Author(s): HANNON, S., HENTHORN, L., VADHER, J., MONTGOMERY, C., PAVIS, G.F., STEPHENS, F.B., WALL, B.T., HODSON, N., MORTON, J.P., KENNERLEY, A.J., HEARRIS, M.A., Institution: MANCHESTER METROPOLITAN UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1106

INTRODUCTION:
Endogenous carbohydrate (CHO) provides the primary energy substrate during high intensity exercise. Whilst muscle glycogen utilisation during such exercise is well documented, little is known about liver glycogen utilisation, largely due to methodological challenges in measuring hepatic glycogen. Furthermore, the influence of CHO feeding on liver glycogen utilisation during high-intensity exercise remains unclear. Accordingly, the primary aim of this study was to quantify liver and muscle glycogen use during a bout of high-intensity interval exercise (HIIE), and to determine the impact of CHO feeding, using non-invasive 13C magnetic resonance spectroscopy (MRS).
METHODS:
In a randomised, repeated-measures crossover design, nine trained male cyclists (age, 29 ± 8 years; V̇O2max, 55 ± 10 ml·kg−1·min−1) completed a bout of HIIE (8 x 5 min at self-selected intensity with 1-min of recovery), ingesting either placebo (PLA), 60 g CHO (CHO), or 60 g CHO co-ingested with a multi-ingredient formulation of caffeine, taurine, l-citrulline, l-theanine and citicoline (CHO+). Liver and muscle glycogen concentrations and liver volume were measured immediately pre- and post- exercise using 13C MRS and magnetic resonance imaging. Blood metabolites and hormones were measured throughout exercise.
RESULTS:
Net liver glycogen utilisation, expressed as both concentrations (mmol.L-1) and total content (g) did not differ between trials (P = 0.002, P <0.001, respectively), despite lower plasma glucagon concentrations in CHO and CHO+ trials compared with PLA (both P = 0.001). In contrast, net muscle glycogen utilisation was reduced with CHO compared with PLA, although this glycogen sparing effect was not evident with CHO+ (P = 0.073). Whilst plasma glucose was comparable between trials (P = 0.175), plasma lactate was significantly higher in the CHO+ vs CHO trial (P = 0.003) and accompanied by lower blood bicarbonate (P = 0.005), base excess (P <0.001) and total CO2 (P = 0.004). Although average power output was similar between trials (P = 0.074), power output during the latter stage intervals (intervals 5-8) was significantly higher when compared to CHO (P = 0.05) and PLA (P = 0.048) in line with elevated plasma caffeine concentrations.
CONCLUSION:
Using non-invasive 13C MRS, we provide the first report to simultaneously characterise muscle and liver glycogen utilisation in response to high intensity cycling. These data demonstrate that muscle glycogen is the predominant energy substrate for energy production during high-intensity interval cycling and its utilisation is spared with CHO feeding. In contrast, liver glycogen utilisation was comparatively small and unaffected by CHO feeding. These findings provide new insight into the glycogen cost of high-intensity interval exercise and may inform practical fuelling strategies to promote training intensity.