Certain behaviors predispose dogs to weight gain. In Part 1 of this two-part series, we explore common causes of weight gain in dogs and the consequences of dogs being overweight or obese. In Part 2, we discuss treatment and monitoring strategies.
DEFINING OVERWEIGHT AND OBESE
Obesity is defined as excessive white adipose tissue.1 Human epidemiologic data show increased morbidity and mortality with increasing body fat mass.2
The most commonly used measure of body fat in people is the body mass index (BMI: weight [kg] divided by height2 [m]). People are defined as
Individuals who are overweight or obese have an increased risk of cardiovascular disease and certain cancers and of overall mortality.7-10
Data from companion animals are more limited, and the definition of obesity is more arbitrary. Dogs are overweight when their weight is > 15% above ideal and are obese when their weight is > 30% of ideal.11,12 However, these criteria have not been confirmed with rigorous epidemiologic studies, and limited data exist on the definition of an optimal body weight. In most animals, obesity results from an imbalance between energy intake and energy expenditure.
PREVALENCE AND CAUSES
Obesity is one of the most important medical problems in dogs. Studies from throughout the world have estimated the prevalence of overweight or obese dogs to range between 22% and 40%.13-17 Cross-sectional data suggest one in three dogs seen by U.S. veterinarians is overweight.18
Recent data in various animal species provide new insight into the genetic basis of obesity.19-21 A significant breed predisposition to obesity has been shown in certain breeds including Cairn terriers, West Highland white terriers, Scottish terriers, Shetland sheepdogs, basset hounds, Cavalier King Charles spaniels, dachshunds, beagles, cocker spaniels, and Labrador retrievers.12,15,22-24 Conversely, certain breeds, particularly sight hounds, appear to be resistant to the development of obesity.24
As an adult animal ages, lean body mass declines, resulting in a decrease in total daily energy needs.25 The loss of lean body mass is exacerbated if aging is accompanied by a decrease in voluntary activity. The total daily energy needs of an average-sized 7-year-old dog may decrease by as much as 20% when compared with its needs as a young adult. If food intake does not decrease proportionately with the decreasing energy needs, weight gain results.26 The end result is an increasing prevalence of obesity in older dogs. In short, a reduced metabolic rate associated with aging predisposes dogs to being overweight or obese.27
Neutering results in the net loss of circulating sex hormones, slowing an animal's metabolism and predisposing the neutered animal to becoming overweight or obese.14,15,28 Changes in sex hormones after neutering directly affect the satiety center in the brain through changes in leptin and ghrelin concentrations and possibly indirectly affect it by altering cell metabolism and hormonal regulators of food.24,29-31
Gonadectomy in dogs also results in modified feeding patterns. Compared with before surgery, four female ovariectomized beagles fed ad libitum for three months after surgery ate significantly more food and gained more weight.28 The study suggested that energy intake should be reduced by 30% after ovariectomy to maintain optimal body weight for a period of six months.28 These results confirmed those of a previous experiment that also used four young adult female beagles and showed that energy requirements are 20% lower after ovariectomy.31
Certain medications may also contribute to obesity in dogs. Phenobarbital, a common anticonvulsant, causes polyphagia at high serum concentrations. The increase in food consumption may contribute to a worsening body condition, resulting in an overweight or obese dog. Glucocorticoids stimulate gluconeogenesis and abdominal lipogenesis, which may contribute to fat deposition and weight gain.
Excessive white adipose tissue impacts canine health through two main mechanisms. First, excessive fat deposition may have physical effects on the body, including excessive weight bearing, which worsens orthopedic diseases; constriction of upper airways, which exacerbates respiratory disorders; an inability to groom, which leads to dermatologic issues; and an insulating effect, which causes reduced heat dissipation. Second, the normal endocrine function of white adipose tissue may be disturbed, a now-recognized significant pathogenetic mechanism for the development and exacerbation of many of the obesity-associated metabolic disorders in both humans and domestic animal species.32
Dietary factors also contribute to excessive weight gain in dogs. The number of meals and snacks fed, the consumption of table scraps, and an animal's presence when owners prepare or eat their own meals all contribute to canine weight gain.41 The cost of pet food has been shown to have a variable effect; a study showed that obese dogs were more likely to be fed a higher volume of cheaper brand foods than premium brands.42 Additionally, dogs with elderly owners were more likely to be overweight or obese, potentially because of poor dietary habits since the nutrition of the elderly owners was not ideal and, thus, supportive of obesity.15
Overweight or obese owners
In people, exposure to environments with a high prevalence of overweight and obese people leads to inaccurate perceptions of what constitutes a normal body shape.41 This misperception may also contribute to the increasing prevalence of obesity within the canine population.
A recent survey study collected data from 829 dog owners through personal interviews.43 Owners were asked to subjectively evaluate their dogs' body condition score (BCS). In addition, both the owner's BMI and the dog's BCS were assessed and recorded by the interviewer. Obese dogs were twice as likely to have obese owners as nonobese dogs were. Owner underestimation of a dog's BCS was nearly 20 times more common in dogs that were obese than in normal or underweight dogs. No significant relationship was found between owner underestimation of a dog's BCS and owner obesity. Interestingly, while obese owners are more likely to have obese dogs, they do not appear to differ in their misperception of their dogs' BCS when compared to nonobese owners.
Owner misperception of an obese dog's BCS presents a major obstacle in weight management. This misperception appears to be independent of whether the owner is obese. That is in contrast to the situation with obese children and parents, where obese parents consistently fail to recognize (or acknowledge) obesity in their children.44 Targeting this misperception of BCS with education is a requisite for successfully preventing and treating canine obesity.
