Use of the oral sugar test in ponies when performed with or without prior fasting
Knowles, E.J., Harris, P.A., Elliot, J. and Menzies-Gow, N.J.
It is not possible for some ponies to be fasted, e.g. those turned out permanently, so this study compared the oral sugar test for diagnosis of insulin dysregulation in fasted and non-fasted ponies.
Ten native pony mares were subjected to four oral sugar tests, two fasted overnight and two fed (left out at pasture). Blood samples for measurement of serum insulin concentration were taken before oral administration of 0.15 ml/kg bwt of Karo syrup (corn oil) and at 30, 60, 75, 90 and 120 min. Insulin concentrations were measured using a radioimmunoassay.
There were significant differences between the insulin response under fasting and fed conditions. Fasting exacerbated the insulin response to sugar and fasting and fed tests cannot be used interchangeably. However, tests under both fasting and fed conditions had similar reliability. Additionally, although insulin values tended to differ fairly widely at the same time point within the same individual, and hence repeatability between individual OSTs was poor, the results of fasted and fed tests were similar in the context of dichotomous interpretation, i.e. whether ponies were defined as having insulin dysregulation or not. Within this context there was good agreement between fasted and fed results within individual ponies.
Therefore a non-fasted test will still be likely to diagnose whether insulin dysregulation is present, but repeating the test to monitor changes in insulin concentrations in an individual may not be very reliable. Peak insulin concentrations were recorded most commonly at 30 min in both fasted and fed conditions, with the majority of the rest occurring at 60 min.
If fasting before an OST is not possible, the test is still likely to identify insulin regulation/dysregulation. The optimal window for sampling is up to 60 minutes.