Application of ¹³ C MRS demonstrates carbohydrate feeding spares muscle but not liver glycogen utilisation during high-intensity interval exercise

We examined liver and muscle glycogen utilisation during high-intensity interval cycling, and the impact of carbohydrate (CHO) feeding, using non-invasive ¹³ C magnetic resonance spectroscopy (MRS). Following 24 h of standardised dietary intake, nine male cyclists completed 8 x 5-min intervals (1-min recovery), ingesting either placebo (PLA), 60 g maltodextrin (CHO) or 60 g maltodextrin plus caffeine, taurine, l-theanine, l-citrulline and citicoline (CHO+) in a randomised crossover design. ¹³ C MRS and ¹ H imaging were performed pre- and post-exercise to determine liver and muscle glycogen and liver volume, respectively. Liver glycogen utilisation was not significantly different between trials ( P = 0.101) despite lower post-exercise plasma glucagon concentrations in CHO and CHO+ ( P = 0.001). In contrast, muscle glycogen utilisation was significantly lower (~40%) with CHO feeding compared to PLA ( P = 0.006) yet this sparing effect was not evident with CHO+ ( P = 0.073) in accordance with a higher mean power output during the late intervals (+ 2.8%, P = 0.046). Plasma glucose was comparable between trials ( P = 0.175) whereas plasma lactate was higher in CHO+ vs CHO ( P = 0.003), alongside lower blood bicarbonate ( P = 0.005), base excess ( P <0.001) and total CO ₂ ( P = 0.004). These findings demonstrate preferential use of skeletal muscle glycogen during HIIT, which is attenuated under conditions of CHO feeding. This sparing effect is, however, not evident with the co-ingestion of a caffeine containing multi-ingredient blend, potentially due to an increased capacity to sustain higher power outputs resulting in greater glycogen utilisation.