Genitourinary disorders
An association between obesity and some cases of urethral sphincter mechanism incompetence has been reported.70 Indeed, weight reduction in overweight dogs with urethral sphincter mechanism incompetence is often all that is required
to restore continence. Although the mechanism is not clear, one possible explanation is that increased retroperitoneal fat
may lead to caudal displacement of the urinary bladder. Obese animals are reported to suffer from an increased risk of dystocia,71 likely related to excess adipose tissue in and around the birth canal.
Neoplasia
Obesity is known to predispose people to various types of neoplasia, including breast, colorectal, renal cell, and esophageal
cancer.8 Recent epidemiologic evidence from dogs and cats suggests overweight and obese animals have an increased risk of developing
neoplasia.12 Further, overweight dogs reportedly have an increased risk of developing transitional cell carcinoma of the bladder.72
A limited number of retrospective studies have evaluated the correlation between specific cancer types and obesity prevalence
in dogs. A large retrospective study of dogs presenting to a referral veterinary teaching hospital showed a lower prevalence
of obesity among dogs when all types of malignant neoplasia were evaluated together but a difference in obesity prevalence
when cancer types were evaluated individually and compared with dogs without cancer.72
Researchers in one study collected owner-reported obesity statuses a year before the dogs were diagnosed with transitional
cell carcinoma of the urinary bladder and found that dogs exposed to topical insecticide use had an increased risk of developing
bladder cancer and that this risk was increased in overweight or obese dogs compared with size- and age-matched control dogs
that had other neoplasms or chronic diseases.73 Other studies showed a positive correlation between mammary tumor development and owner-reported obesity.73-75
Another study found a higher prevalence of overweight or obese dogs with mammary cancers compared with dogs without cancer.72 Conversely, another study found no correlation with obesity and mammary tumor development.76
Mortality data from a long-term prospective study on calorie restriction showed an equal distribution of cancers among 24
pairs of control and restricted-fed Labrador retrievers.46 The variety of cancer types reported, limited sample size, and avoidance of overt obesity in control dogs make direct conclusions
about obesity and cancer development in that study difficult.
Adipocytes' ability to secrete cytokines and inflammatory markers into the circulation has been well-documented in people
and domestic animal species. Proteins including leptin, adiponectin, and retinol-binding protein can induce peripheral insulin-resistance,
inhibit normal apoptotic mechanisms, promote angiogenesis, and decrease circulating HDL concentrations in people. As such,
chronic inflammation associated with obesity has been proposed to result in oxidative injury to DNA and predispose obese people
to increased cancer risk.77
Obese dogs have increased circulating concentrations of adipokines, such as insulin-like growth factor, tumor necrosis factor-alpha,
and leptin.47 Leptin is an in vitro promoter of mammary tumors and hepatocellular carcinomas in people.78 To date, this relationship has not been documented in dogs. Tumor suppressor genes, such as p53, are found in about half
of all cancers in people and in certain canine cancers, such as mammary tumors and osteosarcoma.79-82 Elevated leptin concentrations can directly inhibit p53 expression in human mammary cancer cells in vitro.83
CANINE PATIENT ASSESSMENT
A number of techniques for evaluating canine weight and body composition have been examined in recent years. These include
deuterium oxide dilution, bioelectrical impedance, ultrasonography, dual-energy X-ray absorptiometry, and BCS.84-87 For field purposes, BCS is a simple, expedient, noninvasive, and invaluable tool for assessing obesity during a complete
physical examination.88 BCS provides a subjective yet quantitative way to estimate the amount of excess adipose tissue present. A validated, nine-point
system is a commonly used method for assessing BCS in which each point is 10% to 15% of body weight. Overweight in dogs is
defined as a BCS of 6/9 or 7/9. Canine obesity is defined, as in people, as weighing about 30% or more over ideal (equivalent
to a BCS 8/9 or 9/9).89
By recording both body weight and BCS, ideal body weight may be more easily determined through serial monitoring of trends
in these measures.90 Patients that are overweight will be recognized sooner, as will contributing factors, such as endocrinopathies, and associated
problems, such as hyperlipidemia. Quantifying body condition also facilitates communication with clients—an important aspect
of weight control.
CONCLUSION
Dogs become overweight or obese because of a combination of causes with physical, emotional (owner-related), environmental,
endocrine, or neurologic components. Musculoskeletal problems and developmental abnormalities may also greatly influence a
dog's ability to exercise and may ultimately contribute to weight gain. Indiscriminate feeding habits including feeding table
scraps, poor diet, and constant access to food are significant contributing factors to this most prevalent issue in the canine
population.
Christopher G. Byers, DVM, DACVECC, DACVIM
VCA Veterinary Referral Associates
500 Perry Parkway
Gaithersburg, MD 20877
Cindy C. Wilson, PhD
Mark B. Stephens, MD, MS
Jeffrey Goodie, PhD, ABPP
Department of Family Medicine
School of Medicine Uniformed Services
University of the Health Sciences
Bethesda, MD 20814
F. Ellen Netting, PhD
School of Social Work
Virginia Commonwealth University
Richmond, VA 23284
Cara Olsen, PhD
Department of Preventive Medicine and Biometrics
School of Medicine Uniformed Services
University of the Health Sciences
Bethesda, MD 20814
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