High-intensity interval, but not endurance training induces muscle fiber type-specific subsarcolemmal lipid droplet size reduction in type 2 diabetic patients.

This study compared the effects of moderate-intensity endurance training and high-intensity interval training on fiber type-specific subcellular volumetric content and morphology of lipid droplets and mitochondria in skeletal muscles of type 2 diabetic patients. Sixteen sedentary type 2 diabetic patients (57±7 years old) were randomized to complete 11 weeks of either 40-min cycling at 50% peak workload (Endurance, n = 8) or 10 1-min cycling intervals at 95% peak workload separated by 1 min of recovery (High-intensity Interval, n = 8), 3 times per week. Assessments for cardiorespiratory fitness, body composition, glycemic control, together with muscle biopsies were performed before and after the intervention. Morphometric analyses of lipid droplets and mitochondria were conducted in the subcellular fractions of biopsied muscle fibers using quantitative electron microscopy. The training intervention increased cardiorespiratory fitness, lowered fat mass and improved non-fasting glycemic control (P < 0.05), with no difference between training modalities. In the subsarcolemmal space, training decreased lipid droplet volume (P = 0.003), and high-intensity interval, but not endurance training, reduced the size of lipid droplets, specifically in type 2 fibers (P < 0.001). No training-induced change in intermyofibrillar lipid droplets was observed in both fiber types. Subsarcolemmal mitochondrial volume was increased by high-intensity interval (P = 0.02), but not endurance training (P = 0.79). Along with improvement in glycemic control, low volume high-intensity interval training is an alternative time-saving training modality that affects subcellular morphology and volumetric content of lipid droplets in skeletal muscle of type 2 diabetic patients.