Indexes of Fat Oxidation from Ramp vs. Graded Incremental Protocols in Postmenopausal Women

The maximal rate of fat oxidation (MFO, in g∙min−1) and the relative exercise intensity at which it occurs (FATmax, as %V̇O2max) are indexes of metabolic flexibility. The time-consuming, graded exercise protocol required for MFO/FATmax determination hinders the extensive use of these indexes for individualized exercise prescription and monitoring. Purpose: validate ramp testing for MFO and FATmax measures in postmenopausal women. Methods: Seventeen healthy women (age: 54 ± 4 years, BMI 22 ± 3 kg·m−2, and V̇O2max 36.4 ± 5.3 mL·min−1), who were 4 ± 3 years from menopause, performed on a cycle-ergometer, a ramp, and a graded incremental test. Based on V̇O2 and respiratory exchange ratio from the ramp and graded protocol (i.e., the 5th minute of each step), MFO and FATmax were determined. Data from the two protocols were compared using paired t-tests, linear regression, and Bland–Altman analysis. Results: The MFO measured with a ramp protocol was not different from (0.24 ± 0.09 vs. 0.20 ± 0.08 g·min−1, p = 0.10), and moderately associated with, that of the graded protocol (r2 = 0.46). FATmax occurred at similar exercise intensity for both protocols (47.8 ± 5.1 vs. 47.5 ± 4.3 %V̇O2max, p = 0.91, r2 = 0.52). The comparison of MFO and FATmax across the protocols yields a non-significant bias but a relatively large limit of agreement (respectively, 0.05 g·min−1, LOA = −0.08, and 0.19 g·min−1; 0.3 %V̇O2max, LOA = −7.8, and 10.6 %V̇O2max). Conclusions: In postmenopausal women, ramp testing offers a valid alternative to the graded protocol for identifying MFO and FATmax. The availability of a time- and cost-efficient approach, which can be incorporated into standard ramp incremental testing, can facilitate using these indexes of metabolic flexibility in research and medicine.