Responses of blood glucose, insulin and cortisol concentrations to common equine diets

Two experiments were conducted to determine the response of blood glucose, insulin and cortisol levels to four common equine diets. Experiment 1 was designed to determine the diurnal variation of glucose and two glucoregulatory hormones, insulin and cortisol, in four fasting geldings. No diurnal variation was observed in either glucose or insulin levels during the 24-h sampling period. However, cortisol levels did exhibit a circadian rhythm, with elevated values observed in the morning and low values in the evening. Experiment 2 investigated the response of glucose, insulin, cortisol and selected amino acids to four isoenergetic equine diets. Four 2-yr-old quarter horse geldings were used in a Latin square design. Pelleted isoenergetic diets were composed as follows on a digestible energy basis: 100% alfalfa (diet A), 50% alfalfa and 50% corn (diet AC), 100% corn (diet C) and 90% corn and 10% corn oil (diet CO). A single meal was fed after an overnight fast. Blood samples were taken via a jugular catheter from 0800 to 1700 h. Analysis of variance by repeated measures and mean response area for glucose showed no difference between diets. However, postprandial peak glucose levels were elevated (P less than 0.01) over prefeeding levels in diets AC and C. Analysis of variance by repeated measures and mean response area for insulin showed differences (P less than 0.05) between diets. Cortisol showed no meal-related responses to any of the diets within its expected circadian rhythm.