A recent survey sought to identify environmental risk factors for canine obesity.45 In the study, 829 interviews were conducted (400 at a charity practice, 429 in private practice) in which owners were asked about their feeding and exercise habits, the household income, and their age. The BCSs of 696 dogs 1 year of age or older were assessed by using the seven-point S.H.A.P.E. (size, health, and physical evaluation) morphometric technique. Those dogs with a BCS 5/7 were considered mildly overweight, those with a BCS 6/7 were considered moderately overweight, and those with a BCS 7/7 were designated extremely overweight.
Using the seven-point scale, 35.3% (n=246) of the dogs were above their ideal body condition, 38.9% (n=271) were overweight, 20.4% (n=142) were obese, and 5.3% (n=37) were underweight.
Obese dogs had a higher median age and were more likely to be female and neutered. Obese dogs had significantly less exercise per week compared with nonobese dogs. Socioeconomic class appeared to be a factor in a dog's becoming overweight or obese; the risk of obesity was significantly associated with owner income as pet owners in the highest income bracket were more likely to have obese dogs. Finally, increasing owner age was also associated with increased incidence of overweight or obese dogs.45
CONSEQUENCES OF OBESITY
As in people, obesity adversely impacts longevity in dogs. In one study in which 24 pairs of Labrador retrievers were randomly assigned to an ad libitum feeding group or an energy-restricted group (fed 75% of the amount consumed by the respective pair mate), the energy-restricted group's BCS was closer to the optimal score than the ad libitum feeding group's BCS was.46 The energy-restricted animals lived 1.8 years longer, had improved glucose tolerance (body fat mass and insulin resistance were correlated—the threshold range for increased insulin resistance was 5,000 g of body fat—energy-restricted dogs had lower body fat composition), and had reduced risk of orthopedic disorders such as osteoarthritis.
Diabetes mellitus and hypothyroidism are frequently associated with canine obesity. In people, tissues develop insulin resistance with excessive caloric intake. Plasma concentrations of insulin also increase in direct proportion to increasing BMI in men and women. Abdominal obesity is a major determinant of insulin resistance, hyperinsulinemia, and, subsequently, type-2 diabetes mellitus in people.10,47,48 In people, metabolic syndrome is a group of risk factors associated with insulin resistance, cardiovascular disease, and inflammation.4,49 Canine diabetic patients commonly suffer from diabetes mellitus resembling human type-1 diabetes mellitus. Since type-2 diabetes mellitus is uncommon in dogs, obesity rarely leads to overt clinical signs of diabetes mellitus.46
Although hypothyroidism is commonly considered an underlying cause of obesity, such cases are relatively unusual.50 While hypothyroidism should always be a differential diagnosis in obese dogs, it is rarely the sole cause. Obese dogs have marginally higher concentrations of both total T4 and total T3 concentrations than nonobese dogs do, but free T4 concentrations, thyroid stimulating hormone (TSH) concentrations, and TSH stimulation test results are not significantly different.51,52 Thus, although obesity may have some effect on thyroid homeostasis, such changes are unlikely to affect the interpretation of thyroid function tests.
Hyperadrenocorticism has been associated with increased intra-abdominal adipose deposition. Combined with the weakening of ventral abdominal musculature secondary to endogenous corticosteroids, patients living with this condition certainly may be readily classified as overweight or obese.53,54
In experimental models, obese dogs had mild elevations in cholesterol, triglyceride, and phospholipid concentrations compared with normal-weight dogs.29,55 Laboratory dogs that had been made obese had increased total cholesterol concentrations and increased cholesterol concentrations in very-low-density lipoprotein (VLDL), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) fractions. Additionally, these dogs had higher triglyceride concentrations in total plasma and in the VLDL fractions. These dyslipidemic changes were associated with insulin resistance after lipid infusion.29,55
Orthopedic diseases, including osteoarthritis, osteochondrosis, and osteochondritis dissecans, are the most common obesity-associated problems in dogs.56,57 A recent study demonstrated food intake may have a significant effect on the development of osteoarthritis in dogs, and the disease severity was associated with unrestricted feeding.58
Cardiopulmonary disease and hypertension
Obesity may have a marked effect on respiratory function and is a recognized risk factor for the development of tracheal collapse in small-breed dogs.59 Other respiratory diseases frequently exacerbated by obesity include laryngeal paralysis, asthma, and brachycephalic airway obstruction syndrome; obstruction of thoracic movement related to obesity hypoventilation syndrome (also known as Pickwickian-type syndrome) also occurs.60-63 Obesity hypoventilation syndrome in people is defined as obesity accompanied by lethargy, drowsiness, hypoventilation, hypoxia, and secondary polycythemia.
Obesity may also affect arterial blood pressure in pets, as hypertension has been reported in 23% to 45% of obese dogs.12,64 One study showed that 67% of obese dogs had a systolic blood pressure > 160 mm Hg and that obese dogs had significantly higher blood pressure than control dogs did.65 Hypertension was considered a risk factor associated with BCS in dogs.
Obesity in people reduces systolic and diastolic functions of both the left and right ventricles, and the severity of the dysfunction is related to the magnitude of obesity.66-68 Research has also shown that overweight dogs without overt heart disease have preclinical left myocardial systolic and diastolic function changes.69 Thus, even without a predisposition to ischemic heart disease, obesity has an impact on ventricular function in dogs.
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.
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.
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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