Muscle fiber type-specific autophagy responses following an overnight fast and mixed meal ingestion in human skeletal muscle.

The aim of the present study was to investigate the fiber type-specific abundance of autophagy-related proteins after an overnight fast and following ingestion of a mixed meal in human skeletal muscle. Twelve overweight, healthy young male volunteers underwent a 3-h mixed meal tolerance test following an overnight fast. Blood samples were collected in the overnight-fasted state and throughout the 180-min postmeal period. Skeletal muscle biopsies were collected in the fasted state, and at 30 and 90 min after meal ingestion. Protein content of key autophagy markers and upstream signaling responses were measured in whole muscle and pooled single fibers using immunoblotting. In the fasted state, type I fibers displayed lower LC3B-I but higher LC3B-II abundance and higher LC3B-II/LC3B-I ratio compared with type II fibers (P < 0.05). However, there were no fiber type differences in p62/SQSTM1, unc-51 like autophagy activating kinase (ULK1), ATG5, or ATG12 (P > 0.05). Compared with the fasted state, there was a reduction in LC3B-II abundance, indicative of lower autophagosome content, in whole muscle and in both type I and type II fibers following meal ingestion (P < 0.05). This reduction in autophagosome content occurred alongside similar increases in p-AktS473 and p-mTORS2448 in both type I and type II muscle fibers (P < 0.05). In human skeletal muscle, type I fibers have a greater autophagosome content than type II fibers in the overnight-fasted state despite comparable abundance of other key upstream autophagy proteins. Autophagy is rapidly inhibited in both fiber types following the ingestion of a mixed meal.NEW & NOTEWORTHY This study examined the fiber type-specific content of key autophagy proteins in human muscle. We showed that markers of autophagosome content are higher in type I fibers in the overnight-fasted state, whereas autophagy is rapidly inhibited in both type I and type II fibers after the ingestion of a mixed meal.

Intramuscular lipid utilization during exercise: a systematic review, meta-analysis, and meta-regression.

Intramuscular lipid (IMCL) utilization during exercise was controversial as numerous studies did not observe a decline in IMCL content post-exercise when assessed in muscle biopsies using biochemical techniques. Contemporary techniques including immunofluorescence microscopy and 1H-magnetic resonance spectroscopy (1H-MRS) offer advantages over biochemical techniques. The primary aim of this systematic review, meta-analysis, and meta-regression was to examine the net degradation of IMCL in response to an acute bout of cycling exercise in humans, as assessed with different analytical approaches. A secondary aim was to explore the factors influencing IMCL degradation including feeding status, exercise variables, and participant characteristics. A total of 44 studies met the inclusion criteria using biochemical, immunofluorescence, and 1H-MRS techniques. A meta-analysis was completed using a random effects model and percentage change in IMCL content calculated from the standardized mean difference. Cycling exercise resulted in a net degradation of IMCL regardless of technique (total effect -23.7%, 95% CI = -28.7 to -18.7%) and there was no difference when comparing fasted versus fed-state exercise (P > 0.05). IMCL degradation using immunofluorescence techniques detected larger effects in type I fibers compared with whole muscle using biochemical techniques (P = 0.003) and in type I fibers compared with type II fibers (P < 0.001). Although IMCL degradation was associated with exercise duration, V̇o2max, and BMI, none of these factors independently related to the change in IMCL content. These findings provide strong evidence that the analytical approach can influence the assessment of IMCL degradation in human skeletal muscle in response to exercise.