a).

Macronutrients		Minerals		Vitamins
Protein		Macro		Fat-soluble
Amino acids			Calcium		Vitamin A (retinol)
	Arginine		Phosphorus		Cholecalciferol
	Histidine		Magnesium		Vitamin E (alpha-tocopherol)
	Isoleucine		Sodium		Vitamin K (menadione)b
	Methionine		Potassium	Water-soluble
	Methionine & cystine		Chloride		Thiamine
	Leucine	Trace-elements			Riboflavin
	Lysine		Iron		Pyridoxine
	Phenylalanine		Copper		Niacin
	Phenylalanine & tyrosine		Zinc		Pantothenic acid
	Threonine		Manganese		Cobalamin
	Tryptophan		Selenium		Folic acid
	Valine		Iodine		Biotin c
	Taurine				Choline
Lipids
Fatty acids
	Linoleic acid
	Arachidonic acid
	Alpha-linolenic acid
	EPA + DHA

The values in bold are the nutrients required only for cats.

NRC, National Research Council; EPA, eicosapentaenoic acid; DHA, docosahexaenoic acid.

^aFrom reference [7].

^bFor vitamin K, there is a value, although its essentiality in the diet has not been demonstrated, probably due to microbial synthesis.

^cFor biotin in dogs, there is no value due to a lack of documentation.

In addition to the need to meet nutritional needs, the specificity of digestive competence related to age must be considered. Although currently controversial [29], the ability to utilize food was considered to change slightly with age [76]. Nevertheless, food with good digestibility, i.e., at least 80% of apparent digestibility for proteins, should be the aim. When dealing with a population of animals, averages can be used, which can be adjusted to varying degrees by subpopulation. Nevertheless, the diversity of the canine and feline population and the impact of these characteristics on nutritional needs justify offering more individualized solutions to the animal in the veterinary consulting room for dogs and cats [77,78,79]. This applies to a healthy older animal and to an older animal suffering from one or more diseases [80,81].

The nutritional tables (NRC 2006 [7]) indicate the minimum amounts, nutrient by nutrient, for the entire population of adult animals, and for growth and reproduction. These values are based on published studies, but do not consider the life expectancy or long-term healthy life. No specific requirements for older animals are mentioned. Indeed, an older animal is first and foremost an adult. This does not mean that there are no specific requirements to apply in this case, but the minima, which are otherwise very minimalist, remain the same. Moreover, these are not optimal values for an individual or a subpopulation. Therefore, specific information must be sought elsewhere.

Qualitative nutritional intakes have little controversy. On the other hand, the quantity of each nutrient and the choice of energy sources have attracted considerable interest [72], particularly concerning protein and amino acid intake. For several years, this debate has been underpinned by the possible competition of nutritional sources between humans and animals, but proteins are also the most expensive nutrients in the ration. A wide variety of compositions is available on the market. Most of the complete foods available on the market contain much more than the minimum recommended in the recommendation tables [82].

1) Protein and amino acids

Assessing protein and amino acid requirements is quite complex. The two primary methods used to validate the protein requirements are indirect because proteins are all nitrogen compounds, and animals can only use organic nitrogen to synthesize their proteins: measuring nitrogen balance (amount of nitrogen entering the body equals that leaving the body) and maintaining lean mass (maximum lean mass being genetically determined, and enabled by protein and amino acids intake and their transformation by the body). This last method was not previously possible in vivo, but it is now possible to measure it using dual-energy X-ray absorptiometry [52].

The use of the nitrogen balance is subject to pitfalls; the body must renew itself and lose nitrogen through catabolism. Providing nitrogen, in the form of amino acids, must compensate for this loss. Nevertheless, the nitrogen balance does not appear to correlate with lean mass [83]. Thus, the nitrogen balance can be zero even though the lean mass of an animal is not maximal [84]. Thus, the survival of critically ill human patients is better in the cases of a positive nitrogen balance [85].

At the same time, Keller (2011) [86] reported that, except in individuals with renal failure, a protein intake higher than that required for a zero-nitrogen balance is more than positive, considering the key role of protein in limiting sarcopenia. Therefore, the idea is rather to allow the organism to install and maintain a lean mass permitted by the genetic potential of the animal. Wakshlag et al. [87] evaluated the ability of 12% and 28% protein diets to maintain lean body mass, with four different protein sources for each group, given for 10 weeks. They showed that only one of the eight diets tested (No. 5) maintained a positive lean body mass in adult dogs. Two diets (No. 3 and No. 4, with 12% protein) even led to a significant decrease in lean body mass during this study. Nevertheless, these two diets covered the minimum amino acid intakes of the NRC 1985 [88]. Compared to the NRC 2006 [7] minimal requirements, diet 4 does not appear to cover the lysine requirement. A comparison of the intakes of diets in this study with the minimum amino acid intakes of the NRC 1985 [88] and NRC 2006 [7] (Table 3) suggested that the minimum intakes of these tables do not appear to be sufficient to maintain lean body mass in adult dogs, even small dogs, e.g., 13kg Beagle dogs. This was confirmed in a recent meta-analysis [89]. Similarly, in cats, a study showing the benefits of adding specific nutrients to the immune response in cats showed that a diet rich in protein (115 g protein/kcal ME) is more effective than one that meets the NRC 2006 minimum [90].

Table 3. Mean intake for each group of essential amino acids (tryptophan could not be presented) for adult dogs weighing 13 kg on average.

