The present study tested the hypothesis that insulin sensitivity would be lower in horses adapted to sugar and starch than those adapted to fat and fiber during an endurance race. Forty horses were divided into 3 dietary groups: one group of experimental feed (SS) was rich in starch (33%), low in fat (8%); another (FF) low in starch (6%) and rich in fat (15%); and a third group of commercial concentrates (CC) was intermediate in starch (16%) and fat (11%). Blood samples were taken the day before the race (PRE), within 3 minutes of arrival at each vet check (after 27, 48, 80 km), and three hours post completion or elimination. Plasma samples were analyzed for glucose, insulin, cortisol, glycerol, triglycerides, CK and AST. A proxy for insulin sensitivity as measured by the minimal model was calculated: RISQI = (1/sqrt [insulin]). Also, a proxy for pancreatic β-cell response to plasma glucose was calculated: MIRG =[800-0.30([insulin] -50)2]/[glucose - 30]. Higher insulin in FF and CC horses, and in eliminated horses, combined with a lower RISQI in eliminated horses indicated that insulin resistance (low RISQI) was attenuated by fat and fiber feeding and decreased the likelihood of elimination. Higher insulin sensitivity in finishers and fat and fiber fed horses may have allowed a more efficient glucose uptake by muscles, allowing energy to be obtained through NEFA and TG. Fat and fiber feeding could avoid insulin resistance improving the efficiency of energy utilization and performance of horses during endurance races it also could reduce excitement and increases in muscle enzymes.
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