Changing Tides in Insulin Resistance Interpretation

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In 1999 I was told: “Everyone knows blood insulin and glucose are too variable to be of any use.” However, insulin resistance (IR) and Equine Metabolic Syndrome (EMS) have been the focus of much research for the past 10 years, and blood insulin and glucose levels do have their purposes when appropriately used and interpreted.

A widespread perception exists that any elevation above “normal” in plasma insulin (hyperinsulinemia) indicates EMS and a significant risk of laminitis. However, diagnosis is not quite that simple.

There are indeed a number of factors that influence fasting glucose and insulin responses to dextrose or sugar challenges and tissue sensitivity to the actions of insulin. These need to be taken into consideration when evaluating an IR/EMS suspect. We now know that not all obese horses are IR, nor are truly IR horses/ponies/donkeys at high risk of laminitis always obese. Stress and acute pain can also induce IR.

Hyperinsulinemic responses to sugar challenges and mild fasting hyperinsulinemia are normal in horses adapted to high starch/sugar feeds. This is an adaptation that allows rapid return to normal blood glucose levels after ingestion of high glycemic index feeds and does not pose a health risk to the horse. The return to baseline insulin concentrations is actually faster than in horses not accustomed to ingestion of grain-based concentrates, so previous rations need to be taken into consideration.

Researchers use the modified Frequently Sampled Glucose/Insulin Tolerance (FSGIT) test as the gold standard for determining insulin sensitivity and detection of changes in experimental models. However, this test is impractical for field use. Low dose (0.25 gram dextrose/kg or 0.15 ml Karo Syrup/kg) sugar challenges are now recognized to be more reliable physiologic and sensitive measures of insulin sensitivity. The challenge is given orally in the morning and blood is drawn before dosing and then 60 and 120 minutes later. “Normal” peak glucose and insulin concentrations at 60 minutes are not well established but can be as high as 180 mg glucose/dl and 60 µIU insulin/ml, respectively. At 120 minutes both should be lower, but not necessarily returned to baseline levels. If there is hyperglycemia and hyperinsulinemia with a slow clearance, a horse may be at risk of IR/EMS and starch/sugar intakes may need to be restricted.

Much more research is needed into the physiology and epidemiology of these diseases.

Reprinted from Equine Disease Quarterly.