Department of Veterinary Clinical Science, University of Liverpool, Small Animal Hospital
Obesity is defined as an accumulation of excessive amounts of adipose tissue in the body, and has been defined as a greater than 15% increase above the ideal body weight for the individual. In humans, strict definitions of the degree of adiposity based upon relative mortality risk and risk of developing associated diseases. Although data from companion animals are more limited, some studies do suggest and increase in morbidity when animals are both underweight and overweight. This presentation will summarise the current knowledge on obesity and its co-morbidities in companion animals.
Measurement of Obesity in Companion Animals
All measures of adiposity involve defining body composition, which is the 'relative amounts of the various biological components of the body'. The main conceptual division of importance is between fat mass (FM; the triglyceride component in adipose tissue) and lean body mass (LBM). The various techniques differ in applicability to research, referral veterinary practice and first-opinion practice. Whatever method is used, investigators should be aware of both the precision and accuracy of the chosen method. Ideally, a test which is both accurate and precise should be chosen; however, many tests for body composition are precise but not accurate, whilst some are neither! Other important aspects of a test are cost, ease of use, acceptance by veterinary surgeons and clients, and invasiveness. At the current time, there is no method which cannot be criticised and, therefore, the perfect method for analysis does not yet exist.
Potential research techniques include chemical analysis, densitometry, total body water measurement, absorptiometry (including dual-energy X-ray absorptiometry--DEXA), ultrasonography, electrical conductance, and advanced imaging techniques (CT and MRI). In the clinical setting there is more of a need for quick, cheap and non-invasive methods of body composition measurement. The most widely adopted quantitative procedures include measurement of body weight and morphometry.
Morphometry is defined as the measurement of 'form' and, in relation to body composition analysis, refers to a variety of measured parameters that are used to estimate body composition. The three main approaches are measurement of skinfold thickness, dimensional evaluations (where various measures of stature are combined with weight) and body condition scores.
Dimensional evaluations are usually performed by tape measure, and a number have been reported in dogs and cats. Body condition scoring is a subjective, semi-quantitative method of evaluating body composition. A number of schemes have been devised, with 9-point scheme being the most widely accepted. All systems assess visual and palpable characteristics which correlate subcutaneous fat, abdominal fat and superficial musculature (e.g., ribcage, dorsal spinous processes, and waist). Unfortunately, despite their apparent ease of use, these systems are used all too uncommonly in companion animal practice.
Prevalence of Obesity in Companion Animals
We are all now well-aware of the ever-growing problem of obesity in people, with current estimates in the UK suggesting that over 55% of adults are overweight and a further 22% are obese. As in humans, obesity is one of the most important medical problems in the pet population. Studies, from various parts of the world, have estimated the prevalence of obesity in the pet population to be between 22% and 40%. The most recently published data comes from a large study in Australia, where 33.5% of dogs were classed as overweight, whilst 7.6% were judged to be obese. These figures are similar to recent data from a study in France.
Causes of Obesity
Although some diseases (e.g., hypothyroidism and hyperadrenocortism in dogs), pharmaceuticals (e.g., drug-induced polyphagia caused by glucocorticoids and anti-convulsant drugs) and rare genetic defects (in humans) can cause obesity, the main reason for development of obesity is getting the 'energy balance equation' wrong. In this respect, either excessive dietary intake or inadequate energy utilisation can lead to a state of positive energy balance; numerous factors may be involved including genetics, amount of physical activity, and the calorific content of the diet.
The effect of genetics is illustrated by recognised breed associations (e.g., Labrador retriever, Cairn terrier, cavalier King Charles spaniel, Scottish terrier, cocker spaniel for dogs; domestic shorthair for cats). Neutering is an important risk factor in both species, whilst gender is a predisposing factor in some canine studies, with females over-represented. Other recognised associations in dogs include indoor lifestyle, inactivity, middle age, neutering, have all been associated with obesity in dogs. In cats, middle age, male gender, neutering, and apartment dwelling are possible risk factors.
Dietary factors can also lead to the development of obesity in both species. For instance, in dogs, the number of meals and snacks fed, the feeding of table scraps, and the animal being present when owners prepared or ate their own meal. However, the price of the pet food does have a significant effect, where obese dogs are more likely to have been fed cheaper rather than premium brand foods. Behavioural factors may also play a part; the development of obesity in cats may be caused by anxiety, depression, failure to establish a normal feeding behaviour, and failure to develop control of satiety.
The Pathological Importance of Obesity
In humans, the medical importance of obesity lies in the effect on mortality and morbidity of associated diseases. Obese humans, on average, do not live as long, and are more likely to suffer from diseases such as type II diabetes mellitus, hypertension, coronary heart disease, certain cancers (e.g., breast, ovarian, prostate), osteoarthritis, respiratory disease, and reproductive disorders. Similarly, obesity has detrimental effects on health and longevity of dogs and cats. Problems to which obese companion animals may be predisposed include orthopaedic disease, diabetes mellitus, abnormalities in circulating lipid profiles, cardiorespiratory disease, urinary disorders, reproductive disorders, neoplasia (mammary tumours, transitional cell carcinoma), dermatological diseases, and anaesthetic complications.
Treatment of Obesity
In humans, current therapeutic options for obesity include dietary management, exercise, psychological and behavioural modification, drug therapy, and surgery. Many of these options are available for companion animals, although it is not ethically justifiable to consider surgical approaches. Further, no pharmaceutical compounds have, as of yet, been licensed for weight loss in dogs and cats.
Dietary therapy forms the cornerstone to weight management in dogs and cats, and will be covered in detail in another lecture. Increasing exercise and behavioural management form useful adjuncts. Increasing physical activity may only increase energy expenditure by a modest amount it has other benefits such as promoting fatty acid oxidation and preserving lean tissue during weight loss. The exact programme must be tailored to the individual, and take account of any concurrent medical concerns. Suitable exercise strategies in dogs include lead walking, swimming, hydrotherapy, and treadmills. Exercise in cats can be encouraged by increasing play activity, using cat toys (e.g., fishing rod toys), motorised units and feeding toys. Cats can also be encouraged to 'work' for their food by moving the food bowl between rooms prior to feeding, or by the use of feeding toys.
In addition to the above strategies, it is essential that the whole weight reduction regime be supervised. This is labour intensive, requires some degree or expertise and training in owner counseling, and often requires a dedicated member of staff. Nevertheless, in the author's opinion, this is the single most important component to the weight loss strategy. A recent study has demonstrated that weight loss is more successful if an organised strategy is followed, which includes owner education.
References are available upon request.