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Introduction

Obesity is defined as an accumulation of excessive amounts of adipose tissue in the body. In humans, strict definitions of the degree of adiposity exist, and these are based upon epidemiological data. Overweight cats and dogs are greater than 15% above their ideal weight, whilst the term 'obese' is used when current weight is 30% above ideal. Disease associations have been proven for companion animals that are overweight as well as those that are obese.

Causes of Obesity

The main reason for development of obesity is an imbalance in the 'energy balance equation', in that either excessive dietary intake or inadequate energy utilisation causes positive energy balance leading to increased body fat stores. Numerous factors may influence the relative ease with which weight is gained, and these include genetics, age, neuter status, amount of physical activity and the caloric content of the diet.

It is known that a number of canine breeds are predisposed to develop obesity, e.g., Labrador Retriever, Cairn Terrier, Cavalier King Charles Spaniel, Scottish Terrier, Cocker Spaniel. Neutering is also an important risk factor in both cats and dogs, by causing behavioural changes, which lead both to an increased food intake and to decreased activity. Gender itself is also a predisposing factor in some canine studies, with females over-represented. Other recognised associations in dogs include indoor lifestyle, inactivity and middle age. In cats, middle age and apartment dwelling are possible risk factors.

Dietary factors can also predispose to obesity in both species, with both the number of meals and snacks fed and the feeding of table scraps being key. Further, obese cats more commonly have free choice of food intake. Behavioural factors also play a part in the development of obesity. For cats, possible factors involved in development of obesity include anxiety, depression, failure to establish a normal feeding behaviour and failure to develop control of satiety. The human-animal relationship is also of importance, and has been shown to be more intense in owners of obese cats. Further, misinterpretation of feline behaviour on the part of the owner is relevant; in this regard, many owners misread signals about the behaviour of their cat with regard to eating. In contrast to humans and dogs where eating is a social function, cats do not have any inherent need for social interaction during feeding times. When the cat initiates contact, owners often assume that they are hungry and are asking for food when they are not. Nevertheless, if food is provided at such times, the cat soon learns that initiating contact results in a food reward.

Disease Risks Associated with Obesity

In humans, the medical importance of obesity lies in its effects on lifespan and on predisposing to other diseases. Obese humans, on average, do not live as long, and are more likely to suffer from conditions such as diabetes, high blood pressure, coronary heart disease, certain types of cancer (e.g., breast, ovarian, prostate), arthritis and respiratory disease. Similarly, obesity has detrimental effects on health and well-being of dogs and cats.

Clinical Evaluation, Physiology and Anaesthesia

Overall, clinical evaluation is more difficult in an obese patient compared with a patient in ideal body condition. Techniques that are complicated by obesity include physical examination, thoracic auscultation, palpation and aspiration of peripheral lymph nodes, abdominal palpation, blood sampling, cystocentesis and diagnostic imaging (especially ultrasonography). Anaesthetic risk is also reportedly increased in obese companion animals, and problems include estimation of anaesthetic dose, catheter placement and operating time. Finally, decreased heat tolerance and stamina have also been reported in obese animals.

Longevity

A recent prospective study has demonstrated an effect of obesity on lifespan in dogs. Twenty-four pairs of Labrador Retrievers (48 total) were used, and one dog in each pair was randomly assigned to one of two groups. The dogs in one group were fed ad libitum, whilst the dogs in the other group were fed 75% of the amount consumed by the respective pair. In the energy-restricted group, body condition score was closer to 'optimal' (e.g., group mean 4.5/9) than in the ad libitum feeding group (e.g., group mean 6.8/9); lifespan was also increased (e.g., 13 years with energy restriction c.f. 11.2 years when fed ad lib). Other beneficial effects included reduced risk of orthopaedic disorders such as osteoarthritis and hip dysplasia, and improved glucose tolerance was also seen.

Endocrine and Metabolic Diseases

Hormonal diseases with a reported association with obesity include diabetes mellitus, hypothyroidism and insulinoma.

