In humans, the medical importance of obesity relates to the effect on mortality and morbidity. Obese humans tend to have a shorter life span and are more likely to develop diabetes mellitus, hypertension, coronary heart disease, cancer (eg, esophageal, breast, ovarian, prostate), osteoarthritis, respiratory disease, and reproductive disorders. Similarly, obesity has detrimental effects on the health and longevity of dogs and cats.
Clinical Evaluation & Anesthesia
Overall, clinical evaluation is more difficult in obese dogs and cats than in those in ideal body condition. Anesthetic risk is also increased because there may be problems with estimating anesthetic dose, respiratory compromise may occur, and operating times are often prolonged.
Longevity
A recent prospective study showed that abnormally high weight may shorten life span in dogs.6 One group of dogs was fed ad libitum,* and the second had food intake restricted. Dogs in the ad libitum group tended to be overweight, whereas those in the restricted group remained in ideal condition. By the end of the study, on average, those in the restricted group lived almost 2 years longer.
Endocrine & Metabolic Diseases
Hormonal diseases with a reported association with obesity include diabetes mellitus, hyper-adrenocorticism, and hypothyroidism.
Insulin resistance & diabetes mellitus. Cats most often develop diabetes mellitus (a form that resembles type 2 diabetes mellitus in humans) and obesity is a major risk factor. The most common type of canine diabetes mellitus resembles human type 1 diabetes mellitus, although obesity is a known risk factor in this species.2 However, the reasons for this association are not currently known.
Hypothyroidism. Although hypothyroidism is commonly cited as an underlying cause of obesity, such cases are the exception rather than the rule. The prevalence of hypothyroidism is under 1%, with less than half of such patients presenting with obesity; in contrast, the prevalence of obesity is 30% to 50%. Thus, although hypothyroidism should always be considered, it is rarely the reason for the obesity.
Hyperlipidemia & Dyslipidemia
The link between hepatic lipidosis and obesity in cats is well known. Experimental studies in obese dogs have demonstrated mild (insignificant) elevations in cholesterol, triglyceride, and phospholipid levels, whereas laboratory dogs made obese have increased plasma levels of nonesterified fatty acid and triglycerides.9 Whether lipid alterations account for the increased incidence of pancreatitis in obese dogs requires further study.
Orthopedic Disorders
Obesity is a major risk factor for orthopedic diseases in companion animals, especially dogs.1 Reported associations include cruciate ligament disease, hip dysplasia, osteoarthritis (Figure 2), condylar fractures, and thoracolumbar disk disease.
Cardiorespiratory Disease & Hypertension
Obesity can have a profound effect on respiratory system function, most notably as a risk factor for such diseases as tracheal collapse (Figure 3), laryngeal paralysis, and brachycephalic airway obstruction syndrome. Obesity can also affect cardiac function, cardiac rhythm, and left ventricular volume, leading to hypertension and increased plasma volume.
Urinary Tract Disorders
An association between obesity and some cases of urethral sphincter mechanism incompetence has been reported. Weight reduction in overweight dogs with urethral sphincter mechanism incompetence can lead to complete resolution of the sign, probably by reducing the amount of retroperitoneal fat. This reduction, in turn, decreases intraabdominal pressure while the animal is lying down, and reduces the chance of postural incontinence.
Neoplasia
In humans, obesity is known to predispose to various types of neoplasia, including breast (postmenopausal), colonic/rectal, renal cell, and esophageal cancers.10 An association between obesity and neoplasia has recently been reported in a large epidemiologic study in dogs.2
Dermatologic Disorders
Obese animals have been reported to be at increased risk for certain dermatologic disorders. Diffuse scale is commonly observed (especially in cats), most likely due to the reduced ability to groom efficiently. Severely obese animals can develop pressure sores.