The average age of companion animals is increasing with approximately 40% of pet dogs and cats aged 7 years or older (Laflamme, 2012). Continued improvements in veterinary care and disease prevention strategies, veterinary nutrition, breeding and husbandry are just a few of the factors contributing to pet longevity, resulting in a significant population of senior small companion animals (Day, 2010; Biourge and Elliott, 2014; O'Neill et al, 2015).
Cats are considered ‘senior’ at 11–14 years old and ‘geriatric’ at 15 years and above (Caney, 2009), and while age itself is not a disease, special consideration should be given to this when they present for consultation as increased age is associated with a decline in organ function and immune response, as well as the development of physiological changes (Lumbis, 2018). 15% of cats over 12 years of age have a low body condition (Armstrong and Lund, 1996) and after 14 years of age, cats are at 15 times greater risk for being underweight (Courcier et al, 2012).
This article considers the most common causes of weight loss in the older cat through review of the definitions and pathophysiology of muscle loss, and examining the most common concurrent metabolic and endocrine diseases associated with weight loss in the older feline patient.
Assessment of weight loss
History taking is the first opportunity to try and identify changes that the owner attributes to ‘just getting older’ as potentially indicating underlying disease. During the consultation, it is important to try and ask open-ended questions, such as: ‘How has your cat been since we saw you last?’ or ‘Have you noticed any changes in behaviour?’ rather than closed questions such as ‘Is your cat eating less?’ Open questions give the owner greater scope for describing things that are concerning them and responses are usually a more honest description of changes (Esteban et al, 2014).
It is important that owners understand that even apparently small weight variations are significant; expressing the weight loss or gain as a percentage change may have a greater impact, for example a 300 g loss in a 3 kg cat may not sound much to a client but represents a 10% weight loss for the cat. Regular weight checks should be advised with the owner so that a clear record of any changes can be monitored and acted on at the earliest stage if there is any suspicion of the early stages of a disease process warranting further investigation.
The use of body condition scoring and the more recently validated WSAVA muscle condition scoring charts (Figure 1) to make a full record of the cat's weight should be encouraged any time the cat is seen at the practice. It is important to distinguish the weight loss as either fat loss, muscle loss or a combination of the two. It may be that a cat has the same overall weight because they have lost lean body mass and have a decreased metabolic rate (as lean body mass is the main driver of metabolism) and gained fat. Muscle condition scoring (MCS) is subjective, with classifications of normal, mild, moderate and severe. A score is obtained by both visually assessing and palpating the patient, with particular focus on the bony prominences and epaxial muscles, as these tend to be the earliest sites of muscle loss in both dogs and cats (WSAVA, 2016), while inguinal fat takes longer to disappear.
Muscle wastage
Sarcopenia is a reduction in lean muscle mass unrelated to disease, which becomes more evident in the senior pet; it is associated with increased risks for morbidity and mortality (Laflamme, 2018). While there have not been many studies conducted into sarcopenia of client owned cats, those that have been completed demonstrate that apparently healthy cats do lose lean body mass as they age. Laflamme (2018) found that in apparently healthy cats, approximately one third of lean body mass is lost from the age of 10 to 15 years. Peterson and Little (2018) observed that 37.6% of 255 cats over 7 years of age, that presented for a wellness examination, had evidence of muscle wastage. When following these cats over the course of their lives the prevalence of muscle wasting progressively increased, rising from 22% of mature cats (7 to 10 years) to 54% of senior cats (11 to 14 years) and up to 77% of geriatric cats (≥15 years). Muscle wasting is a result of a combination of imbalances between synthetic and degradative protein pathways (Figure 2) coupled with an in-creased rate of myocyte apoptosis (programmed cell death) and a decreased capacity for these cells to regenerate (Argilés et al, 2015).
While the number of obese cats declines after 7 years of age, a significant number of mature cats remain overweight (Williams, 2015); this means that cats may not start exhibiting signs of weight loss until they become older, however sarcopenia should be assessed each time the animal is seen to ensure the process of sarcopenia obesity is not occurring, where an increase in body fat percentage and a concurrent decrease in lean body mass are occurring making the overall body weight appear to be static or even increased. In contrast, senior and geriatric cats are prone to weight loss, justifying an increase, rather than decline, in energy requirements (Gross et al, 2010; Sparks, 2011). Energy requirements and food intake should be adjusted accordingly to maintain a healthy weight and lean body condition.
Decreased nutrient digestibility
Studies in geriatric cats have shown a reduced digestive ability and this may contribute to their increased maintenance energy requirements (Laflamme, 2002; Perez-Camargo, 2004; Sparkes, 2011). Perez-Camargo and Young (2005) showed a reduced ability to digest protein in 20% of geriatric cats, with 33% having a significant reduction in their ability to digest dietary fat; and Williams (2018) found that a fifth of cats aged over 14 have impaired protein digestibility of more than 20%. Although the cause(s) of this poorer digestive ability is still unknown, an idiopathic chronic enteropathy is believed to be involved in many cases. Indeed, many cats with impaired protein digestion also show a reduction in their ability to digest fat, which may contribute to both sarcopenia and a reduction in bodyweight because when dietary protein intake is inadequate, protein will initially be depleted from skeletal muscle, accelerating muscle wastage.
