Effect of Non-Structural Carbohydrate, Fat and Fiber Intake on Glycogen Repletion Following Intense Exercise

Introduction Muscle glycogen is a potentially limiting substrate for horses during intense exercise. Low-starch, high-fat concentrates have become popular for performance horses, but their effect on muscle glycogen usage and repletion is unclear. Methods Six Thoroughbred horses were studied in a 3 x 3 Latin Square design to measure the effect of non-structural carbohydrate (NSC), fat and fiber intake on glycogen repletion following intense exercise. Horses were fed isocaloric, isonitrogenous rations that supplied high (HS), medium (MS) or low (LS) NSC intake. The horses were fed 1.25% BW/d grass-hay and 1.0% BW/d concentrate. These rations provided 45%, 36% and 18% of total digestible energy (DE) from NSC, 11%, 15% and 23% of total DE from fat, and 30%, 33% and 45% of total DE from fiber. During each 1-month period horses were trained for three weeks on a high-speed treadmill followed by a three-day glycogen-depletion period comprising multiple bouts of intense exercise. Muscle biopsies were taken before and 0, 24, 48 and 72 hours post-depletion. A standardized exercise test (SET) was performed on day 1 of the depletion period. Results During the SET, lactate was lower in LS compared to MS or HS (P<0.05). Heart rate, plasma glucose, VO2, VCO2 were unaffected by treatment. Muscle glycogen depletion averaged 33%, 30%, and 36% in the HS, MS and LS treatments. Muscle glycogen repletion was significantly lower in the LS group 72 hours post exercise (P<0.05). Conclusions Low NSC concentrates may not provide enough substrate for glycogen repletion following multiple bouts of intense exercise. Ethical Animal Research Approved by Kentucky Equine Research animal use committee. Sources of funding: Kentucky Equine Research and Cargill. Competing interests: Some of the concentrates tested are owned by the funding sources. Dr Pagan is president of Kentucky Equine Research.