Original ResearchAssessment of Insulin and Glucose Dynamics by Using an Oral Sugar Test in Horses
Introduction
Straightforward testing procedures are needed to facilitate the diagnosis of insulin dysregulation because hyperinsulinemia and insulin resistance (IR) are associated with laminitis in equids [1], [2], [3], [4]. The general term “insulin dysregulation” is used to refer to alterations in insulin metabolism. Current screening for insulin dysregulation includes measurement of resting glucose and insulin concentrations and calculation of glucose-to-insulin and insulin-to-glucose ratios [1]. Proxy measures of insulin sensitivity and secretion, such as the reciprocal of the square root of insulin and modified insulin-to-glucose ratio, respectively, are also calculated from glucose and insulin values [1], [2]. If further evaluation is required, dynamic testing is performed by conducting a euglycemic–hyperinsulinemic clamp, a frequently sampled intravenous glucose tolerance test, or a combined glucose–insulin tolerance test [1]. These procedures provide more direct measures of insulin sensitivity but often require hospitalization, have a higher cost, and take several hours to complete.
Measurement of resting insulin concentrations under fasting conditions is currently recommended as a screening test for insulin dysregulation, with detection of hyperinsulinemia in the fasting state supporting a diagnosis of IR [3]. However, a dynamic test is necessary for the diagnosis of insulin dysregulation when fasting insulin concentrations are within reference ranges. The ideal dynamic test would be easy to perform on the farm and take only a short time to complete. An oral sugar test (OST) was therefore developed by our research group to try and meet these goals. In a pilot study, corn syrup (Karo light corn syrup; ACH Food Companies, Inc, Memphis, TN) was administered orally to horses in different quantities, and glucose and insulin concentrations were measured. Corn syrup was selected because it is readily available for purchase and can be easily administered to horses by owners. The syrup is administered orally by using a dose syringe and is very palatable. Once an OST was developed, we sought to compare results of this test with those of a conventional intravenous glucose tolerance test (IVGTT). We specifically hypothesized that OST and IVGTT area under the curve values for glucose (AUCg) and insulin (AUCi) would be closely correlated, as defined by a correlation coefficient value (CV) of >0.90. Other objectives of this study were to examine OST results from a herd of healthy Quarter Horse crossbred mares and assess test repeatability. A corn syrup containing glucose, maltose, and starch, but not fructose, was selected.
Section snippets
Animals
Ten adult horses with equine metabolic syndrome (EMS) were compared with 8 healthy adult mares (control group). Criteria for inclusion in the EMS group were based on American College of Veterinary Internal Medicine consensus statement recommendations [3] and included obesity, defined by a body condition score (BCS) of ≥7/9 according to the scoring system of 1 to 9 developed by Henneke et al [4] and/or regional adiposity in the form of an enlarged neck crest (subjective assessment); and combined
Comparison of EMS and Control Groups
Body weight did not significantly differ between groups (P = .502). Glucose and insulin concentrations were higher (P < .05) in the EMS group than in the control group at all time points during the IVGTT (Figs. 1 and 2) and OST (Figs. 3 and 4), including time 0. Accordingly, AUCg and AUCi values were higher in the EMS group for both the IVGTT and OST (Figs. 5 and 6). For the IVGTT, median (range) AUCg values were 293.9 (237.5-382.1) and 373.3 (356.4-391.6) mmol/L · min−1 , and AUCi values were
Discussion
An OST was developed using corn syrup, and AUCi values correlated well with those obtained from the IVGTT. Higher OST and IVGTT glucose and insulin concentrations were detected in horses with EMS than in control group mares. Oral sugar test results from 21 adult nonobese mares from the same herd provided 97.5th percentile glucose and insulin values for this specific population. The repeatability of the OST was better for glucose concentrations than insulin concentrations.
When the OST is
Conclusions
In conclusion, OST and IVGTT results were positively correlated, and horses in EMS and control groups differed significantly with respect to glucose and insulin concentrations for both tests. Further studies are required to establish reference intervals for the OST by examining larger populations of horses and separating breed groups. The repeatability of diagnostic interpretations from OST results must also be assessed in the future.
Acknowledgments
Funding was provided by the University of Tennessee College of Veterinary Medicine Center of Excellence. The test protocol was first presented as a poster at the American College of Veterinary Internal Medicine 2010 conference, Anaheim, CA. It was also presented orally as an abstract at the December 2012 American Association of Equine Practitioners Convention, Anaheim, CA. The authors declare no conflicts of interest.
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Dr. Schuver's present address is PO Box 826 Washington, OK 73093.