It is important to remember that some cats may compensate for this by eating more (Williams, 2018). As protein in-take is dependent on overall food intake, as well as the nutritional composition of the diet, and is closely linked to energy intake, if a cat cannot meet their energy requirement, they will be unable to meet their protein needs. When considering diet, many geriatric cats may benefit from both an increased energy level and protein content within their diet in order to compensate for poorer digestive ability and no reduction in metabolic energy requirements. Micronutrient absorption (potassium, phosphorus, sodium, choline, B vitamins and vitamin E) is also decreased in healthy cats as they age (Perez-Camargo and Young, 2005).
Unless a condition requiring protein restriction such as renal disease is present, protein restriction is believed to be undesirable (Churchill, 2018) as protein requirements are increased in the cat's senior life stage compared with the adult. Quality of protein is also an important consideration: high biological value allows the protein source to be used in the most efficient manner and has reduced bacterial metabolites (Fascetti, 2010; Hutchinson et al, 2011). Biological value measures protein quality by calculating the nitrogen used for tissue formation divided by the nitrogen absorbed from food, and provides a measurement of how efficiently the body utilises protein consumed in the diet (Hoffman and Falvo, 2004).
While sarcopenia is generally regarded as the loss of muscle tissue in the absence of an underlying disease process, it is important to work up any suspected weight loss in an ageing cat because of the prevalence of numerous other causes of weight loss.
Concurrent disease
The presence of concurrent disease may be a contributing factor to muscle wastage in some cats (Freeman et al, 2016). While there are many concurrent diseases which may contribute to muscle wastage in cats, this article aims to make observations on the most commonly documented of these conditions. Chronic kidney disease (CKD) is the most common metabolic disease of domesticated cats, with the frequency of diagnosis having increased in recent decades, and with most affected cats being geriatric (Brown et al, 2016). The most common endocrine diseases seen in the cat are diabetes mellitus and hyperthyroidism (Peterson et al, 2016)
Feline diabetes mellitus
O'Brien (2002) observed that the common form of spontaneous diabetes mellitus that occurs in domestic cats bears close resemblance clinically and pathologically to human type 2 diabetes mellitus. Given that lean body mass is a primary driver of metabolism, a decrease in this associated with sarcopenia in the absence of any disease process, can result in weight gain in some cats, generally, those under 12 years of age (Scherk, 2018). Weight gain and obesity are a risk factor for the development of diabetes mellitus in predisposed individuals, which is a hypermetabolic endocrine disease that may compound the sarcopenia already associated with increasing age (Petersen and Little, 2018). Studies in cats consistently show diabetes to be a disease of senior cats with approximately 70% of those affected being more than 10 years of age at the time of diagnosis (Rand, 2012). It has been observed in human studies that type 2 diabetes is associated with an increased risk of concurrent sarcopenia, and because skeletal muscle is a primary site for insulin-mediated glucose uptake and deposition, sarcopenia may contribute to insulin resistance (Kim et al, 2010). This has yet to be identified within the feline disease process of diabetes mellitus, however it may be theorised that sarcopenia can act both as a contributing factor to insulin resistance, and as a result of protein catabolism associated with insulin resistance and prolonged hyperglycaemia.
Hyperthyroidism
In addition to diabetes mellitus, the other most common endocrine disease seen in the cat is hyperthyroidism (Peterson et al, 2016). One of the most common presenting complaints in hyperthyroidism by the owners of such cats is weight loss despite an increased appetite; with some cats being described as ravenous. This is because, in addition to being a highly multifactorial disease, these cats lose weight as hyperthyroidism accelerates their metabolic rate to such a degree that energy demand exceeds the energy the cat can consume; the progressive weight loss and muscle wasting that is characteristic of feline hyperthyroid disease is caused by increased protein breakdown (catabolism) leading to a negative nitrogen balance (Peterson and Eirmann, 2014).
Renal disease
Weight loss is common in cats with CKD. The prevalence reported in the literature ranges from 42–82% and depends on factors such as stage of disease, concurrent diseases, and treatment (Freeman et al, 2016). Mechanisms for weight loss in CKD are multifactorial and include inflammation, malabsorption, increased energy requirements, and decreased appetite (Freeman, 2012). Weight loss in cats with CKD is important because it can negatively impact survival time through direct effects of weight and muscle loss (e.g. weakness and impaired immune function) (Lumbis, 2018). A commonly reported complication of renal disease because of the associated hypertension is the potential for retinal detachment and subsequent blindness, which may result in stress-induced anorexia (Sandmeyer and Grahn, 2008). It is important to note that other secondary complications such as stomatitis may result in decreased appetite (Langston and Eatroff, 2016).
