Managing body composition in senior pets

Introduction

The life expectancy of our pets is increasing (1); the average life expectancy at birth for cats and dogs in the US has been estimated to be 11.2 years and 12.7 years respectively (1), although many will live much longer than this. In the UK alone there are an estimated 10.6 million cats (2), and around 30% of these may be older than 10 years of age. Meanwhile, nearly 50% of the UK dog population is aged seven or older, with around 2 million being 12 years or above (3).

With a growing population of aging pets comes a growing appreciation of the need to support them and improve their welfare as they grow older. Cats are considered to be mature between 7-10 years of age, and senior when more than 10 years (4). Due to differences with breed and size, dogs are deemed to be senior when they are in the last 25% of their estimated lifespan (4). An increased lifespan, however, does not necessarily equate to an increase in healthspan; this is defined as the length of time living in good health. Senior pets are at increased risk of developing multiple age-related diseases that can obviously impact on their health and quality of life. However, they are also more likely to exhibit changes in their body composition, which can contribute to frailty, and this also affects their quality of life (5,6). Understanding how both age and age-related morbidity affect body composition is important if we want to improve the healthspan of our pets.

This article will consider approaches to supporting and maintaining healthy weight in senior cats and dogs. Firstly, it will offer an overview of the methods used to measure body composition, followed by consideration of how age and age-related diseases affect body composition, along with a proposal of how a “healthy weight” can be defined. Finally, some practical tips for supporting healthy weight in senior pets will be discussed.

How do we measure body composition?

Three main metrics are used to assess body composition in clinical practice: body weight, body condition score (BCS) and muscle condition score (MCS), the first two being most often utilized. For example, body weight was recorded in over 95% of cats attending UK primary care practices in a recent study of electronic health records, although BCS was only recorded in 22.5% of cases (7). MCS is performed less often but gives valuable information that can provide insights into healthy aging.

Body weight has the benefits of being a quick, readily obtainable objective measurement that is easily reproducible and which enables small changes to be identified and monitored. However, body weight differs by breed and overall stature of the animal, and cannot distinguish between tissue types, such as fat and lean mass. Conversely, BCS and MCS are more subjective, since each requires a clinician to make judgements against a rubric of descriptors, but they provide a greater insight into aspects of body composition than body weight can offer. Although different systems have been developed, the WSAVA recommends a 9-point BCS, not least because it correlates with body fat percentage as measured by dual-energy X-ray absorptiometry (DEXA), with low inter-observer variation. In this respect, one point change on the BCS scale is equivalent to approximately 10% change in body weight.

MCS is a measurement of the amount of muscle coverage an animal has, and WSAVA provides appropriate charts for both cats and dogs in their nutrition toolkit. Using a four-point scale, MCS can be graded from no muscle wastage (3/3) through mild and moderate wastage to severe muscle wastage (0/3), identifiable over different anatomical sites (Figure 1), and the score is easily incorporated into clinical notes. This scoring system has been validated against objective measurements of lean mass made both by DEXA (8) or ultrasonography by measuring epaxial muscle height (9). Scores from different body regions have also been summed to produce a composite score of up to 30, and used in epidemiological studies in cats (6). Recording muscle mass is useful, not least because BCS and MCS do not always correlate; it is possible to have an overweight animal with muscle wastage.

Figure 1. The skeletal landmarks that can be palpated to ascertain muscle condition in the cat; the same landmarks are applicable to the dog. 

©From (6). Licensed under CC BY/redrawn by Sandrine Fontègne

 

How does aging affect body composition?

In humans, there tends to be an increase in body fat during middle age years before a decrease in both body weight and body fat in older age. The same is true for cats and dogs (6,10), but muscle condition tends to decline earlier, with decreases in lean body mass occurring from middle age onwards (6,10) – again similar to the human situation. Understanding that a decline in muscle mass begins earlier than a decline in fat mass emphasizes the importance of recording muscle condition in older pets. Age-related diseases will impact body composition, but decreases in body fat and lean mass are observed in old age even in animals with no diagnosed disease, alluding to the effect of underlying aging mechanisms on body composition (6,11).

The reasons for this age-related change in body composition are complex and multifactorial. They include a loss in the ability to maintain energy balance, with the decline in energy requirements in middle age predisposing to obesity, before the raised energy requirements in senior years increases the risk of weight loss (5). Aging also affects nutrient digestibility and absorption, with decreased protein and fat digestibility reported in older cats and dogs further contributing to weight and muscle loss (11). At a cellular level, mitochondrial dysfunction, oxidative stress and chronic low-grade inflammation (known as “inflammaging”) play important roles in driving loss of lean mass and changes in fat distribution (11). Together, these mechanisms explain why body composition shifts are a feature of aging, even in the absence of overt disease.

