Obesity is becoming increasingly prevalent in cats and dogs. It is a major risk factor in the development of diabetes and other diseases not only in man but also in cats (1,2). Specifically, in a recent study, obese cats were found to have a three to five times greater risk of developing skin problems, lameness and diabetes (1). Obesity refers to the excessive storage of energy as fat. Several theories have been proposed to explain the development of obesity which is believed to be of both genetic and environmental origin. Excess caloric intake, decreased physical activity, and metabolic and endocrine abnormalities may be involved in the pathogenesis of obesity in many species, including the cat. As an example, it was found that the presence of dogs in the household made it less likely for cats to become obese, illustrating how important exercise is in the prevention of obesity not only in people but also in cats (3).
We have recently examined records of cats admitted to our hospital. Approximately 4.5 % of all cats seen for medical reasons were obese. This observation is virtually identical to incidence data reported from Australia (4) and elsewhere in the USA (5). In these studies an additional approximately 20% were labeled "heavy." This figure likely underestimates the true number of obese cases because most clinics do not always record body scores and frequently even weights are not recorded.
Routine blood examinations
Complete blood count
In a study of 50 obese research cats, we did not find a change in any of the parameters that are routinely measured including hematocrit, white blood cell count and platelet count. However, 9 of 12 obese client-owned cats had a higher than normal hematocrit. This is similar to findings in obese people (6,7).
The concentrations of cholesterol and triglycerides were significantly increased in obese cats which is similar to what has been reported in obese dogs and man as well as in other species. The increase in cholesterol and triglycerides is associated with an increased risk for cardiovascular disease and stroke in people (8); however, this has not been examined in obese cats. It is also thought that an increase in triglycerides may be a cause in the deterioration of beta cell function and eventually lead to diabetes. It is unknown if a similar mechanism plays a role in the increased risk of obese cats to become diabetic. While most obese cats also show an increase in non-esterified fatty acids, this change is not consistently seen. Similar to triglycerides, non-esterified fatty acids also have been implicated in the deterioration of beta cell function which leads to the development of diabetes in many obese patients (9,10).
It is interesting that baseline glucose concentrations are not usually increased in obese cats, dogs or man.
Glucose tolerance testing
Many of the problems seen in obesity are due to hyperinsulinemia. In the progression from the normal to the obese state in human patients and in the cat, the beta cell changes from normal secretion to hypersecretion (11-13). The insulin secretion pattern during the hypersecretory phase becomes abnormal (12). The hypersecretion is likely caused by insulin resistance; i.e., the tissues become less sensitive to insulin and beta cells try to overcome this defect by increasing their output. We have recently shown that persistent hypersecretion of insulin in the insulin resistant state eventually leads to hyposecretion (12).The insulin release becomes insufficient to maintain the glucose at a normal or slightly hyperglycemic range and overt diabetes ensues. The result may even be absolute insulin deficiency. Diabetes seems to occur as a late phenomenon in obese people and is preceded by years of impaired glucose intolerance. Obese but healthy cats are not usually screened for glucose intolerance and the true incidence of glucose intolerance with obesity is, therefore, unknown in cats. It is known, however, that not every obese cat suffers from glucose intolerance.
Glycosylated hemoglobin measurements
Glycosylated hemoglobin (gHb) is produced by a reaction between glucose and hemoglobin. The glycosylation of hemoglobin is dependent on the concentration of blood glucose. The reaction is not reversible and therefore the half-life of glycosylated hemoglobin relates to the life span of the red blood cell (2-3 months). An increased glycohemoglobin therefore would indicate a high glucose during the preceding months. In a study of a large number of well-monitored obese cats, all gHb concentrations remained within the normal range. This is in agreement with our findings of normal basal glucose concentrations in obese cats.
Leptin concentrations are increased about 2-3 fold in obese cats as has been described in other species. Because leptin is a hormone that leads to a decrease in appetite and an increase in energy expenditure in lean subjects, it is said that obesity leads to leptin resistance (11-13).
Contrary to the effect of obesity in people and to the dog, obesity does not lead to a change in the density of lipoprotein in cats. It is unknown whether this lack of lipoprotein changes may protect the cats from cardiovascular changes that are risk factors in people.
An interesting change seen in cats studied as they became obese was an increase in free thyroxine by dialysis (FT4D) within the physiological range, correlating with most indices of obesity. Total thyroxine (TT4) and total triiodothyronine (TT3) concentrations did not change significantly. Possible mechanisms for the increase in FT4D include displacement of T4 by NEFA on serum and/or cellular binding proteins, or by the intermediary effects of leptin. While we postulate an increased FT4 fraction in serum, the elevated absolute FT4D concentration in obese cats may be an indication of relative thyroid hormone resistance (14).
Obesity has just recently gained more attention in the veterinary field because of the increasing prevalence of obese pets and their increased risk of developing diabetes and other diseases. Most of the information available in the literature originates from well-controlled research animals. It is, however, important to gain more knowledge about the general presentation of this disease in client owned animals so that we might identify factors that lead to its progression to diabetes and other diseases. Practitioners should increase their efforts to identify and follow obese patients.