Cachexia
Cachexia refers to a complicated metabolic syndrome resulting in weight and muscle loss secondary to chronic inflammation or disease. The causes of cachexia are not fully understood. Inflammatory mediators, oxidative stress, upregulation of protein turnover pathways and hormonal mediators appear to play a role in this complex process. Heart failure, neoplasia, advanced kidney disease and failure are often associated with cachexia (Finn et al, 2010). Muscle loss is more rapid and therefore, more obvious in cachexia compared with sarcopenia, and may also include fat loss. The most effective approach to cachexia is through early treatment of disease and nutritional intervention directed towards preventing muscle wasting. As older cats are predisposed to those conditions in which cachexia occurs, a goal in this population is to achieve and maintain a slightly higher body condition score (e.g. 6–7/9 or 4/5) as this appears to reduce the risk for mortality (Scherk, 2018). The weight loss associated with cachexia is different to weight loss seen in a healthy cat that loses weight; for example, in a healthy cat that is receiving too few calories to meet their metabolic energy requirement, their metabolism adapts to allow fat reserves to be used as the primary fuel source, which means that muscle (lean body mass) is preserved as much as possible. Conversely, the primary fuel source in cats exhibiting cachexia seems to be amino acids, meaning that these patients break down lean body mass (Freeman, 2009). The loss of lean body mass has been shown to have a direct negative effect on strength, immune function and survival (Lumbis, 2018).
Other reasons for weight loss
In addition to the above-mentioned conditions, there are numerous other reasons why an ageing cat may present with weight loss. A conversation should always be had with the owner in the first instance to decipher if the cat can physically eat, i.e. does it have chronic oral pain as a result of stomatitis, oral tumour, feline resorptive oral lesions, or acute pain as the result of traumatic injury, such as a collision with a car resulting in jaw fracture. If the former is ruled out on physical examination and/or radiography, then attention should be turned to why the cat does not wish to eat and several age-related changes should be considered at this point, including age-related loss of olfactory receptors and fibres, thereby reducing sense of smell (Boyce and Shone, 2006). Older cats have been shown to be at statistically higher risk of constipation, which may result in inappetence because of the associated discomfort (Benjamin and Drobatz, 2020). The environment should be considered when taking the history; perhaps the cat can no longer easily jump up to access the feeding bowl because of osteoarthritis (OA). Recognising chronic pain caused by OA can present a challenge in that most cats will not exhibit lameness (Slingerland et al, 2011). The main features of feline OA are insidious changes in behaviour and lifestyle, which owners often perceive as ‘getting older’ when they present for consultation. Despite overt lameness not being a commonly reported sign, one study of 100 cats found 61% of cats had OA in at least one joint and 48% in multiple joints (Slingerland et al, 2011). Decreased activity as a result of pain attributed to arthritis, leading to reduced lean body mass, should be considered when assessing an older cat for weight loss (Bennett et al, 2012).
Studies have shown that old age in the cat may produce behavioural changes associated with cognitive dysfunction syndrome. This may manifest in signs such as vocalisation, night-time wandering confusion regarding their location or location of feeding stations and litter boxes, and inappropriate elimination. An older cat that may be displaying characteristics of this syndrome may be losing weight as they can no longer remember where they are fed (Landsberg et al, 2010).
As well as anatomical and physiological causes of anorexia, environmental causes should also be considered, for example stress caused by a new baby, moving house, building work etc (Amat et al, 2016).
Conclusion
While senior and geriatric cats can be at increased risk from disease processes which result in weight loss, it has been found that muscle loss can occur independently in older, but otherwise healthy cats. It is recommended that a full history is taken at every consultation to identify reasons for any potential change in bodyweight, and that the use of client questionnaires and nutritional assessment checklists form a routine part of every visit (Rollins and Murphy, 2019). The outcome of such an assessment will dictate what, if any, dietary changes are required. Such checklists can be found on the WSAVA global nutrition committee toolkit (2016). When considering nutrition in these cats, protein and fat restriction may be contraindicated unless the cat has a specific clinical disease requiring such an adjustment (which should be done in line with available PARNUTS (feed intended for PARticular NUTritional purposes) from the European Pet Food Industry Federation (FEDIAF)). Many geriatric cats may benefit from both an increased energy level and high-quality protein content within their diet in order to compensate for poorer digestive ability and no reduction in meta-bolic energy requirements (Sparkes, 2011).
KEY POINTS
- Special consideration should be given the patient's age when they present for consultation, as increased age is associated with a decline in organ function and immune response, as well as the development of physiological changes.
- The use of body condition scoring and muscle condition scoring charts to make a full record of the cat's weight should be encouraged any time the cat is seen at the practice. This is to distinguish the weight loss as either fat loss, muscle loss or a combination of the two.
- Sarcopenia is a reduction in lean muscle mass unrelated to disease which becomes more evident in the senior pet; it is associated with increased risks for morbidity and mortality.
- When considering diet, many geriatric cats may benefit from both an increased energy level and protein content within their diet in order to compensate for poorer digestive ability and no reduction in metabolic energy requirements.
- Unless a condition requiring protein restriction such as renal disease is present, protein restriction is believed to be undesirable, as protein requirements are increased in the cat's senior life stage compared with the adult.
- While sarcopenia is generally regarded as the loss of muscle tissue in the absence of an underlying disease process, it is important to work up any suspected weight loss in an ageing cat because of the prevalence of numerous other causes of weight loss.