Energy from protein (% Protein/DM)	Diets 1 to 4: 12.3%ME (12.1%–14.4%DM)				Diets 5 to 8: 28.3%ME (29.4%–31.5%DM)				NRC Minimal amount (mg/kg BW/d)
Animal:vegetable	100:0	67:33	33:67	0:100	100:0	67:33	33:67	0:100
Diet n°	1	2	3a	4a	5	6	7	8	[88]	[7]
ARG	148.0 ± 21.3	99.0 ± 19.3	77.1 ± 15.2	81.4 ± 13.8	321.6 ± 64.7	303.9 ± 52.5	192.8 ± 24.9	178.6 ± 29.2	21.0	58.0
HIS	56.9 ± 8.2	66.7 ± 13.0	47.7 ± 9.4	57.8 ± 9.8	117.6 ± 23.6	117.5 ± 20.3	97.3 ± 12.6	115.1 ± 18.8	22.0	32.0
ISO	91.1 ± 13.1	82.7 ± 16.1	56.9 ± 11.2	81.4 ± 13.8	213.7 ± 43.0	227.4 ± 39.3	169.4 ± 21.9	200.4 ± 32.7	48.0	63.0
LEU	197.3 ± 28.4	216.3 ± 42.0	247.8 ± 48.9	344.9 ± 58.3	396.7 ± 79.8	596.6 ± 103	639.6 ± 82.5	880.9 ± 144.0	84.0	115.0
LYS	151.8 ± 21.8	119.3 ± 23.2	75.2 ± 14.9	50.1 ± 7.4	348.5 ± 70.1	289.0 ± 49.9	145.9 ± 18.8	97.2 ± 15.9	50.0	58.0
MET	51.2 ± 7.4	48.5 ± 9.4	51.4 ± 10.1	79.3 ± 13.4	104.0 ± 20.9	130.5 ± 22.5	147.7 ± 19.1	144.0 ± 39.9	26.7	58.0
CYS	43.6 ± 6.3	58.6 ± 11.4	55.1 ± 10.9	77.1 ± 13.0	115.5 ± 23.2	141.7 ± 24.5	158.6 ± 20.5	174.6 ± 28.5	3.3	53.0
PHE	94.9 ± 13	95.0 ± 18.5	95.4 ± 18.8	130.7 ± 22.1	200.3 ± 40.3	262.9 ± 45.4	252.2 ± 32.5	333.3 ± 54.6	51.0	60.0
TYR	41.7 ± 6.0	32.2 ± 6.3	44.0 ± 8.7	72.8 ± 12.3	107.8 ± 21.7	151.0 ± 26.1	153.1 ± 19.8	200.4 ± 32.7	35.0	47.0
THR	115.7 ± 16.6	97.0 ± 18.9	78.9 ± 15.6	92.1 ± 15.6	233.0 ± 46.9	238.6 ± 41.2	191.0 ± 24.6	206.3 ± 33.7	44.0	73.0
VAL	100.6 ± 14.5	97.0 ± 18.9	82.6 ± 16.3	98.5 ± 16.7	227.2 ± 42.8	248.0 ± 42.8	201.8 ± 26.0	228.2 ± 37.3	60.0	84.0

Diets differed by Energy from protein (as % of Metabolizable Energy (ME), by % Protein per Dry Matter (range as %DM), and by origin of protein (as % of Animal and Vegetal protein).

Values are presented as mg/kg/day per group for each amino acid as mean ± SD.

The values in bold are diet with a positive lean mass, thus not significantly different from diets 1,2,6,7,8.

Adapted from Wakshlag et al. [87].

DM, dry matter; BW, body weight; NRC, National Research Council; ARG, arginine; HIS, histidine; ISO, isoleucine; LEU, leucine; LYS, lysine; MET, methionine; CYS, cysteine; PHE, phenylalanine; TYR, tyrosine; THR, threonine; VAL, valine.

^aDiets for which lean mass was significantly decreased.

Aging in dogs is sometimes associated with a loss of lean mass and an increase in fat mass [91], but this is not inevitable [92]. Furthermore, maintaining lean mass, particularly between the ages of 8 and 10, appears to be associated with a longer life expectancy [91]. Finally, in a longitudinal study of Labradors throughout their lives, with one group fed to maintain the optimal weight and one fed ad libitum and becoming obese, it was reported that the factor explaining the best the two-year longer life expectancy of Labradors with optimal body condition is the preservation of lean mass and the control of the increase in fat mass [74]. Kawauchi et al. [93] compared diets containing 51 g and 94 g protein/Mcal ME in dogs weighing approximately 12 kg, for 26 weeks post-neutering, maintained at a constant weight by controlled caloric intake. They reported that the 94 g protein/Mcal diet allowed the maintenance of lean body mass without increasing the fat mass.

No consensus is available for adult dogs. Nevertheless, the idea would be to limit energy overfeeding by adjusting the caloric intake to the dog's needs, while providing sufficient protein and amino acids to allow the preservation of lean body mass throughout adulthood.

In cats, an intake of at least 5 g of protein/kg of BW/day from a food with more than 80% digestibility appears to help maintain lean body mass (i.e., 5.2 g to 6 g protein/kg/day [94]). This minimal value of 5 g/kg/day currently appears to be widely accepted. This value can be increased for cats with reduced energy needs [78], particularly in older cats [95].

In dogs, Wannemacher and McCoy [96] reported in 1966 that older dogs required 50% more calories than young adults. Laflamme (2005) [75] suggested that older dogs should receive at least 25% of their calories from protein. These values may be adjusted in the future by considering the individual characteristics of the dog that impact its energy needs, and lead to adjusting the nutrient: energy ratio of the food to continue to cover the nutrient need when the energy need is increased or decreased because of these individual factors.

