The term insulin kinetics refers to the biological behavior of an insulin product. As a species, cats have unique responses to both food and insulin;
treating a cat like a small dog often results in poor regulation. For example, postprandial hyperglycemia does not occur in
cats fed typical cat foods, so feline diabetics are often fed ad libitum.8 In addition, the calorie source within a feline diabetic's diet has a profound influence on its insulin requirements, and
cats eating high-protein diets require less insulin.9
Table 2. Case Example
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Although most diabetic cats respond to Lente insulin, this type generally has a shorter duration of effect in cats than in
dogs. Consequently, twice-daily dosing may result in long periods of inadequate glycemic control and substantial hyperglycemia
before the next injection.10 A switch to a long-acting product such as insulin glargine is an appropriate option for this problem. (See Table 2 for an example.)
Traditionally, PZI products are a popular choice for feline diabetics and are often regarded as appropriate for once-daily
dosing. Some cats do appear adequately controlled with once-daily therapy, which may be regarded as an advantage by some clients.11 However, in many individuals twice-daily therapy seems to provide the best glycemic control.12 A 24-hour glucose curve is the best way to identify the true behavior of this product in an individual cat, as the absorption
kinetics, peak effect, and duration of action are unpredictable.7
The very long-acting products such as insulin glargine are now emerging as a suitable choice for feline diabetics.10 Insulin glargine has a gradual onset of effect but often lasts more than 24 hours.13 However, twice-daily dosing is suggested to maintain a euglycemic state. Because of its specific biochemical attributes,
practitioners need to learn new parameters for dose adjustments with this product. One of the crucial concepts is the slow
response; it may take two to three days to see a change in blood glucose concentrations after initiation of therapy or after
a dose increase. Serious problems can occur if the dose is stepped up aggressively without allowing time to determine the
effect. One of the advantages of insulin glargine is that most cats experience only minor fluctuations in blood glucose concentrations
over the course of the day, and a reliable picture of glycemic control can be obtained with a blood glucose measurement taken
every four hours. If possible, have the owner check blood glucose concentrations at home for the first three days so that
hypoglycemia can be identified.
Little clinical data address the issue of anti-insulin antibodies in feline diabetics. In theory, antibodies generated against
heterologous insulin may cause trapping of insulin at the injection site with subsequent compromised release from the subcutaneous
tissues or overt insulin resistance (see below).14 Feline insulin is most similar to bovine insulin, whereas human insulin is less similar. However, as many cats are adequately
regulated for long periods on insulin glargine (which is based on the human molecule) or porcine Lente insulin, it seems unlikely
that anti-insulin antibodies have a marked impact on most feline diabetics.6 The only beef insulins currently on the market are compounded PZI products. Although there are concerns about quality control
with these products, it may be useful to try these in a poorly regulated cat if the common causes of insulin resistance have
been excluded.
Insulin resistance. There is no clear definition of insulin resistance, but most clinicians regard a cat as resistant to insulin if a dose exceeding
1.5 units/kg fails to bring the blood glucose concentration below 300 mg/dl.14 There are several different reasons for insulin resistance in feline patients, and various different biochemical mechanisms
may be involved (Table 1).
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