How does age-related disease affect body composition? 

Apart from changes in body composition caused by aging itself, many age-related diseases will impact body composition in cats and dogs, with some of the more common ones discussed here. 

Chronic kidney disease

In a study of 569 cats with chronic kidney disease (CKD), cats lost weight for three years prior to diagnosis, but lost weight more rapidly after diagnosis; weight loss was also associated with shorter survival (12). In dogs with CKD, decreased BCS and MCS has also been associated with shorter survival (13).

Endocrine disorders

Hyperthyroidism in cats leads to loss of body weight, primarily due to loss of muscle mass rather than body fat; successful treatment of the condition will lead to weight gain and increased BCS, but many animals fail to regain normal muscle mass (14). Hypothyroidism, more common in dogs, is typically associated with an increase in body weight and body fat, mainly because of a reduction in the resting metabolic rate, but it has also been correlated with muscle atrophy, likely due to peripheral neuropathies and hypothyroid myopathies.

Overweight and obesity are risk factors for the development of diabetes mellitus in both cats and dogs, due to increased risk of insulin resistance, which compounds pancreatic beta cell dysfunction. Weight loss and muscle atrophy are commonly seen in untreated animals, although many affected individuals will still have excess body fat at the time of diagnosis (15).

Cardiovascular disease

Loss of muscle mass is also associated with cardiac disease (termed cardiac cachexia) in both dogs and cats. This has been associated with shorter survival times in pets diagnosed with congestive heart failure compared to those without muscle loss (5,16).

Musculoskeletal disorders

Conditions such as osteoarthritis are associated with accompanying muscle wastage of the affected limbs due to disuse atrophy. A reduction in activity levels can also lead to an increased risk of gaining body fat, which in turn can compound the condition by increasing body weight. Increased body fat is a risk factor for osteoarthritis in both cats and dogs, and weight management can improve lameness and quality of life in these animals (17).

How do we define “healthy weight” in aging pets?

Before discussing how to best support maintenance of a healthy weight in aging pets, we must first consider what a “healthy weight” actually is. To the authors’ knowledge no such definition currently exists; therefore, in our opinion, a “healthy weight” is the weight (or, strictly speaking, weight range) at which healthspan is maintained for the longest possible time. More specifically, at such a weight, the amount of muscle mass is optimal, and the amount of adipose tissue should not be excessive (Figure 2). In humans, such a weight range can be determined with reference to the body mass index (BMI), which is associated with all-cause mortality by a U-shaped trajectory. For an adult, Caucasian population, a “normal” BMI (whereby all-cause mortality is lowest) is between 20-25: at BMI’s below or above this range, the risk of mortality increases (18). However, BMI has known limitations, including its inability to differentiate between lean mass and fat mass, and its applicability may vary across populations.

Figure 2. The concept of “healthy weight” in cats and dogs (strictly speaking a weight range rather than a single value) is where muscle mass is optimal and adipose tissue mass is not excessive. Healthy weight is the weight range at which “healthspan” (defined as the length of time living in good health) is maintained for the longest possible time. 

© Redrawn by Sandrine Fontègne

 

In a cross-sectional study of over 2,000 cats attending a primary care practice in Sydney, Australia, cats with a maximum BCS of 6/9 had the longest lifespans, whilst those with a maximum BCS of ≤ 4/9 or 9/9 had the shortest lifespans, with an increased risk of death (19). However, in a different study involving the same cohort of cats, animals with a BCS of ≥ 6/9 were associated with an increased risk of various morbidities, including dermatological problems, musculoskeletal disorders, hypertension, asthma, oral disease, diarrhea, urinary tract conditions and diabetes mellitus (20). Cats with a BCS of 5/9 had the smallest odds of gastrointestinal and upper urinary tract conditions. Besides these data, in a longitudinal cohort study of aging cats (6), body weight, BCS and MCS decreased after ten years of age, but there were steeper declines in cats that developed chronic diseases or subsequently died over the study period (Figure 3). In the same cohort, any amount of muscle wastage at middle age was associated with a shorter lifespan. Therefore, current evidence suggests that the “healthy weight” range in cats corresponds to those weights where the BCS is either 5/9 or 6/9, with an MCS of 3/3.

