Optimisation of carbohydrate intake for skeletal muscle glycogen synthesis and endurance cycling performance

Jones, RO (2025) Optimisation of carbohydrate intake for skeletal muscle glycogen synthesis and endurance cycling performance. Doctoral thesis, Liverpool John Moores University.

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Abstract

The importance of a high pre-exercise CHO intake for increased skeletal muscle glycogen concentrations and improved endurance capacity has long been understood. However, there is high variability in reported muscle glycogen concentrations following similar CHO intakes, and increased glycogen stores have not always translated into improved endurance performance. Therefore, this thesis aimed to investigate the relationship between CHO feeding, CHO availability, whole muscle glycogen, and endurance cycling performance.

Accordingly, Chapter 4 aimed to describe and quantify the relationship between dietary CHO intake and muscle glycogen in previous research with a systematic review and meta-analysis. Studies were sourced from 5 electronic databases and were assessed using tally markings (56 trials, n = 571 participants), linear regression and a generic inverse-variance random effects meta-analysis with subgroups used to investigate heterogeneity (17 trials). Linear regression indicated a significant linear relationship between relative CHO intake and whole muscle glycogen for endurance trained individuals (r2 = 0.493, P < 0.001). Meta-analyses revealed increased dietary CHO intake from a low-moderate to high-very high quantity (< and > 6.5 g⋅kg-1⋅day-1, respectively) significantly increased muscle glycogen concentrations (198.8 mmol⋅kg-1 DM, 95% CI from 148.0 to 249.7 mmol⋅kg-1 DM, P < 0.0001) but displayed significant heterogeneity (P < 0.0001, I2 = 94%). Glycogen concentration primarily depends on quantity of CHO ingested, with period of increased CHO intake, participant training status, and exercise all being potent effectors. Based on current findings to optimise pre-competition muscle glycogen stores athletes should consume > 8 g⋅kg-1⋅day-1 for 36-72 h pre-competition, with the use of an exercise stimulus combined with consumption of high glycaemic CHO allowing more rapid enhancement of stores (24-36 h). However, significant heterogeneity in meta-analysis results raises uncertainty regarding recommendations.

As such, Chapter 5 aimed to determine the dose-response between dietary CHO intake and muscle glycogen in endurance trained individuals, whilst controlling the previously determined key effectors of glycogen storage and replicating real world pre-competition training and nutrition practices of athletes. Following two days of standardised dietary intake and exercise prescription, 11 endurance trained participants (8 males, 3 females; age, 24 ± 5 years; body mass, 71.2 ± 12.0 kg; V̇O2max, 56 ± 6 mL⋅kg-1 ⋅min-1; PPO, 306 ± 54 W; LT1, 164 ± 49 W) consumed either 6, 8 or 10 g⋅kg-1⋅day-1 of CHO for the following 48 h, before returning to the laboratory (day 5) for a high CHO breakfast and muscle biopsy 2 h post-prandial. Muscle glycogen was significantly higher following 10 compared to 6 and 8 g⋅kg-1⋅day-1 of CHO (635.5 ± 78.0, 460.9 ± 100.7 and 506.1 ± 124.0 mmol⋅kg-1 DM, respectively, P < 0.03), with no difference between 6 and 8 g⋅kg-1⋅day-1 (P = 1.00). There was a significant strong positive correlation between relative (r = 0.71, P < 0.001), absolute CHO intake (r = 0.64, P < 0.001) and whole muscle glycogen concentration. In agreement with Chapter 4, there was a strong linear dose-response between dietary CHO intake and muscle glycogen concentrations, however study data suggest intakes ≥10 g⋅kg-1⋅day-1 are necessary to maximise muscle glycogen stores in real world training conditions.

