Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle

Key points Muscle glycogen and intramuscular triglycerides (IMTG, stored in lipid droplets) are important energy substrates during prolonged exercise. Exercise-induced changes in lipid droplet (LD) morphology (i.e. LD size and number) have not yet been studied under nutritional conditions typically adopted by elite endurance athletes, that is, after carbohydrate (CHO) loading and CHO feeding during exercise. We report for the first time that exercise reduces IMTG content in both central and peripheral regions of type I and IIa fibres, reflective of decreased LD number in both fibre types whereas reductions in LD size were exclusive to type I fibres. Additionally, CHO feeding does not alter subcellular IMTG utilisation, LD morphology or muscle glycogen utilisation in type I or IIa/II fibres. In the absence of alterations to muscle fuel selection, CHO feeding does not attenuate cell signalling pathways with regulatory roles in mitochondrial biogenesis. We examined the effects of carbohydrate (CHO) feeding on lipid droplet (LD) morphology, muscle glycogen utilisation and exercise-induced skeletal muscle cell signalling. After a 36 h CHO loading protocol and pre-exercise meal (12 and 2 g kg(-1), respectively), eight trained males ingested 0, 45 or 90 g CHO h(-1) during 180 min cycling at lactate threshold followed by an exercise capacity test (150% lactate threshold). Muscle biopsies were obtained pre- and post-completion of submaximal exercise. Exercise decreased (P < 0.01) glycogen concentration to comparable levels (similar to 700 to 250 mmol kg(-1) DW), though utilisation was greater in type I (similar to 40%) versus type II fibres (similar to 10%) (P < 0.01). LD content decreased in type I (similar to 50%) and type IIa fibres (similar to 30%) (P < 0.01), with greater utilisation in type I fibres (P < 0.01). CHO feeding did not affect glycogen or IMTG utilisation in type I or II fibres (all P > 0.05). Exercise decreased LD number within central and peripheral regions of both type I and IIa fibres, though reduced LD size was exclusive to type I fibres. Exercise induced (all P < 0.05) comparable AMPK(Thr172) (similar to 4-fold), p53(Ser15) (similar to 2-fold) and CaMKIIThr268 phosphorylation (similar to 2-fold) with no effects of CHO feeding (all P > 0.05). CHO increased exercise capacity where 90 g h(-1) (233 +/- 133 s) > 45 g h(-1) (156 +/- 66 s; P = 0.06) > 0 g h(-1) (108 +/- 54 s; P = 0.03). In conditions of high pre-exercise CHO availability, we conclude CHO feeding does not influence exercise-induced changes in LD morphology, glycogen utilisation or cell signalling pathways with regulatory roles in mitochondrial biogenesis.

Automated summary

Muscle glycogen and intramuscular triglycerides are important energy substrates during prolonged exercise. Exercise-induced changes in lipid droplet morphology have been found to be influenced by exercise and CHO feeding in trained males. CHO feeding did not affect glycogen or IMTG utilization in muscle fibres. Moreover, CHO feeding did not attenuate cell signalling pathways related to mitochondrial biogenesis during exercise.