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

Reasons For Performing Study: The 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.

Objectives: To compare the insulin response to OGT and OST in 8 ponies and 5 horses of unknown insulin sensitivity.

Study Design: Experimental, randomised crossover study.

Methods: Animals were fasted for 8 h overnight before and throughout testing. They were fed 1 g/kg bwt glucose powder with chaff (OGT) or 0.15 ml/kg bwt corn syrup (Karo™ Light Syrup; OST) was administered per os in a randomised crossover study with 48 h between tests. Blood samples were obtained at 0, 30, 60, 75, 90, 120 and 180 min. The maximal insulin concentration (Cmaxi ), time to maximal insulin concentration (Tmaxi ) and area under the curve of insulin concentration over time (AUCi ) for the tests were compared using Student's paired t test. The effect of individual subject, horse or pony and test were analysed using a linear mixed model.

Results: The OGT Cmaxi (mean ± s.d.; 154 ± 116 μiu/ml), Tmaxi (136 ± 52 min) and AUCi (15,308 ± 9886 μiu/ml/min) were significantly (P<0.05) greater compared with the OST Cmaxi (72 ± 55 μiu/ml), Tmaxi (63 ± 25 min) and AUCi (5980 ± 4151 μiu/ml/min). The Cmaxi , Tmaxi and AUCi varied significantly between individual subjects. The Tmaxi 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.

Conclusions: The 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.