Whether maximisation of muscle glycogen stores is required to optimise endurance performance remains unclear, as almost all CHO loading studies have failed to adequately blind study participants to CHO intake. As such Chapter 6 aimed to determine whether increased glycogen stores translated into improved endurance performance. In a repeated measures double blind design, 9 endurance trained males (V̇O2max, 63.4 ± 5.2 mL⋅kg-1⋅min-1; PPO, 367 ± 37 W) completed 3 x 4 days of dietary control and prescribed exercise designed to mimic pre-competition practices of endurance cyclists, consuming 6, 8 or 10 g⋅kg-1⋅day-1 of CHO for 48 h, before completing a performance test the following morning (2 h steady state pre-load and ~30 min cycling time trial). There was no significant difference in TT completion time (36 min 46 s ± 4 min 18 s, 34 min 55 s ± 5 min 12 s and 35 min 46 s ± 5 min 56 s; P = 0.16) or mean power output (226 ± 22, 239 ± 29 and 234 ± 29 W; P = 0.10) between 6 vs 8 and 10 g⋅kg-1⋅day-1, respectively. However, there was a large effect size (η2p = 0.21 and η2p = 0.26, respectively) possibly biased by the placebo effect, as 3 participants who correctly identified the placebo condition had the greatest improvements in cycling performance. In conclusion, current data suggests CHO loading with a high or very high CHO intake (8 or 10 g⋅kg-1⋅day-1) provides no benefit to endurance cycling performances lasting ~2.5 h under real world conditions of high exogenous CHO availability.

Lastly, fuelling during endurance exercise has evolved towards greater amounts of CHO ingested per hour, however the effects of different CHO ingestion patterns during exercise have scarcely been investigated in cycling (Chapter 7). In a randomised counterbalanced order, 20 recreationally active males (V̇O2max, 50.4 ± 3.8 mL⋅kg-1⋅min-1; LT1, 139 ± 29 W) cycled for 180 min at LT1 and consumed 90 g⋅h-1 of CHO, either as 22.5g every 15 min or 45g every 30 min. Physiological responses showed no difference between conditions (P > 0.20) or significant interactions (P > 0.30), except for blood glucose which saw a transient difference during the first 30 min (interaction; P = 0.03). Whole body CHO and fat oxidation were not different between conditions (2.38 ± 0.37 and 2.33 ± 0.39 g⋅min-1, P = 0.25, and 0.19 ± 0.07 vs 0.22 ± 0.08 g⋅min-1, P = 0.10, respectively). Ingesting a larger CHO amount at less regular intervals during prolonged cycling had minimal impact on physiological responses to exercise, whole-body substrate oxidation and gut discomfort, allowing athletes to freely select their preferred strategy.

In conclusion, this thesis provides novel data describing the linear dose-response relationship between CHO intake and muscle glycogen, indicating that higher CHO intakes may be better to maximise muscle glycogen concentrations under real world training conditions. However, under conditions studied in this thesis (2.5 h endurance cycling), it appears unnecessary for endurance trained cyclists to CHO load with ≥10 g⋅kg-1⋅day-1 of CHO prior to ~2.5 h of endurance cycling, provided exogenous CHO provision pre and during exercise is optimal. However, greater exercise demands (increased intensity and duration), compared to the current exercise protocol, may require adjustments to CHO availability, possibly achieved with CHO loading to maximise glycogen stores.

Item Type:	Thesis (Doctoral)
Uncontrolled Keywords:	Nutrition; Exercise; Metabolism; Performance
Subjects:	T Technology > TX Home economics > TX341 Nutrition. Foods and food supply R Medicine > RC Internal medicine > RC1200 Sports Medicine
Divisions:	Sport and Exercise Sciences
Date of acceptance:	23 October 2025
Date of first compliant Open Access:	29 October 2025
Date Deposited:	29 Oct 2025 13:34
Last Modified:	29 Oct 2025 13:34
DOI or ID number:	10.24377/LJMU.t.00027420
Supervisors:	Louis, J, Areta, J and Pugh, J
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27420

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