Factors Affecting the Glucose Response to Insulin Injection in Mares: Epinephrine, Short- and Long-Term Prior Feed Intake, Cinnamon Extract, and Omega-3 Fatty Acid Supplementation

Five experiments were conducted with mares to better define factors that might affect the assessment of insulin sensitivity via direct insulin injection, and to then apply this method of assessing insulin sensitivity to trials which tested two potential supplements for improving poor insulin sensitivity in horses. The experiments assessed the effects of the following: (1) previous administration of epinephrine, (2) overnight feed deprivation versus hay or pasture consumption, (3) 10-day acclimatization to hay in a dry lot versus pasture grazing, (4) cinnamon extract supplementation, and (5) fish oil supplementation on insulin sensitivity. Mares of known high and low insulin sensitivities were used in the first three experiments, whereas mares with low insulin sensitivities were used in the supplement trials. Epinephrine administration increased blood glucose concentrations (P < .05) and prevented the insulin-induced decrease in blood glucose concentrations in both sensitive and insensitive mares. Overnight feed deprivation decreased (P < .06) insulin sensitivity relative to overnight ad libitum access to hay, and both regimens resulted in reduced insulin sensitivity relative to overnight pasture availability; sensitive and insensitive mares responded similarly except when kept on pasture (P = .0854). Ten days of hay consumption in a dry lot reduced (P < .05) insulin sensitivity in insensitive mares, but not in sensitive mares, relative to pasture grazing. Supplementation with cinnamon extract or fish oil had no effect on insulin sensitivity of mares with known low insulin sensitivity under the conditions of these experiments.