Comparison of the in-feed glucose test and the oral sugar test

Reasons for performing studyThe in-feed oral glucose test (OGT) and oral sugar test (OST) are advocated as field tests of insulin sensitivity in horses and ponies but have not been directly compared. ObjectivesTo compare the insulin response to OGT and OST in 8 ponies and 5 horses of unknown insulin sensitivity. Study designExperimental, randomised crossover study. MethodsAnimals were fasted for 8h overnight before and throughout testing. They were fed 1g/kg bwt glucose powder with chaff (OGT) or 0.15ml/kg bwt corn syrup (Karo Light Syrup; OST) was administered per os in a randomised crossover study with 48h between tests. Blood samples were obtained at 0, 30, 60, 75, 90, 120 and 180min. The maximal insulin concentration (Cmax(i)), time to maximal insulin concentration (Tmax(i)) and area under the curve of insulin concentration over time (AUC(i)) for the tests were compared using Student's paired ttest. The effect of individual subject, horse or pony and test were analysed using a linear mixed model. ResultsThe OGTCmax(i) (mean s.d.; 154 +/- 116iu/ml), Tmax(i) (136 +/- 52min) and AUC(i) (15,308 +/- 9886iu/ml/min) were significantly (P<0.05) greater compared with the OSTCmax(i) (72 +/- 55iu/ml), Tmax(i) (63 +/- 25min) and AUC(i) (5980 +/- 4151iu/ml/min). The Cmax(i), Tmax(i) and AUC(i) varied significantly between individual subjects. The Tmax(i) was significantly different between horses and ponies during OGT and OST. Using previously defined criteria of insulin dysregulation, OGT identified 7/13 animals as insulin resistant, whereas OST identified 5/13 animals as insulin resistant. ConclusionsThe OGT and OST showed agreement in identification of insulin dysregulation in 85% of equine subjects. Results of the OGT and OST are not comparable in all cases. Further work is required to establish which test more accurately diagnoses insulin dysregulation in horses and ponies.