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

Authors

  • L. STEWART-HUNT,

    1. Departments of Animal and Poultry Science, Pathobiology and Biomedical Science, University of Guelph, Guelph, Ontario, Canada.
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  • S. PRATT-PHILLIPS,

    Corresponding author
    1. Departments of Animal and Poultry Science, Pathobiology and Biomedical Science, University of Guelph, Guelph, Ontario, Canada.
      Present addresses: S. Pratt-Phillips, Department of Animal Science, North Carolina State University, Raleigh, North Carolina, 27608, USA; L. J. McCutcheon, Department of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, Lansing, Michigan 48910, USA; R. J. Geor, Department of Large Animal Clinical Science, College of Veterinary Medicine, Michigan State University, Lansing, Michigan 48910, USA. Email: shannon_pratt@ncsu.edu
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  • L. J. McCUTCHEON,

    1. Departments of Animal and Poultry Science, Pathobiology and Biomedical Science, University of Guelph, Guelph, Ontario, Canada.
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  • R. J. GEOR

    1. Departments of Animal and Poultry Science, Pathobiology and Biomedical Science, University of Guelph, Guelph, Ontario, Canada.
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Present addresses: S. Pratt-Phillips, Department of Animal Science, North Carolina State University, Raleigh, North Carolina, 27608, USA; L. J. McCutcheon, Department of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, Lansing, Michigan 48910, USA; R. J. Geor, Department of Large Animal Clinical Science, College of Veterinary Medicine, Michigan State University, Lansing, Michigan 48910, USA. Email: shannon_pratt@ncsu.edu

Summary

Reasons for performing study: 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.

Objectives: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

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