Correspondence Between Indexes Of Maximal Fat Oxidation From Ramp Vs Steady-state Protocols In Postmenopausal Women

Abstract

Maximal fat oxidation (MFO, in g-1·min-1) is an index of metabolic flexibility and the relative exercise intensity at MFO (FATmax, as % maximum oxygen consumption (VO2max)) is used for exercise prescription for weight loss and metabolic health. The time-consuming, steady-state protocol required for MFO/FATmax determination hinders the extensive use of these indexes. Alternative, ramp testing has been validated for MFO/FATmax determination in healthy, young males only. PURPOSE: to validate ramp testing for MFO/FATmax measure in post-menopausal women. METHODS: 18 sedentary, postmenopausal women (54±4 years; 4±3 years from menopause; 22±3 BMI) performed on a cycle ergometer: i) a ramp incremental test; ii) steady-state trials at 40, 50, 60, 70 and 80% of the VO2max as determined by the incremental test. We measured VO2, respiratory exchange ratio (R) and heart rate (HR). Based on VO2 and R from the ramp test (breath-by breath data) and steady-state protocol (5th minute of each trial) we measured absolute fat oxidation (FO, in g-1·min-1); then, we determined MFO in absolute units (in g-1·min-1) and the relative intensity corresponding to MFO, i.e. FATmax, that was expressed relative to VO2max and directly determined maximal HR (HRmax). MFO and FATmax from the ramp (rMFO and rFATmax) were compared to the values from the steady-state protocol (ssMFO and ssFATmax) by paired t-Test. RESULTS: The ssMFO was significantly lower than rMFO (0.39±0.13 vs 0.19±0.07 g.min-1 p<0.001). On the contrary, the rFATmax and ssFATmax occurred at an identical %VO2max (45±6 vs 47±7%VO2max p=0.17) and %HRmax (61±6 vs 62±6% HRmax p=0.54). CONCLUSIONS: In agreement with previous studies conducted in young males, our data confirm that only steady-state protocols allow accurate MFO quantification. On the contrary, accurate measures of FATmax can be obtained indifferently from both ramp and steady-state protocols in post-menopausal women. Therefore, ramp testing offers a valid alternative to more time-consuming steady-state protocols for the identification of optimal exercise intensity for weight loss and metabolic health. Time-efficient testing strategies have a high practical value for longitudinal/large-scale evaluation of fat metabolism in postmenopausal women offering the means for individualized exercise prescription.