BACKGROUND:Increased muscle mitochondrial mass is characteristic of elite professional endurance athletes (PAs), whereas increased blood lactate levels (lactatemia) at the same absolute submaximal exercise intensities and decreased mitochondrial oxidative capacity are characteristics of individuals with low aerobic power. In contrast to PAs, patients with metabolic syndrome (MtS) are characterized by a decreased capacity to oxidize lipids and by early transition from fat to carbohydrate oxidation (FATox/CHOox), as well as elevated blood lactate concentration [La-] as exercise power output (PO) increases, a condition termed 'metabolic inflexibility'. OBJECTIVE:The aim of this study was to assess metabolic flexibility across populations with different metabolic characteristics. METHODS:We used indirect calorimetry and [La-] measurements to study the metabolic responses to exercise in PAs, moderately active individuals (MAs), and MtS individuals. RESULTS:FATox was significantly higher in PAs than MAs and patients with MtS (p < 0.01), while [La-] was significantly lower in PAs compared with MAs and patients with MtS. FATox and [La-] were inversely correlated in all three groups (PA: r = -0.97, p < 0.01; MA: r = -0.98, p < 0.01; MtS: r = -0.92, p < 0.01). The correlation between FATox and [La-] for all data points corresponding to all populations studied was r = -0.76 (p < 0.01). CONCLUSIONS:Blood lactate accumulation is negatively correlated with FATox and positively correlated with CHOox during exercise across populations with widely ranging metabolic capabilities. Because both lactate and fatty acids are mitochondrial substrates, we believe that measurements of [La-] and FATox rate during exercise provide an indirect method to assess metabolic flexibility and oxidative capacity across individuals of widely different metabolic capabilities.