CONSEQUENCES OF OBESITY
It is generally well-known that obesity has many consequences in people. We are now beginning to recognize the many potential
consequences obesity has in dogs, as well (Table 2).
Table 2: Potential Consequences of Overweight or Obesity in Dogs
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
Thus, even without a predisposition to ischemic heart disease, obesity has an impact on ventricular function in dogs.