Lower serum riboflavin concentrations are associated with the severity of metabolic impairment in atypical myopathy

Background: Training-induced improved insulin sensitivity and increased GLUT4 expression is well known in humans.A downregulation in total GLUT4 expression with training was recently reported in horses.Objectives: To investigate the dynamics and relationship between insulin-dependent GLUT4 expression and insulin sensitivity in equine athletes.Study design: Prospective study.Methods: Eleven untrained Standardbred mares performed a standardised exercise test (SET) before and after 8 weeks of standardised training.Muscle biopsies (m.Vastus Lateralis) were taken before and after each SET, and dynamic insulin testing was performed.Total GLUT4 expression was determined using Western blot.Insulin sensitivity indices were calculated with an in-feed oral glucose tolerance test (OGTT) (Maximum glucose , Time to peak glucose , AUC glucose , Maximum insulin , AUC insulin , Time to peak insulin ) and a frequently sampled intravenous glucose tolerance test (FSIGTT) (Acute insulin response to glucose (AIRg), disposition index (DI), insulin sensitivity (SI) and glucose effectiveness (Sg)).The correlation between total GLUT4 expression and insulin sensitivity was determined with the Spearman correlation, with significance set at p 0.05.Results: Training decreased Max insulin concentration (from 56.07 37.63 mU/L with 33.84-78.395% CI to 30.0723.83mU/L with 15.99-44.1695% CI; p = 0.02); AUC insulin (from 7049.325099.89(mU/L)xmin with 4035.53-10063.1195% CI to 3861.553183.34(mU/L)xmin with 1980.34-5742.75 95% CI; p = 0.003) and GLUT4 expression after acute exercise (Fold change of 0.950.11;p = 0.02).There was a very strong negative Spearman correlation ( = 0.86) between GLUT4 expression in acute samples with training and DI (p = 0.02).Main limitation: Small study size.Conclusion: This study shows training increased insulin sensitivity with downregulation of GLUT4 in locomotory muscles and a significant negative correlation between GLUT4 expression after acute exercise with training and DI.