2) Lipids and polyunsaturated fatty acids (PUFAs) and antioxidants

Lipids are well tolerated in older dogs or cats. Their digestibility remains excellent [29]. Lipids are a concentrated source of energy that is well utilized by carnivores. Beyond the minimum amount of lipids required to be the vehicle for essential fatty acids, the absorption of fat-soluble vitamins, and a palatability factor, the proportion depends more on the desired energy density for a given animal.

The requirement for essential fatty acids is at least the same in aged as in adult dogs and cats, for omega-6 and omega-3. Nevertheless, certain fatty acids, long-chain omega-3s, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), can play specific roles, and a minimum of 30 mg EPA + DHA/kg MW is suggested by the NRC 2006 [7]. Nevertheless, because of their special benefit on immunity [97] and, at relatively high doses (between 33 and 67 mg DHA/kg and 27 to 43 mg EPA/kg) [98,99,100], on cognitive function in both cats and dogs, Blanchard et al. [101] suggest an intake above the minimum. This additional intake of PUFA must necessarily be associated with a higher intake of antioxidants, particularly vitamin E [102,103] (up to 1.8 mg RRR-alpha-tocopherol/g of fatty acid added to the food) (NRC 2006 [7]).

3) Calcium and phosphorus...

Phosphorus intake should be reduced in cases of kidney disease [104,105,106], which is more common in older animals. Excess phosphorus, particularly soluble phosphates [107], and any diet with a Ca:P < 1 [108], are toxic to the kidneys. In addition, the risk of urolithiasis is also associated with certain configurations, with more nephrolithiasis during the test periods (Ca:P = 0.59 and Ca:P = 0.93) [109].

On the other hand, the benefit of drastic phosphorus restriction during the cat's lifetime on the prevalence of kidney disease has not been demonstrated [110]. The difficulty is that phosphorus concentration accompanies the protein concentration, in most commercially available processed feeds. Nevertheless, protein restriction is not beneficial in older animals, even in cases of early kidney disease [104,105]. Emphasis must be placed on this duality because the goal is to nourish an individual, and not just consider their kidneys, brain, or lean body mass. Therefore, the goal should be to cover the protein requirement while providing a reasonable amount of phosphorus, and a Ca:P ratio between 1 and 2.

4) Other essential nutrients

No specific information is available on the requirements for potassium, magnesium, sodium, and chloride with aging. Little original research supports a possible change in the requirements or intake recommendations for other nutrients, whether trace elements or vitamins. Finally, most authors agree that, when interpreting the available data and applying scientific, biological, and nutritional logic to older dogs and cats, the fiber intake should be adjusted according to the energy needs, body condition, and digestive and transit capacities. Fiber, particularly insoluble fiber, reduces the energy density of the food, stimulates transit, textures stools and increases their volume, and reduces digestibility [29]. A minimal intake is essential. If the animal has very low caloric needs, a higher concentration can be a real advantage by allowing for a larger ration. On the other hand, if the animal has a reduced appetite, it is preferable to limit the fiber intake to what is strictly necessary to ensure transit.

5) Water

One specific nutrient can be addressed: water. Water will be consumed alongside dry food. To increase water consumption in dogs, consider distributing the kibble meal in a large volume of water (three times the weight of the kibble, for example). In cats, however, this method can lead to food refusal, and the consumption of wet food is the most effective solution [111]. Thus, with the same composition, a food moistened to 75% moisture allows for significantly more water consumption than the same food distributed dry or less moistened [112].

Number of meals

No data is currently available that would show an advantage in splitting meals differently in older dogs and cats. The best urinary stability in cats is achieved with four meals per day [113,114]. This is relatively easy with kibble dispensers equipped with a timer, even when restricting the daily amount distributed. In the case of wet food, this means distributing one meal in the morning upon waking, one before leaving the house, one upon returning, regardless of the time, and a final meal at bedtime.

Dietary transition

Age does not appear to have a particular impact on the acceptance of new food. Nevertheless, it is prudent to implement a gradual dietary transition, over a week for dogs to limit digestive disturbances, and more gradually for cats to avoid psychogenic anorexia.

Therefore, for cats, a small amount of new ingredients or food may be introduced once a day, day after day, while continuing to distribute the usual food in quantities limited to the ER. The quantity will increase when this new food is consumed for several days in a row. The amount of usual food is gradually decreased [115].

Section 3. Nutritional assessment and monitoring in clinical practice

Without any calculation, a first approach can be obtained for any vet practitioner, even if not trained in nutrition. The nutritional assessment may allow the vet to answer the question: Does this pet medically require nutrition changes and a nutritional prescription?

The way to realize this prescription may be assisted by digital tools or by a board-certified vet nutritionist, but the diagnosis may be straightforward in most situations.

The nutritional assessment (WSAVA)

The WSAVA has issued nutritional assessment guidelines (2011) [116] to introduce nutritional assessments as one of the five vital signs to be included in each standard physical examination. The entire action may be difficult for general practitioners with no nutrition training. Practically, a simplified option may be offered, which can lead to the conclusion “a nutritional prescription is required, with the assistance of a vet trained in nutrition, ideally a board-certified vet nutritionist” [117,118].

This method can include diet and pets, and may be described as follows:

1) The diet assessment

Diet: recording of the entire food intake, including treats.

This requires that the diet has been steady for at least a month. 4 questions may be helpful to obtain more information from interviewing pet owners:

- What food do you give to your pet?

- What else? In what amount daily?

- Are you happy with it?

- What would you like to do?

Two main situations arise for the food assessment and the pet assessment (Freeman et al. [116]).

