Obesity in pets is a clinically significant problem, often predisposing the animal to a variety of health disorders. A comprehensive survey of pet cats in the Northeastern United States revealed that 29% of cats were overweight. Factors that have been identified as being associated with an overweight body condition include breed, age, gender/neuter, diet, and physical activity. A variety of health problems in cats have definitively been shown to be associated with obesity. These include lameness, diabetes mellitus, and non-allergic skin disease.

Gonadectomy appears to substantially increase the risk for weight gain and obesity in cats. Ovariohysterectomy of female cats results in approximately 20% increases in ad libitum food intake, 20% decreases in fasting metabolic rate, and a 40% increase in body weight in as little as 3 months. Likewise, orchiectomy of male cats increases ad libitum food intake approximately 25%, decreases resting metabolic rate 30% and leads to a weight gain of about 30% in 3 months.

In several independent studies, overweight, neutered male and female cats fed a commercial low calorie, low fat, and high fiber diet underwent significant weight loss (~20%) over 3–4 months. Quantitative evaluation of body composition demonstrated that this weight loss was predominantly, but not entirely, composed of fat mass. In one of these studies, although serum triiodothyronine values decreased significantly following weight loss, there were no concomitant changes in fasting or resting metabolic rates. Normalization of insulin sensitivity during an intravenous glucose tolerance test was evidenced by decreased resting serum insulin concentrations and reduced insulin peak responses.

The most effective, experimentally validated method to induce weight loss in overweight cats is to restrict food intake to weighed portions of food commensurate with a calculated reduction in caloric intake. Water dilution of a diet to as little as 8% solids results in voluntary caloric intake only 10% less than normal. Dietary dilution with solids like cellulose appears to be more effective at inducing satiety in dogs when caloric intake is restricted. This appears to be due to factors such as physical distention of the stomach, osmotic effects of fermentation products on intestinal fill, and stimulation of satiety-promoting gastrointestinal neural and hormonal signals. This has not been adequately evaluated as yet in cats.

Recent anecdotal evidence indicates that a low carbohydrate, high protein, and moderate fat-containing diet may be more effective at inducing weight loss in some obese cats while maintaining better satiety than higher carbohydrate and fiber-containing weight loss diets. Dietary fat is generally more effective on a caloric basis at inducing satiety than carbohydrate. Further, cats appear to be unlike any other species, in that increased dietary fat intake increases rates of fat oxidation. This may explain the observation that growth diets can be used effectively to induce weight loss and maintain satiety in cats. In rodents and humans, the satiety-inducing effects of dietary fat typically decline after several months, but whether this waning effect occurs in cats is not yet known. Nevertheless, feeding high carbohydrate/low fat diets can mitigate excess post-prandial insulin release and reduce circulating levels of glycosylated proteins in obese, non-insulin dependent diabetic cats. Thus, it is not clear which approach should be routinely used to promote weight loss in obese cats.

Regardless of the approach chosen for inducing weight loss in obese cats, owner comprehension & participation are absolutely essential, and behavioral modifications should be promoted that can help improve both the pet's physical health and its relationship with the owner.