Figure 3. Changes in body weight, BCS and MCS in male cats enrolled on the Cat Prospective Aging and Welfare Study, separated by whether a cat aged without (“Healthy”, orange line) or with (“Not healthy”, purple line) the development of diagnosed morbidities. Cats that developed age-related morbidities lost body weight, BCS and MCS at a greater rate than those cats that aged healthily. MCS was measured using ten skeletal landmarks, each graded 0 (severe muscle loss) to 3 (no muscle loss), up to a combined score of up to 30. Note that a gender difference (not shown here) was observed with body weight and BCS but not with MCS. 

© From (6). Licensed under CC-BY/redrawn by Sandrine Fontègne

 

In a study in dogs, life expectancy was significantly greater for animals with a median of 4-5/9 and less for those with BCS 1-3/9 or 8-9/9 (1). In dogs aged 6.5 to 8.5 years grouped into “overweight” or “normal” body condition, animals in the former category had a shorter median lifespan compared with those in the latter category for all 12 breeds studied, but particularly Yorkshire Terriers (21). Muscle loss contributes to the frailty phenotype in dogs, which also corresponds to reduced lifespan. There are several canine frailty-related phenotype clinical tools available, and dogs scoring more on these scales at clinical assessment from age 5 onwards had decreased lifespans (22). Severe muscle wastage of the epaxial muscles at age 12 (range 9-17 years) was associated with decreased lifespan in various dog breeds (23). Therefore “healthy” weight in dogs corresponds to a BCS of 4/9 to 5/9, with an MCS of 3/3. 

How do we maintain or support healthy weight in senior pets?

Table 1 gives an overview of the steps required to maintain or support healthy weight in senior cats and dogs, with each point discussed in more detail below.

 

Table 1. Key steps to maintain or support healthy weight, depending on an animal’s current body condition.

Cats and dogs at healthy weight	Cats and dogs that are underweight	Cats and dogs that are overweight
Recognize, treat & manage any chronic disease	Recognize, treat & manage any chronic disease	Recognize, treat & manage any chronic disease
Regular nutrition assessments	Attempt modest weight gain towards healthy weight	Attempt modest weight loss towards healthy weight
Regular healthy weight checks	Maintain muscle mass	Minimize lean tissue loss
	Use food, drugs and nutritional support when needed	Compromise with the chosen target weight set

 

 

Cats and dogs at healthy weight

Recognize, treat and manage any chronic disease. Age is a risk factor for disease and, as such, it is important to examine for possible chronic diseases in senior animals, regardless of their body composition. A thorough history and complete physical examination should be undertaken, including oral and orthopedic examinations (if possible), given the high prevalence of dental and musculoskeletal disease in older animals (4). Investigations including systolic blood pressure measurement, hematology, serum biochemistry and urinalysis will enable existing conditions to be identified, whilst routine investigations from middle age onwards will facilitate the early detection of such conditions as soon as they develop. In cats over 9 years of age, measurement of serum thyroxine concentration should also be considered.

Regular nutrition assessments. Nutritional assessment is one of the five vital signs that should be reviewed at every consultation. To this end, the WSAVA provides a checklist that can aid in screening those animals at risk of not having their nutritional requirements met. The current recommendation is that such a nutritional assessment, along with accompanying specific dietary recommendations, be conducted at every veterinarian visit. All cats and dogs in their senior life stage require close scrutiny, although if there are no other causes for concern, an extended nutritional assessment is not necessarily required.

Regular healthy weight checks. As discussed above, body weight is a key metric of health, and regularly weighing of cats and dogs throughout their life is a sensitive way of detecting possible health issues as they develop. The authors recommend that veterinary professionals set a healthy weight in early adult life for all patients, which is recorded in the clinical notes, and then endeavor to weigh cats and dogs under their care at least twice a year throughout adult life (and more often during senior years). A nutritional assessment can also be performed at the same visit. The calculation for percentage body weight change is shown in Box 1; any deviation of ±5% from this healthy weight should be flagged and investigated for the cause. Whilst body weight changes are the most sensitive for monitoring purposes, it is also recommended that BCS and MCS be monitored and recorded at least annually, with any changes in these metrics prompting further investigation.

 

Box 1. Calculation of percentage (%) change in body weight.