Diet (1) is regular daily food composed of only commercial complete food, dry and or wet, chosen and designed for the target, and a maximum of 10% of the amount from all treats delivered in the day (amount may be considered wet, wet–dry, or dry). This amount of treats shall be considered dry for dry, wet for wet. Indeed, it is not considered ideal because it may unbalance the diet [115]. The negative effect is attenuated if the amount of calories provided by such treats is considered in the overall daily food intake, and the main food is adjusted accordingly.

Diet (2) comprises regular daily food composed of commercial foods not designed for the target pet, or unfamiliar foods (vegan, barf, raw, homemade…) at sanitary risk or risk nutritional unbalance [117,119,120,121,122,123,124], or of complementary commercial food, or including daily treats representing more than 10% of the total food intake or includes skin-based treats or dry meat/live.

The consequences of energy malnutrition can be seen in a few days or weeks by the changes in BW, but malnutrition or nutritional imbalance for other nutrients generally does not appear that quickly. The best way is to evaluate the composition of the diet and compare it with individual requirements [125].

2) The pet assessment

Pet: assessment of the pet characteristics (Table 4) includes

Table 4. Pet assessment of the nutrition-related risk factors: optimal situation and situation requiring action.

Pet assessment	(A) Shall be	(B) Requires action if
Body weight and condition	Optimal	Underweight or overweight
Muscle condition	Optimal	Muscle loss (above the eyes, lumbar area…)
Hair coat and skin condition	Bright, strong, clear skin	Sticky, dandruff, dull, rough, hair loss, hair pulling out easily
		Reactive skin, oily or dry skin, erythema, which seems to bother the animal
Dental health	Healthy gums, little tartar	Abnormality, excessive tartar, loose teeth, swollen or inflamed gums…
Digestive health	Well-textured stools, 1 to 3 stools per day	Any sign of digestive discomfort (e.g., vomiting, diarrhea, nausea, flatulence, constipation, colitis with or without blood) or recurrent parasitism
Urinary health	Proper urination, fasting urine strip clean of abnormalitiesa	Fasting urine strip with abnormality such as urinary pH < 6 or > 7, crystals, or blood in urine…
New disease		In case of metabolic disease, any disease diagnosed requiring nutritional adaptation as part of the treatment or support

Adapted from Freeman et al. [116].

^aRecommended in case of doubt on the diet.

- BW

- Body conditions, evaluation based on a five or nine-point scale, 4–5/9 being optimal

- Muscle conditions, either normal, mild, moderate, or marked muscle wasting

- Hair coat and skin condition

- Gastrointestinal function6

- Dental health

- Urinary health

3) What to do after this nutritional assessment?

Using the results of diet and pet assessments, the action may be implemented as described in Table 5 and monitored.

Table 5. Action considering the diet and pet nutritional assessments.

Assessment	Action
Diet (1) + Pet (A)	No action required so far, new nutritional assessment at the next consultation or if the situation changes.
Diet (2) + Pet (A)	Action required to balance and adjust the diet according to age.
	Not only prescribing a diet for senior pets with high digestibility, but choosing it appropriately, to cover the energy requirement, the protein requirement, not too much phosphorus, and adding EPA + DHA in purpose, and prescribing this diet on a prescription form, with the amount and the distribution directions.
	This diet can be commercially available, or the recipe for a balanced home cooked diet, or even a mixed diet, including apart from commercial food and apart from homemade diet, the latest being balanced considering the pet and the commercial food chosen.
Diet (1) + Pet (B)	Even if the diet might be ok for another pet, for this one, nutrition shall be adjusted, at least by a nutrition-trained veterinarian, and if possible, by a board-certified vet nutritionist.
Diet (2) + Pet (B)	Action required, to balance and adjust the diet, including according to the age and the specific condition, as for situation 2-A.

EPA, eicosapentaenoic acid; DHA, docosahexaenoic acid.

4) Monitoring

Following any nutrition change, it is recommended to monitor the animal for approximately one month after the first prescription, even with an intermediate check in cats, in the case of food transition, because food acceptance of the cat may be critical. In any case, this monitoring may include a new nutritional assessment and adjustment of the food or the amount if needed. The follow-up may be planned depending on the pet's health status. More generally, at each later consultation, the nutritional assessment, as considered by Freeman et al. [116] as one of the five vital signs to be evaluated, must be included in the routine clinical examination. Any unexpected change shall be explored, and the expected changes recorded. In the case a target has been identified at a previous consultation, a change may be operated once the target is reached.

Diets for aged dogs and cats

Observed feeding methods have evolved over the past decade [126], and commercial foods alone are no longer the preferred method for feeding dogs and cats. Home-made and raw diets have become very popular. The nutritional and health risks associated with raw or vegan diets have previously been discussed. Home-made diets can be at risk (nutritionally imbalanced because of inappropriate recipes and a lack of compliance from the pet owner). On the other hand, if they are properly formulated and prepared, they are of high quality and can be individually adjusted to the animal [115,127,128]. The principle of a mixed diet is also discussed because it appears to represent the main way of feeding pets, as discussed in France in the 2000s [129,130].

1) Common strategies for tailoring individualized diets

As described below, the food provided can be individualized using all available information about the pet, including its individual characteristics. The strategy to tailor diets considering all characteristics of dogs or cats is as follows [128]. The first steps are common to all ways of feeding.

(1) Individual daily ER

The daily ER is evaluated using the opt BW and condition:

In cats: i.e., 50 kcal/kg opt BW for a neutered indoor cat, 60 kcal/kg opt BW for an intact indoor cat, 70 kcal/kg opt BW for an intact outdoor/active cat.

In dogs: ER = K × MER = K × 130 kcal/kg opt BW^0.75 [7].

