Effects of dietary energy source and physical conditioning on insulin sensitivity and glucose tolerance in standardbred horses.

REASONS FOR PERFORMING STUDY: There is evidence that adaptation to diets rich in nonstructural carbohydrates (NSC) contributes to the development of insulin resistance in horses. To date, however, no study in horses has examined the effects of physical conditioning on diet-associated alterations in insulin sensitivity and glucose tolerance.
OBJECTIVES: To examine the effects of adaptation to concentrate feeds rich in NSC or fat on insulin sensitivity and glucose tolerance in horses, both in the sedentary state and after a subsequent period of physical conditioning.
METHODS: Fourteen mature Standardbred horses underwent both a euglycaemic-hyperinsulinaemic clamp (EHC) and oral glucose tolerance test (OGTT) after each of the following phases: Baseline - fed only forage cubes for 3 weeks; Diet horses were randomly assigned to receive either a high NSC (S) concentrate or a high fat concentrate (F) with forage cubes for 6 weeks; and Diet x Exercise - horses remained on the assigned ration and underwent a 7 week period of physical conditioning. An incremental exercise test was performed before, and after, the Diet x Exercise phase for measurement of the peak rate of oxygen consumption (VO2peak).
RESULTS: In both diet groups, there was an approximately 10% increase in mean VO2peak after physical conditioning. The mean rate of glucose disposal (M) per unit of serum insulin (I) during the EHC [M/I ratio] in S horses was 30% lower (P<0.05) in the Diet phase when compared to Baseline, but not different from Baseline after physical conditioning. The S diet also resulted in a greater (P<0.05) OGTT insulin response (area under the insulin vs. time curve, AUC(INS)) in both Diet and Diet x Exercise phases when compared to Baseline. In F, insulin sensitivity (mean M/I ratio) and glucose tolerance were unchanged during the study. CONCLUSIONS AND POTENTIAL RELEVANCE: Feeding a diet rich in NSC for 6 weeks resulted in decreased insulin sensitivity and impaired glucose tolerance. Physical conditioning lessened the effects of the high NSC diet on insulin sensitivity, as evidenced by the return to baseline M/I, but did not mitigate the impaired glucose tolerance. Decreased insulin sensitivity has been implicated in the development of obesity and laminitis in horses and the present findings provide support for avoidance of concentrates with high NSC in the dietary management of horses at risk for the development of these conditions.