% change in body weight = (New body weight / Previous body weight) × 100 - 100

For example, a dog whose body weight is now 23.5kg, but was 25.7kg at the previous visit, has a % change in body weight of (23.5 ÷25.7) × 100 - 100 = -8.56%. Any unexplained change of >±5% body weight should be investigated.

 

 

Cats and dogs that are underweight

Recognize, treat and manage any chronic disease. Underweight animals are more likely to have certain age-related diseases, as outlined above. Therefore, for animals in this category, comprehensive diagnostic investigations are usually necessary in order to identify and diagnose possible underlying causes of weight loss, poor body condition or muscle wastage, with subsequent treatment and management tailored accordingly (4). The exact investigations will depend upon the individual case and clinical signs, but will likely involve a combination of clinicopathological tests and diagnostic imaging in the first instance; treatment options are beyond the scope of this article. It is recommended that veterinary professionals consult relevant staging systems and guidelines (e.g., guidelines of the International Renal Interest Society) when deciding on treatment and any necessary follow-up monitoring.

Attempt modest weight gain towards healthy weight. In addition to appropriate therapy for the underlying condition(s), it is sensible to increase food intake to facilitate weight gain and return the pet to its healthy weight range. The aim is steady, sustainable progress, since rapid weight gain may exacerbate concurrent disease or predispose to metabolic disturbances. However, strategies to help weight gain must be carefully tailored to the pet’s life stage and health status. Specific guidance for achieving such weight gain involves:

a. Evaluating current food intake and compare with energy requirements. Estimating maintenance energy requirement (MER) and adjust for lifestyle, disease, or recovery status provides a foundation for planning. 

b. Creating an individualized monitoring plan. Frequent weight checks (e.g., every 2-4 weeks) as well as tracking of BCS and MCS are essential. Owners can be encouraged to monitor appetite and record food intake at home.

c. Offering energy-dense, highly digestible diets. Senior cats and dogs have an increased requirement for dietary energy and protein. Diets should also be highly digestible to compensate for age-related changes in gastrointestinal function. Occasionally, it can be challenging to increase protein intake if an animal is suffering from CKD, since raw materials that provide protein usually also increase phosphorus content. Therefore, where possible, diets specifically formulated for senior underweight pets are ideal.

d. Addressing behavioral or environmental barriers. Older cats and dogs may be more sensitive to environmental stressors, such as noise, competition for food in multi-pet households, or unappealing food placement (e.g. near litter trays). Cognitive decline can also impact feeding routines, as affected individuals can become disorientated and confused as well as having altered sleep:wake cycles. Mobility limitations may prevent older pets from reaching feeding areas, climbing stairs, or standing comfortably to eat. Practical adjustments, e.g., by providing quiet feeding areas, raised bowls for pets with arthritis, or multiple feeding stations in multi-pet homes, can improve intake. For visually impaired pets, keeping feeding stations in a consistent location is particularly important.

e. Optimizing palatability and feeding frequency. Senior pets may have diminished senses of taste or smell, reducing appetite. Warming food, offering wet diets, or dividing daily rations into several small meals can encourage better intake.

Maintain muscle mass. Preserving lean tissue is critical, so adequate dietary protein should be provided, and where appropriate, owners can be encouraged to support gentle strength-building activities, such as interactive play or low-impact exercise tailored to the pet’s ability.

Use food, drugs and nutritional support when needed. Dietary adjustments should also account for concurrent disease (e.g., renal, hepatic or gastrointestinal conditions). Including long-chain omega-3 fatty acids, antioxidants and medium-chain triglycerides might help to support metabolism and cognitive function (11). If voluntary intake remains insufficient, nutritional support may include appetite stimulants, assisted feeding, or even feeding tubes in select cases, always balancing the benefits against the pet’s overall quality of life. 

A case example for methods employed to support healthy weight in a senior cat with CKD and hyperthyroidism is provided in Figure 4. 

Figure 4. Progression of body weight, BCS and MCS in a 9-year-old neutered female domestic shorthair cat that presented with weight loss after a diagnosis of IRIS stage 2 chronic kidney disease. An esophageal tube was initially placed for a short time as a supportive measure, leading to an increase in body weight and BCS. A low phosphorous renal diet was fed, but the cat gradually lost weight and BCS over a 12-month period. Mirtazapine was then prescribed as an appetite stimulant, and body weight stabilized for a few months before decreasing again. At this point normal cat food was added to the renal diet to encourage weight gain, and the cat gained 11% of her body weight over 8 months. There was then a noticeable drop in body weight of 24%, with a decrease in one BCS point and a decrease in MCS. Hyperthyroidism was diagnosed and treatment started with carbimazole, leading to an improved body weight and BCS (but not MCS). After 8 months there was further deterioration, with decreases in body weight, BCS and MCS, and despite providing support with a naso-esophageal feeding tube for a 2-week period to improve quality of life, ultimately euthanasia was the preferred option. 