With K a coefficient = k (breed) × k (neutering) × k (lifestyle, sedentarity) × k (activity, exercise) × k (ambient temperature) × k (disease)… (Table 1).

‘K’ is the product of all k coefficient, with a minimal value of 0.5.

‘k’ are coefficients depending on breed, neutering, lifestyle, activity, ambient temperature, and disease, if any.

(2) Individual daily protein requirement

The daily protein amount is evaluated using the opt BW and condition (i.e., at least 5 g protein/kg opt BW in cats, and 60 g protein/Mcal MER in dogs > which allows the amount of protein in grams to be defined as 60 × MER expressed in Mcal ME.

As an example, consider an average dog with a MER of 1,000 kcal and a protein requirement of 60 g per day. If this dog is neutered (k = 0.8) and sedentary (k = 0.8) (living indoors with a 1-hour walk per day), k = 0.8 × 0.8 = 0.64 > instead of 1,000 kcal, it must receive only ER = 640 kcal to prevent weight gain. The dog would still need to receive 60 g of protein to maintain its fat-free mass.

This means a diet with a protein:energy ratio of 60:0.64 = 94 g protein/Mcal ME.

This data may help choose an adapted commercial diet or formulate a home-made diet or a combined option, a mixed diet.

2) Feeding using a commercial diet

Most of the time, the information offered about complete commercial diets does not include all nutrients, but it is always possible to calculate the protein:energy ratio and energy density. The individual target protein:energy ratio, considering protein and ER for a specific pet, may be used to choose a commercial complete diet, whether dry or wet, adapted for this dog or cat. The daily amount is calculated using the energy density (kcal/g as fed) of the chosen food. For instance, consider a dry food with 3.5 kcal ME/g as feed. In this example, 640/3.5 = 182 g, rounded to 180 g, is recommended on the prescription form.

3) Feeding a homemade diet-and principles of calculation

Homemade diets were simplified to make them easier to prepare, which may help pet owners’ compliance with the recipe to achieve proper coverage of nutrition requirements. Therefore, they will mostly include 4 to 5 ingredients: a source of animal protein with adequate amino acids profile (muscle), a source of essential fatty acids (canola oil), a source of fiber and volume (vegetables), a source of calcium, 6 trace elements, 12 vitamins (specific mineral vitamin supplement), and in most cases a source of energy from starch (carbohydrates, such as rice and pasta). Treats and supplements may be added.

From the energy and protein requirements and the target protein:energy ratio in the final diet, the protein source is chosen with a protein:energy ratio higher than the final target in the entire food. The amount meat/fish is calculated considering that it will bring 80% to 100% of protein, depending on the species (higher in cats than in dogs) and on the other components of the diet (more legumes as veggies and a significant part of energy from carbohydrates allows the lowest, while almost all protein will come from an animal source when low energy vegetables and no specific carbohydrate source are included).

Muscle meat/fish filet provides essential amino acids, arachidonic acid, phosphorus, and vitamin B12. Meat/fish will be cut into pieces in accordance with the pet format and steamed or cooked with water in a pan for a few minutes to reach 70°C, suppressing any sanitary risk.

·Partition between Carbs and Fat: depending on the dog’s tolerance to carbohydrates [99,131], the ER, the appetite, and more energy will be obtained from carbohydrates or fat.

·A% of the MER from vegetables to provide fiber: the minimum would be 4% of the MER, but can go up to 10%–15% in the case of low ER and high appetite. Vegetables are boiled or steamed, cooked enough to be easily mashed. In cats that refuse vegetables or show a poor appetite, vegetables can even be replaced with 2 g of wheat bran per 4 kg cat per day to provide fiber without volume.

·A% of MER from canola or soybean oil to provide sufficient linoleic and alpha-linolenic acids. The vegetable oil is included raw. This % may be more, up to 10% to 15% if more fat is required. In such cases, fat can also be included using a fatter meat—saturated fat being very well used and tolerated by dogs and cats.

·The sum of energy, protein, and all nutrients brought by all 3 first ingredients may show the nutrients still to be included. A mineral vitamin supplement containing at least calcium, 6 trace elements, and 12 vitamins may, at this stage, be included.

In case a treat is added to the daily recipe, the corresponding energy is retrieved at this stage. The remaining energy will be brought either by a source of carbohydrates or by the addition of animal fat. All ingredients except carbohydrates may be mixed and frozen into daily portions and thawed in the fridge when needed. Carbohydrates must be cooked on the day of distribution.

The sum of all ingredients as prescribed, considering the rounding of quantities, and all nutrients, is then controlled to ensure the diet covers the target food recommendation.

The entire recipe will be provided on a prescription form, including detailed information on the ingredients and directions on how to prepare, cook, preserve, and distribute [128].

4) Feeding a mixed diet

A mixed diet is an option in several situations, mainly when the owner is requesting it, but also when no commercial diet is available for a specific situation. The daily nutrition prescription may then include a portion of energy from the commercial diet, even with part in the form of dry and/or wet commercial foods and part in the form of a home-cooked diet. The detailed composition of commercial foods for micronutrients is rarely available. Hence, the best approach is to choose a diet formulated for the target, either for seniors or for a specific condition.

The home-made part can be formulated based on the requirements of the cat or dog, and the composition of the commercial foods chosen. This way, the total energy and protein requirements may be easily covered. The home-made part must be balanced for micronutrients because the commercial part does not compensate for the deficiencies when the home-made part contains only meat, veggies, and rice. Some canola oil is added to bring essential fatty acids. The mineral content, particularly calcium, is adjusted, and trace elements and vitamins are added according to the mineral vitamin supplement chosen to balance the home-made part of the diet.

