Insulin sensitivity measured by the minimal model: no associations with fasting respiratory exchange ratio in trained athletes.

The aim of this study was to examine the role of fasting insulin concentrations and tissue insulin sensitivity on whole-body substrate oxidation in 61 well-trained subjects. Subjects underwent a frequently sampled intravenous glucose tolerance test (FSIVGT) after a 10- to 12-hour overnight fast. Minimal model analysis was used to determine insulin sensitivity (S(i)). A week later, fasting (10- to 12-hour) respiratory exchange ratio (RER) was measured at rest and during exercise at 25%, 50%, and 70% of peak power output (W(peak)). Prior to these measurements, training volume, dietary intake, and muscle fiber composition, substrate concentrations, and enzyme activities were determined. The average fasting plasma insulin concentration was 7.3 +/- 2.4 microU/mL (4.0 to 10.5 microU/mL), and the mean S(i) was 14.0 +/- 6.1 x (10(-4) min(-1) x microU(-1) x mL(-1)) (2.6 to 26.3 x 10(-4) min(-1) x microU(-1) x mL(-1)). There was no significant correlation between fasting plasma insulin concentration and S(i) (r = -.14, P =.336) or between these measurements and fasting RER, measured at rest and during exercise at 25%, 50%, and 70% W(peak). Only VO(2max) and the proportion of type 1 muscle fibers were significantly correlated with S(i) (r =.30, P =.045 and r =.34, P =.026, respectively), and waist-to-hip ratio (WHR) was significantly correlated with fasting plasma insulin concentration (r =.35, P =.006). In conclusion, S(i) and fasting plasma insulin concentration were not associated with fasting RER at rest and during exercise of increasing intensity in trained athletes who have high S(i).