Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes

We examined the variability and determinants of the respiratory exchange ra tio (RER) at rest and during exercise in 61 trained cyclists. Fasting (10-1 2 h) RER was measured at rest and during exercise at 25, 50, and 70% of pea k power output (W-peak), during which blood samples were drawn for [lactate ] and [free fatty acid] ([FFA]). Before these measurements, training volume , dietary intake and muscle fiber composition, [substrate], and enzyme acti vities were determined. There was large interindividual variability in rest ing RER (0.718-0.927) that persisted during exercise of increasing intensit y. The major determinants of resting RER included muscle glycogen content, training volume, proportion of type 1 fibers, [FFA] and [lactate], and %die tary fat intake (adjusted r(2) = 0.59, P < 0.001). Except for muscle fiber composition, these variables also predicted RER at 25, 50, and 70% W-peak t o different extents. The key determinant at 25% W-peak was blood-borne [sub strate], at 50% was muscle [substrate] and glycolytic enzyme activities, an d at 70% was [lactate]. Resting RER was also a significant determinant of R ER at 25 (r = 0.60) and 50% (r = 0.44) W-peak.