This way of feeding allows an individualized adaptation of the sources of energy to the pet's capacity because the home-made part may include more carbohydrates or more lipids. The volume of the diet may also be adjusted to the appetite because of the variable portion of vegetables included. All information shall be provided in a prescription form.

Section 4. Nutritional adaptation in some age-associated diseases

Beyond the signs of aging, the prevalence of certain diseases increases with age, even in apparently healthy animals [6,13,132], such as cognitive decline, signs of OA, and cancer. In other cases, the treatment of some conditions may differ in aged pets compared to young adults [133], considering the condition or frailty at this stage.

Regardless of the condition considered, it is of high interest to the pet to always consider the 3 major nutritional considerations that improve the aging process: energy intake to maintain or reach optimal weight, protein intake to maintain lean mass and immunity, and provide all essential nutrients to cover requirements.

Cognitive decline

Aged canines naturally accumulate several types of neuropathology that may have links to cognitive decline [134] and lead to cognitive dysfunction syndrome (CDS) in dogs [135]. Parenchymal lesions and vascular pathology lead to a general cognitive decline in aging dogs, as in humans [136]. The risk factors associated with brain aging and CDS have not been fully explored, but the prevalence does differ among breeds, even if larger dogs have a smaller lifespan than small ones [137,138].

Studies have shown that the signs of CDS were improved after three months in dogs given 6.5% of medium-chain triacylglycerol (MCTs) in the diet [135]. In this study, the diet contains approximately 4.3 kcal ME/g as fed, 70 g protein/Mcal ME, and 12% of ME from mcTs (as 97% as caprylic acid and 3% of capric acid).

The precise mechanisms are not fully understood in dogs. On the other hand, MCTs at least provide an alternative energy source for neurons with an impaired ability to use glucose as an energy source. Several other dietary modifications improved the illness or delayed progression of cognitive decline, like omega-3-fatty acids, as well as antioxidants, vitamins B, and mitochondrial cofactors (e.g., L-carnitine, DL-a-lipoic acid) [139]. A systematic review of enriched diets or supplements, including plant and algae extracts, that were tested to improve the cognitive function of dogs was recently presented [101]. Thus far, information on cats does not allow clear nutritional recommendations.

OA and orthopedic troubles

OA is the inflammation and ultimate breakdown of the primary components of the joints, including cartilage, joint capsule, and underlying/surrounding bone. Inflammation in the joint causes damage and ultimately erosion of the protective layer of cartilage in the joint, and pain. The main macroscopic nutritional action is to maintain the optimal body condition and muscle mass, treating overweight if present, and maintaining non-traumatic exercise as much as possible [140,141,142].

Adding long-chain polyunsaturated fatty acids (LC-PUFAs) ω3 in OA, e.g., EPA and DHA, for their anti-inflammatory benefits is well documented in dogs [143,144,145,146,147,148], even if the optimal dosage is unclear: from 266 mg [144,145,146] to 310 mg EPA + DHA/MW (Bauer 2011 [143]). Alternatives such as green lip mussel extract are suitable even if the dosage is unclear [149]. The addition of alpha-linoleic acid, a shorter chain omega-3, does not have the same anti-inflammatory effects as EPA and DHA, even for high dosage and comparable omega 6:omega 3 ratios [144,146].

In addition to LC-PUFA ω3, very few molecules have an effective role in OA in dogs. A recent meta-analysis has shown, in addition to the role of omega-3 fatty acids sensu stricto, the potential role of the same omega-3 with cannabidiol (to a lesser degree), the relative effectiveness of collagen derivatives, and the ineffectiveness of supplements containing chondroitin or glucosamine or both [150]. This work also shows that collagen derivatives (undenatured type II collagen) or eggshell membrane might be of interest, but these studies were carried out with a small number of dogs and evaluations.

Less information is available for cats, but the addition of fish oil, a source of EPA and DHA, is also considered as effective as in dogs [151,152], at a dosage of 1.88 g EPA + DHA/Mcal ME (equivalent to 94 mg/kg BW for a neutered sedentary cat) in improving the clinical signs of OA.

Cancer

Cancer is among the most common causes of death for dogs and humans in the developed world, even though it is uncommon in other domestic animals [153]. In dogs, size influences the age at diagnosis. For example, larger dogs have shorter life expectancy and increased risk for osteosarcoma than smaller dogs [154]. This observation is repeated for other cancers: dogs weighing 2.5–5 kg had a median age at cancer diagnosis of 11 years, compared to 5 years for dogs weighing more than 75 kg [155]. No evidence exists showing that nutrition can influence the incidence of cancer in dogs, but feeding plays a crucial role in the management of oncology in dogs.

First, the main risk of cancer is cachexia, which is characterized primarily by a loss of muscle mass. This result may be due to complex metabolic changes induced by cancer, a loss of appetite caused by the disease itself (increasing cytokines such as interleukin (IL)-1, IL-6, and tumor necrosis factor-α playing a role in central anorexia) and by the side effects of medications. Second, in dogs affected by cancer, there is an inflammatory state, with an increase in the plasma concentration of inflammatory catecholamines and cytokines, stress-related hormones, such as cortisol, insulin, and glucagon, which interfere with the ability to use fat as the primary energy source. Third, cancer cells consume glucose to generate energy through anaerobic metabolism, resulting in lactate as the final product. This results in a negative energy balance to the host and production of reactive oxygen species that harm healthy tissue but not the tumor cells, which have a high ability to produce antioxidants through an intermediate metabolite from the lactate formation [156]. These facts suggest that diets with low carbohydrate levels and high lipid and protein contents should be the best strategy because they can provide fat as the primary energy source and protein to support the maintenance and reconstruction of lean mass [157]. Despite the limited recent information published on cats, a similar recommendation for dogs is warranted [157]. For other conditions, regardless of the age of the pet, nutritional adaptation is an integral part of treatment, and it should be considered as it is for adult animals.

