Increased contact between lipid droplets and mitochondria in skeletal muscles of male elite endurance athletes

Endurance athletes exhibit higher skeletal muscle mitochondrial and lipid droplet (LD) content compared with recreationally active individuals, along with greater whole body oxygen uptake and maximal fat oxidation rates. In this study, we investigated whether these differences manifest in a greater LD-mitochondria contact and how this may relate to the organelles' size, shape, and numerical densities. We obtained skeletal muscle biopsies from 17 male elite triathletes and road cyclists and seven recreationally active men. Using quantitative transmission electron microscopy, we found that the endurance athletes had two- to threefold greater LD-mitochondria total contact length than the recreationally active individuals. This was related to higher numerical densities of both mitochondria (+30%) and LDs (+100%) in the intermyofibrillar space. Adding data from untrained individuals with equally high intermyofibrillar LD density as the endurance athletes revealed a 24% greater total LD-mitochondria contact length in the endurance athletes. We observed small trivial differences in the shape of both organelles between populations. However, large mitochondrial profiles were more elongated and irregular in shape than small mitochondrial profiles, whereas large LD profiles were more circular and less irregular than small LD profiles. Within athletes, large intermyofibrillar LD profiles correlated (r = 0.72) with a high fraction of PLIN5-positive LDs and their maximal fat oxidation rate was positively associated with an interaction between the profile size of both intermyofibrillar LDs and mitochondria. In conclusion, male endurance athletes have a greater LD-mitochondria contact than recreationally active and untrained individuals. This muscular phenotype is restricted to the intermyofibrillar space and to fibers rich in mitochondria.NEW & NOTEWORTHY Lipid droplets and mitochondria form membrane contacts in skeletal muscle fibers, but the quantitative magnitude of these contacts remains unknown. Our study reveals that endurance athletes exhibit greater contact length between lipid droplets and mitochondria compared with recreationally active and untrained individuals. This difference is influenced by the content and spatial distribution of mitochondria and lipid droplets. The results provide quantitative insights into lipid droplet and mitochondria morphology across various human populations, including endurance athletes.