Insulin sensitivity and postprandial insulin response in equines

Sammanfattning: A metabolic syndrome in equines was first proposed in 2002, describing a cluster of clinical abnormalities including obesity, insulin resistance (IR), fasting- and postprandial hyperinsulinemia and a predisposition for laminitis. It has been established that intake of grain-based diets rich in non-structural carbohydrates (NSC) contribute to development of IR and hyperinsulinemia. A causal link between hyperinsulinemia and laminitis in horses has also been proven. A correct diagnosis of IR requires the use of complex and expensive tests, which complicates clinical identification of horses at risk for laminitis. The overall aim for this thesis was to study factors that influence insulin sensitivity (IS) in horses and to study the relationship between IS and the postprandial insulin response. In addition, a simple field test for identification of horses with postprandial hyperinsulinemia was evaluated. By feeding horses increasing amounts of a forage-based diet low in NSC, a 9% increase in body weight was obtained, but there was no decrease in the horses’ IS. This indicates that the dietary content of NSC has a greater impact on alterations in IS in horses than short-term weight gain. An oral sugar test (OST) was developed and was found to be practical for diagnosis of postprandial hyperinsulinemia in horses under field settings. A hyperbolic relationship was found between OST-derived indices of β-cell response (based on postprandial insulin data) and quantitative measures of IS in horses. This implies that the β-cell response depend on the prevailing IS. Results also confirm that the OST primarily is a test that estimates the β-cell response rather than IS. The postprandial insulin response was evaluated using forage diets differing in NSC content. It was found that the postprandial insulin response depended on both the NSC content in forage and the horse’s IS, but the effect of IS was diminished when the forage NSC content was low. This indicates that horses with IR and an augmented β-cell response should be a fed forage diet with low content of NSC, in order to attenuate the postprandial insulin response and thereby decrease the risk for laminitis. This work adds to the knowledge about the pathophysiology, clinical management and recognition of horses with IR and hyperinsulinemia.