Hyperthyroidism in cats

Hyperthyroidism is more common in older cats [158]. Medical treatment is possible, but when it is not, a diet with a reduced iodine content down to 30 or 40 µg I/Mcal ME) may be effective [159]. This goal can be reached with commercial foods and with properly formulated home-made diets. This last option allows the adaptation of the diet to a situation where hyperthyroidism is not the only condition (i.e., associated with CKD).

Chronic kidney disease

CKD is the most common kidney disease in dogs, with an estimated prevalence of up to 7% [160,161], especially in older animals.

Many elderly cats also develop the clinical signs of CKD, with a reported prevalence in the general feline population of 1%–3%, and as high as 35% in populations presenting to referral centers [162].

In healthy cats, excess phosphorus, particularly inorganic phosphorus, could be responsible for the earlier acquisition of CKD [107,163], especially when the calcium to phosphorus ratio (Ca:P) is under 1:0. When the Ca:P ratio is over 1, the dose of organic or inorganic phosphorus necessary to cause renal toxicity is unclear [109,164].

For managing CKD, dietary phosphorus restriction to 0.4%DM (~ 1 g P/Mcal ME) has been shown for a long time to slow the progression of renal fibrosis and improve the survival rates [104,105,106,165].

Accordingly, the phosphorus level is low in prescription diets for CKD, and calcium is often adjusted for a calcium:phosphorus ratio of approximately 2 because it lowers phosphorus excretion [166] due to decreased absorption.

Most studies published have shown the overall benefit of prescription diets in pets with CKD, but many add the restriction of both protein and phosphorus [167,168,169]. Diets with a restricted amount of protein and phosphorus show decreased azotemia, less uremic crisis, decrease serum phosphorus, and parathyroid hormones, compared to a diet with high protein and very high phosphorus (e.g., 0.67–0.71 g protein/Mcal ME, Ca:P = 1.6–1.9, and 55–60 g protein/Mcal ME versus 4.77 g protein/Mcal, Ca:P = 1.1 and 120 g protein/Mcal ME). In such studies, as in this case, control diets may also bring very high amounts of protein, much more than the minimal requirement to maintain lean body mass, leading to more nitrogen catabolites, which is deleterious in case of CKD.

Recently, hypercalcemia has been observed in some studies [29] on cats with early renal failure (International Renal Interest Society [IRIS] stages 1–2) induced by diets high in soluble phosphorus and with a Ca:P < 1, and who were then fed a low-phosphorus (0.84 g protein/Mcal ME) renal failure diet with Ca:P = 1.9, but extremely low in protein (59 g protein/Mcal). This hypercalcemia resolved and creatinine normalized when the diet was changed to a less restricted phosphorus diet (1.4–1.6 g protein/Mcal) with the same calcium content (leading to a Ca:P ratio = 1.4–1.6 instead of 1.9) and less restricted protein (76–98 g protein/Mcal), without worsening the renal parameters after 17 months. These data are consistent with previous recommendations for protein in cats.

A summary of the published data so far shows that, at least in early stages of CKD, such a limited amount of protein (5 g protein/kg BW) is compatible with the maintenance of lean mass, and a reduced amount of phosphorus (~ 1 g/Mcal [105,106,107]) and a Ca:P ≈ 2 may still be suggested.

Such a commercial food is currently impossible to offer in Europe because the constraints proposed in the EU regulation [170] for “Support of renal function in case of chronic renal insufficiency” limits the protein content to a maximum equivalent to 90 g protein/Mcal for cats and 55 g/Mcal for dogs.

A food with a protein:energy ratio of 100 g protein/Mcal ME is needed to provide the 5 g protein/kg to a neutered and sedentary cat with an ER of 50 kcal/Mcal. The reasoning is the same for a dog that needs 60 g of protein per day for an MER of 1,000 kcal. Assume the dog has an early to moderate stage of renal insufficiency, and the aim is to provide the dog with 20% less protein, i.e., 60/1.2 = 50 g of protein. If this dog is neutered with reduced activity, its ER can drop to 640 kcal. Therefore, a food with a protein:energy ratio of 50 g/0.640 Mcal = 78 g/Mcal EM is needed. This is higher than 55, and no commercial diet for CKD will be available for this dog. Nevertheless, coupled with a phosphorus restriction to 1 g/Mcal, and added EPA and DHA, this food would be well-suited to this dog with IRIS stage 2, or even 3.

Therefore, dogs and cats must consume sufficient calories from an appropriate diet to maintain a body condition score of 4 or 5/9 because underweight dogs have been shown to have a smaller survival rate in cases of CKD [171]. Protein is a factor of palatability, especially in cats [172]. In these cases, offering a home-cooked or mixed ration adjusted to the animal is possible and may even be wise in this context, especially if this dog has difficulty consuming enough of the prescription food for CKD.

High EPA and DHA have been associated with longer survival in cats with CKD, with a recommended dosage of 2 g/Mcal in cats, equivalent to approximately 100 mg (EPA + DHA)/kg BW [173] and 40 mg (EPA + DHA)/kg MW in dogs [143].

