Effect of Dose and Fasting on Oral Sugar Test Responses in Insulin Dysregulated Horses.
Authors: Lyn Macon Erica, Harris Patricia, Partridge Emma, Day Barker Virginia, Adams Amanda
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Oral Sugar Testing Protocols in Insulin Dysregulated Horses The oral sugar test remains a cornerstone diagnostic tool for identifying insulin dysregulation in equines, yet standardisation of testing protocols has been limited, particularly in horses as opposed to ponies. Macon and colleagues compared two critical variables affecting OST interpretation—pre-test fasting duration and sugar dose—in 14–16 horses per group (insulin dysregulated and control) using randomised crossover designs with either 0.15 mL/kg bodyweight administered after a 3-hour fast versus direct from pasture, and separately comparing low-dose (0.15 mL/kg) against high-dose (0.45 mL/kg) protocols. Insulin dysregulated horses demonstrated significantly elevated responses across all test conditions; critically, however, tripling the sugar dose produced no measurable difference in insulin concentrations at baseline, 60-minute post-administration, or delta insulin change in either population, whilst fasting affected only resting insulin levels rather than the dynamic response to the sugar challenge. These findings suggest that practitioners can apply the standard low-dose OST protocol regardless of immediate pre-test feeding management without compromising diagnostic sensitivity, though the absence of a fasting effect on the incremental insulin rise may indicate that baseline metabolic state matters less than the oral glucose stimulus itself for identifying dysregulation.
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Practical Takeaways
- •The standard low-dose OST (0.15 mL/kg BW) is sufficient to differentiate ID from NID horses; practitioners do not need to use higher doses for better diagnostic discrimination
- •Fasting status affects baseline insulin levels but not the diagnostic response to the OST, allowing more flexibility in test timing and management protocols
- •ID horses consistently show exaggerated insulin responses regardless of fasting or dose variation, making the OST a robust diagnostic tool for identifying metabolic dysfunction in practice
Key Findings
- •ID horses demonstrated significantly greater insulin responses than NID horses across all OST protocols (P < 0.05)
- •High dose OST (0.45 mL/kg BW) and low dose OST (0.15 mL/kg BW) produced no statistically significant differences in mean insulin concentrations for either ID or NID horses
- •Fasting for 3 hours prior to OST reduced basal insulin (T0) in ID horses compared to fed state (P < 0.01), but did not significantly affect post-test insulin response (T60) or delta insulin
- •Continued forage access did not significantly alter post-OST insulin responses or insulin change from baseline