Insulin resistance is becoming more prevalent among horses worldwide and is associated with a range of metabolic disturbances (Geor and Frank, 2009). Nutrition has a major impact on insulin kinetics and metabolism (Schmidt and Hickey, 2009), but most research has concentrated on the effect of dietary carbohydrate on insulin metabolism and much less emphasis has been placed on the effects of dietary protein and fat (Geor and Frank, 2009; Schmidt and Hickey, 2009). Performance horses require energy in excess of maintenance and in some circumstances, dietary protein intake may be in excess of recommended requirements (NRC, 2007). The objective of this study was to evaluate the effects of elevated dietary protein content on glucose and insulin dynamics in horses using the modified frequently sampled intravenous glucose tolerance test (M-FSIGT) protocol and minimal model analysis (Boston et al., 2003).
Twelve mature Standardbred geldings of similar weight (428.3 ± 10.9 kg) and body condition score (BCS; 2 ± 0.5) that had recently been retired from the racetrack were used in this study. The initial M-FSIGT was undertaken after feeding the horses a hay basal diet for 12 days (daily intake = 650 g) and the second M-FSIGT was undertaken after feeding the horses either a low- or a high-protein diet for 6 weeks (daily crude protein intakes were 650 g and 1600 g, respectively). The horses were matched according to weight and allocated to treatments at random. The horses received the experimental diets from the morning after the first M-FSIGT until the afternoon prior to the second M-FSIGT. Oaten hay (89 g crude protein per kg) was provided ad libitum after the first 45 min of the test period. Jugular catheters were ed 45–60 min prior to collection of the baseline sample. A glucose bolus (0.3 g/kg body weight) was administered, followed by collection of blood samples at 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 19 min. At 20 min, an insulin bolus (20 mIU/ kg body weight) was administered, followed by blood sampling at 22, 23, 24, 25, 27, 30, 35, 40, 50, 60, 70, 80, 90, 100, 120, 150, 180, 210 and 240 min. Results obtained from the M-FSIGT were used for minimal model analysis (Boston et al., 2003) of glucose and insulin dynamics to determine insulin sensitivity (SI), glucose effectiveness (Sg), acute insulin response to glucose (AIRg) and disposition index (DI).
Plasma insulin concentrations were assayed using a radioimmunoassay (Coat-A-Count Insulin, Diagnostic Products Corp., Los Angeles, CA) validated for use with equine plasma (Sessions et al., 2003). Plasma glucose concentrations were analysed enzymatically using a hexokinase-based Olympus kit and an Olympus AU 400 analyser (Olympus Diagnostics Systems Division, Melville, NY). Means (± SE) were calculated for plasma insulin and glucose concentration, body weight, BCS and minimal model parameters derived from M-FSIGT analysis. Statistical significance was assessed using parametric tests including one-way ANOVA with repeated measures for treatment and time comparisons and an unpaired t-test for analysis of individual time points.
No significant differences were shown in the minimal model parameters for horses fed low or high protein diets. However, the glucose concentrations of all horses fell below their baseline concentrations for a considerable period during both M-FSIGTs. The reason for this in circulating glucose levels is unclear but may be related to the low BCS of these horses and may reflect increased insulin sensitivity. Hypoglycaemic responses necessitated the administration of glucose to six horses during the tests and this was not related to protein intake. Investigation into hypoglycaemia in horses and its potential effects on the minimal model analysis would be useful in proving the robustness of the use of the minimal model for horses. Development of a recommended small but effective insulin dose for use during the M-FSIGT for insulin sensitive individuals would be useful to help prevent hypoglycaemia during testing.
Interestingly, the glucose concentrations of the horses that received the low protein diet ped below baseline earlier (P = 0.009) and remained low for longer (P = 0.043) than those of horses that received the high protein diet. This appears to indicate that dietary protein may perturb the insulin/glucose axis. Further studies on the mechanism by which protein and amino acids could alter insulin sensitivity in horses are required.
Funding was received from the Rural Industries Research and Development Corporation, Canberra, Australia.
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This paper was presented at the Recent Advances in Animal Nutrition, Australia Conference, July 2011. Engormix.com thanks the authors and the organizing committee for this huge contribution.