Abbreviations: BCS = body condition score, measured on a 9-point scale. MCS = muscle condition score, measured on a 4-point scale from no muscle loss (A) to severe muscle loss (D). N-Oe = naso-esophageal tube, Oe = esophageal tube. 

NB: The dates shown in the figure are presented in US format (MM/DD/YYYY). For the MCS, the letters B, C, and D above correspond to the numbers 1, 2, and 3 respectively, as outlined in the text.

© Redrawn by Sandrine Fontègne

 

Cats and dogs that are overweight

Recognize, treat and manage any chronic disease. As discussed above, obesity increases the risk of conditions such as diabetes mellitus, osteoarthritis, and hypertension. Screening for these comorbidities is advisable before initiating a therapeutic weight reduction program.

Attempt modest therapeutic weight reduction towards healthy weight. In some situations, modest weight reduction can produce health benefits in overweight senior dogs and cats, for example by improving quality of life, improving function, decreasing chronic pain or reducing the impact of an associated disease (24). Typically, a partial, rather than complete, therapeutic weight reduction plan is advisable, with the target weight set at a point that maximizes clinical benefits, whilst minimizing negative consequences (such as lean tissue loss). Such partial weight reduction protocols lead to quicker average weight loss whilst minimizing lean tissue loss (25). Veterinary therapeutic diets designed for controlled weight reduction are preferable, because they provide balanced nutrition, despite the energy restriction. The target for such a partial plan should be tailored to the individual animal (24), but an initial goal of losing one BCS score (approximately 10-15% of body weight) is realistic and motivating for many animals and caregivers.

Minimize lean tissue loss. Even during therapeutic weight reduction, maintaining muscle mass is vital. Feeding an appropriate dietetic food with an increased protein content (to provide essential amino acids) and additives, such as L-carnitine, can help maintain lean mass during the process. Regularly monitoring MCS helps to identify any concerns with excessive lean tissue mass loss.

Compromise with the set target weight. Owner engagement is key to success, and setting overly ambitious targets risks frustration and loss of compliance. Compromise, realistic goals and regular positive reinforcement help keep caregivers motivated, improving the likelihood of long-term weight management success (24,25).

A case study of therapeutic weight reduction in a senior dog is provided in Figure 5. 

Figure 5. Therapeutic weight reduction in a 12-year-old Beagle dog that had concurrent bilateral stifle osteoarthritis secondary to cranial cruciate ligament disease, with the aim of improving mobility after surgery (tibial plateau levelling osteotomy; TPLO) on his right pelvic limb. Although the dog’s “ideal weight” was estimated to be 15kg, a partial weight reduction plan was decided, with a target weight of ~ 20 kg. This was chosen as a compromise between the benefits (improved mobility and quality of life) and risks (Iean tissue loss) of weight reduction, as well as aiming to ensure owner compliance and motivation. Over 201 days the body weight loss was 27%, averaging 0.9% per week. (a) The dog before therapeutic weight reduction, with an initial weight of 27.2 kg. (b) The same dog after therapeutic weight reduction, with a final weight of 20.6 kg. 

Credit Royal Canin Obesity Care Clinic, University of Liverpool, UK.

 

Conclusion

Maintaining healthy weight in senior cats and dogs is an important factor in supporting healthspan. The aging process, as well as age-related disease, leads to shifts in body composition, including muscle loss and changes in fat distribution, that can impact a pet’s quality of life. Body weight alone is not sufficient to monitor these changes, and body condition score and muscle condition score should also be recorded. Supporting healthy weight involves proactive screening for chronic disease, regular nutritional assessments, and consistent monitoring of weight and body composition. Ultimately, the goal is not only to manage body composition but to optimize healthspan in these animals. Through early recognition of changes in body composition metrics, practical intervention and collaboration with owners, clinicians can significantly improve the wellbeing of aging pets.

 

Conflict of interest statement Both authors are employees of the University of Liverpool, but their positions are funded by Royal Canin. AJG has received financial remuneration and gifts for providing educational material, speaking at conferences, and consultancy work.

 

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