Assessment of metabolic flexibility by measuring maximal fat oxidation during submaximal intensity exercise: Can we improve the analytical procedures?

Assessment of maximal fat oxidation rate (MFO) during a submaximal exercise test has been employed by many studies to investigate the differences in metabolic flexibility (MetFlex) across several populations. Nevertheless, many incorrect assumptions and methodological limitations exist in the procedures employed by previous studies, which might lead to misinterpretation of the reported findings. Considering the data retrieved from 19 trained men (Age: [27 ​± ​4] years; %Body fat: [16.4 ​± ​4.5]%; maximal oxygen consumption: [55.8 ​± ​5.3] mL·kg⁻¹·min⁻¹) who performed a graded exercise test over a motor-driven treadmill, this opinion paper shows that MFO alone does not perfectly capture the MetFlex in response to submaximal intensity exercise and recommend a novel index that considers both fat oxidation and energy expenditure modifications for an accurate examination of MetFlex.