Comparison of Three Methods for Evaluation of Equine Insulin Regulation in Horses of Varied Body Condition Score

Multiple dynamic field tests are used for assessment of equine insulin resistance or altered insulin regulation. However, the relationship between markers of glucose homeostasis and insulin disposal obtained by different testing protocols is unknown. We hypothesized that two recently developed field tests for evaluation of equine insulin dysregulation, the insulin response to dexamethasone test (IRDT) and oral sugar test (OST), would yield comparable results to the hyperinsulinemic euglycemic clamp (HEC). Fifteen light breed horses with body condition scores (BCS) 3 of 9 to 8 of 9 were used in this study. Eight horses (BCS, 5 of 9 to 7 of 9) underwent an OST under two different housing conditions, pasture, and stall (experiment 1). These eight horses also underwent an HEC and IRDT over a 4-week period (experiment 2), and results were compared with the OST stall. Finally, eight horses (BCS, 3 of 9 to 8 of 9), including one horse from experiments 1 and 2, underwent an OST on pasture three times over a 14–16-week period during the summer and the fall (experiment 3). The HEC did not correlate with either the OST or IRDT. The OST was not different when performed in the pasture compared within a stall but did change significantly over time on pasture. These results suggest that in insulin-sensitive horses, the OST and IRDT results are not primarily determined by tissue insulin sensitivity in horses of varying BCS. Furthermore, OST results may vary depending on pasture composition or season.