Skeletal muscle proteomics links mitochondrial abundance with peak fat oxidation in physically active young males

The interindividual variability in peak fat oxidation (PFO) and the intensity at which this occurs (Fat_max) has been attributed to physiological factors, diet and physical activity; however, few studies have examined the contribution of skeletal muscle characteristics. The present study examined the relationship between PFO, Fat_max and the skeletal muscle proteome in young, physically active males. Thirty-four young, lean males were phenotyped through assessment of aerobic capacity, PFO, body composition, fasting blood samples and a muscle biopsy. Liquid chromatography mass spectrometry based proteomics was used to assess skeletal muscle protein abundance. Only absolute PFO (g min^-1) was positively correlated with ${\dot{V}}_{O_2\mathrm{peak}}$ (r = 0.496, P = 0.003). Few skeletal muscle proteins correlated with absolute PFO, whereas relative PFO and Fat_max were positively associated with numerous mitochondrial proteins enriched in metabolic pathways, oxidative phosphorylation and other mitochondrial processes. Mitochondrial proteome abundance was positively correlated with both relative PFO (r = 0.633, P < 0.001) and Fat_max (r = 0.595, P < 0.001). Mitochondrial complex-specific analysis demonstrated that respiratory complex V was associated with both relative PFO and Fat_max. Multiple regression analyses indicated that mitochondrial abundance and muscle glycogen explained 55% of the variability in relative PFO, whereas mitochondrial abundance alone explained 43% of the variability in Fat_max. Absolute PFO was explained by a combination of ${\dot{V}}_{O_2\mathrm{peak}}$ , mitochondrial abundance and muscle glycogen content (r² = 0.562). This untargeted proteomic approach highlights that skeletal muscle mitochondrial content contributes to the interindividual variability in PFO and Fat_max in lean, active young males. KEY POINTS: This study used an untargeted proteomics approach to explore the links between the skeletal muscle proteome and peak fat oxidation (PFO) in young, physically active males. Absolute PFO was primarily associated with maximal aerobic capacity. When expressed relative to fat-free mass, PFO was closely associated with skeletal muscle proteins enriched in oxidative metabolism and mitochondrial pathways. Mitochondrial abundance assessed by mitochondrial proteome content and citrate synthase activity was positively related to relative PFO and the intensity at which this occurs (Fat_max). Mitochondrial respiratory complex V was consistently related to both relative PFO and Fat_max. Mitochondrial content independently contributed to both PFO and Fat_max, highlighting mitochondrial content as a key determinant of the maximal capacity for fat oxidation.