Dietary energy source and physical conditioning affect insulin sensitivity and skeletal muscle glucose metabolism in...

Starch rich (S) feeds reduce insulin sensitivity in untrained horses when compared to high fat (F) feeds, but insulin sensitivity is not affected when S or F are fed during exercise training. The effects of S vs. F on training-associated alterations in skeletal muscle glucose metabolism are unknown.

To determine the effects of dietary energy source on training-associated changes in insulin sensitivity, skeletal muscle GLUT4 protein and hexokinase (HK) and glycogen synthase (GS) activities in horses.

After a baseline period on an all forage diet (Phase 1), horses were adapted to high starch (S) or high fat (F) diets (n = 7/group) for 6 weeks (Phase 2) and then completed 7 weeks of exercise training (Phase 3) on the same diets. To measure insulin sensitivity (SI), minimal model analysis of a frequently-sampled i.v. glucose tolerance test was performed at the end of each phase. Middle gluteal muscle biopsies to measure GLUT-4 protein content, muscle glycogen and HK and GS activities were taken before and after euglycaemic-hyperinsulinaemic clamps administered after each phase. Data were analysed by repeated measures ANOVA.

In S, SI was 36% lower (P < 0.05) after Phase 2 when compared to Phase 1 but was unchanged in F. After Phase 3, SI was increased (P < 0.01) in S and F compared to Phase 2 and did not differ (P > 0.05) between diets. Middle gluteal muscle GLUT-4 protein and post clamp HK activity were increased (P < 0.05) in S after Phase 3, with higher (P < 0.01) GLUT4 in S than in F. GS activities were unchanged in both diets.

Adaptation to S resulted in decreased SI mitigated by moderate physical conditioning. Increased GLUT-4 protein content and HK activity in S may have contributed to higher SI after training.