 Insulin resistance, diabetes mellitus and the metabolic syndrome. Insulin, secreted by cells in the islets of Langerhans of the pancreas, controls uptake and use of glucose in peripheral tissues. In humans, tissues become 'insulin resistant' with obesity, and this can lead to type II diabetes and a condition called the metabolic syndrome. The latter is an associated group of disorders including diabetes, cardiovascular disease, high blood pressure and thrombosis (which can lead to heart attacks and stroke). Since cats most often suffer from a similar type of diabetes mellitus (DM) to humans, obesity is similarly a major cause of this condition in the feline species. Interestingly, although dogs tend to suffer from diabetes that resembles human type I DM, obesity is still known to increase the risk of dogs developing the condition. Like with other species, overweight dogs develop insulin resistance and this improves with weight loss

 Hypothyroidism and thyroid function. Although hypothyroidism is commonly cited as an underlying cause for obesity, such cases are the exception rather than the rule. The prevalence of hypothyroidism is reportedly <1%, with under half of such cases presenting with obesity; in contrast, the prevalence of obesity is far greater (see above). Hypothyroidism is extremely rare in cats. Thus, whilst hypothyroidism should always be considered, it is rarely the reason for the obesity in most cases. Obese dogs have marginally higher (but within the reference range) concentrations of both total T4 and total T3 concentrations than non-obese dogs, but other parameters (e.g., free T4, thyroid stimulating hormone (cTSH), TSH stimulation tests) are not significantly different. Thus, although obesity may have some effects on thyroid homeostasis, such changes are unlikely to affect the interpretation of thyroid function tests

Orthopaedic Disorders

At the author's clinic, orthopaedic diseases are the most commonly recognised obesity-associated problems in dogs. The types of problem to which dogs are known to be predisposed include osteoarthritis, hip dysplasia, disc disease and some types of fracture. Again, weight loss is an important part of the management of these problems.

Cardiorespiratory Disease and High Blood Pressure

Obesity can have a profound effect on the functioning of the respiratory system. Most notably, obesity is an important risk factor for development of tracheal collapse in small dogs; other respiratory diseases that can be exacerbated by obesity include laryngeal paralysis and brachycephalic airway obstruction syndrome. Obesity can also affect cardiac function; increased bodyweight can result in effects on cardiac rhythm, and increase left ventricular volume, blood pressure and plasma volume. Finally, obesity has been shown to have a significant, but minor, effect on the development of hypertension. Obesity may also be associated with portal vein thrombosis and myocardial hypoxia.

Urinary Tract and Reproductive Disorders

An association between obesity and some cases of urethral sphincter mechanism incompetence (USMI) has been reported. Weight reduction in overweight dogs with USMI can often be all that is required to allow continence to be restored. The reason for the association is not clear, although the effect may be purely mechanical, e.g., increased retroperitoneal fat leading to caudal displacement of the bladder. Although the risk of developing calcium oxalate urolithiasis has been reported to be higher in obese dogs, this may be related to dietary factors. Obese animals are reported to suffer from increased risk of dystocia, probably related to excess adipose tissue in and around the birth canal.

Neoplasia

In humans, obesity is known to predispose to various types of cancer, including breast (postmenopausal), colonic/rectal, renal cell and oesophageal cancer. It is estimated that if this link is entirely causal, being overweight or obese may account for one in seven cancer deaths in both men and women in the USA alone. Epidemiological studies have shown that obese dogs and cats are more likely to suffer from neoplasia than animals in ideal weight. An association between mammary carcinoma and obesity has also been reported in some but not all reported canine cases. Further, overweight dogs reportedly have an increased risk of developing transitional cell carcinoma of the bladder.

Skin Problems

Obese animals have been reported to be at increased risk of certain skin disorders. Seborrhoea is commonly observed (especially in cats), most likely due to reduced ability to groom efficiently, and this can also cause perineal soiling. Animals that are severely obese can develop pressure sores.

Why Does Obesity Lead to Other Diseases?

There are two main mechanisms by which excessive body fat in obesity can predispose to the various associated disorders. First, deposition of excessive fat can exert 'mechanical' or 'physical' effects, e.g., excessive weightbearing exacerbating orthopaedic diseases, constriction of upper airways exacerbating respiratory disorders, inability to groom in dermatological complaints and reduced heat dissipation due to the insulating effect of fat for heat stroke. Second, alterations can occur in normal endocrine functions of adipose tissue. In this respect, human fat cells have recently been shown to be able to produce a number of factors, termed 'adipokines', which can have a regulatory effect on many body systems. Examples include hormones (e.g., leptin and adiponectin), cytokines (e.g., tumour necrosis factor alpha and interleukin 6), 'chemokines' (which attract inflammatory cells into tissues; e.g., macrophage-chemotactic protein 1) and other inflammatory factors (e.g., haptoglobin and C-reactive protein). In obesity, adipose tissue not only expands (increased numbers of fat cells and increased size of cell) but also becomes unhealthy. In short, it is in a state of subclinical inflammation, and this then leads to disturbances in production of adipokines. It is now known that alterations in the levels of many of these factors play a key role in the development of many disorders linked to the obese state.

References

References are available upon request.