Miscellaneous

The usual recommendations for any disease should be adjusted to the requirements of the aged pet on a case-by-case basis. If the pet has a dental problem, it is obviously advisable to offer wet, soft, even mashed food. If the appetite is poor, selecting food with sufficient energy density allows a reduction in the quantity of food consumed. It is also possible to divide the daily ration into several meals. As the ability to smell may be impaired, heating the food may increase the flavor and encourage appetite.

In aged dogs suffering from obesity, which is quite a common condition, the treatment of obesity is conducted as if the dog is an adult [140,141,174,175,176,177]. Even late in life, the return to optimal body condition will be beneficial if the protein supply is sufficient to preserve fat-free mass, and the diet is balanced.

For example, in aged cats with diabetes mellitus [178,179], or in aged dogs with refractory chronic enteropathy [132], a home-made diet will be formulated with no source of carbohydrate and more energy from protein and fat, and low phosphorus, considering ageing [180].

In the case of clinically diagnosed CKD, considering the IRIS stage, the amount of protein may be adjusted by a mixed diet when commercial diets are too restrictive in protein. Such diets will be balanced and strictly adjusted to the minimal amount in cats (5 g protein/kg opt BW), and in dogs, to the minimum in the early stages, and below this minimum, depending on the severity of the stage (i.e., 45 to 50 g protein/Mcal MER instead of 60), in more advanced stages. The mineral vitamin supplement, with calcium carbonate [181] and no added phosphorus, will be adjusted to target a reduced phosphorus intake of approximately 1 g protein/Mcal ME [165,182], and to increase the Ca:P ratio to approximately 2 to lower the absorption of phosphorus [181]. EPA and DHA will be added with fish oil capsules to reach the recommended dosage in cats (approximately 100 mg (EPA + DHA)/kg BW [173]) and dogs (40 mg (EPA + DHA)/kg MW [143]).

Many combinations are possible, allowing for great flexibility. Once again, formulating homemade diets can be the best or worst and should be realized by a board-certified vet nutritionist. In addition, it must be accompanied by systematic information for pet owners, providing warnings about the risk of changing the recipe, including all ingredients and the recommended amounts of each.

Multi-pathologies

Resorting to an adapted and specific nutritional prescription is possible when several conditions or diseases are present. This does not mean mixing two foods, one for one disease, the other for another. It is then a question of considering all the constraints, from the pet characteristics and each disease, to propose an adapted nutritional strategy, either commercial foods or formulation of a home-made diet, or even a mixed diet. The prescription of a board-certified vet nutritionist may be more helpful at this stage.

DISCUSSION

Based on current knowledge, veterinarians can act on certain aspects of aging, but sometimes it is very late. Systematizing assessment and developing personalized tools could improve care, particularly by making it available earlier.

An evaluation of the condition of aged dogs and cats will certainly include more systematic interviews in the future, even with a questionnaire that could be sent to the pet owner before the annual or the geriatric visit to the vet clinic. These data may even include a nutritional assessment. The veterinarian could thus confirm during the consultation and input with clinical data, to better evaluate the need for nutritional changes. An evaluation of the individual situation of dogs and cats could also start early in aged pets, before aging becomes obvious, and in obviously aged pets, whether suffering from additional disease or not. Veterinarians should provide appropriate information and prescriptions early in pets' lives and maintain this as a lifelong concern, considering the high impact of the optimal body condition score and lifelong fat-free mass preservation on life expectancy and the risk of overweight associated with neutering and sedentarity lifestyles. The first author already offers, through an online nutrition coaching service open to pet owners and to vets, such follow-up for managing weight loss. A wider coaching service may be developed for pets' lifelong care or for aged pets...

Proper education of the pet owner about the way to preserve health and life expectancy together may include maintaining exercise, evaluating the body condition, and understanding what constitutes optimal body condition and lean mass. Pet owners may be trained to self-evaluate the optimal body condition means. When the nutritional assessment necessitates action, the veterinarian has all the tools to indicate that it is a medical reason and not a quick piece of advice between two doors. The food may be changed and chosen carefully, based on the individual requirements of the pet and not based on the brand or the food. Any nutritional prescription shall include precise feeding directions, including the foods and treats, as well as a clear description of the modalities for food transition, especially in cats.

Once a nutritional prescription is applied, monitoring is strongly recommended, at least one month later, and every month if necessary, when a clinical follow-up is needed. With this digital tool, a monthly reminder or follow-up shall be automatically sent to the pet owners when a nutrition prescription has been settled. A digital “life-long pet coaching” could include all relevant information and warnings, information to fulfill before the consultation, regular reminders, and a constant warning option, to inform the pet owner of events that should motivate a consultation, including nutrition adaptation.

The impact of age on the disease risk in humans and dogs shares some parallels. An analysis of 73,835 dogs from the US Veterinary Medical Database identified cancers, cognitive decline, renal and endocrine diseases, as well as obesity-related conditions like arthrosis, as age-associated issues [182,183]. Nevertheless, it is still too early to propose correlations between the potential behavioral, environmental, and toxicological impacts of the presence of a particular pathology. Moreover, early medical management can influence the outcomes in these populations. Advances in digital tools facilitating data collection and AI are expected to further enhance large-scale data analysis in this field in the future.

In conclusion, nutrition is a vital function that can be adapted at each stage of life, including the latest. Proper nutrition is necessary to avoid nutritional diseases at any age, including in senior cats and dogs. Veterinarians can easily use the nutritional assessment presented here as a diagnostic tool for malnutrition to explain to the owner that the situation of the pet requires a nutritional intervention. The way of feeding can be adjusted to the pet owner’s preference and the pet’s situation and tolerance, either by a change of commercial food, an appropriate homemade diet recipe, or even a mixed diet, prescribed as a medical act.