How much muscle glycogen is needed for high-intensity exercise performance? Original paper

In this crossover trial in recreationally active men, short-duration/high-intensity exercise performance was similar after 3 days of either a moderate-carbohydrate or high-carbohydrate diet, despite the participants having higher muscle glycogen levels after the high-carbohydrate diet.

The effect of short-duration manipulation of carbohydrate intake on muscle glycogen content, body mass, performance during 1-minute and 15-minute maximal cycling tests, and sprint ability.

22 recreationally active men (average age of 24, average VO₂max of 56.5 mL/kg/min).

In a randomized crossover design, the participants completed two 5-day interventions separated by a 10-day washout period. All of the participants started the intervention with a rest day and moderate carbohydrate consumption (4 grams per kilogram of body mass per day). The next day, the participants performed an exercise protocol consisting of arm cranking and cycling exercise to reduce muscle glycogen content in both the upper and lower body.

The participants then continued the moderate-carbohydrate diet or consumed a high-carbohydrate diet (10 grams of carbohydrate per kilogram of body mass per day) for 3 days. The two diets included three identical meals and snacks that contained the same amounts of protein, fat, and dietary fiber but different amounts of carbohydrate and total energy. Additionally, physical activity was strictly controlled during the interventions.

At the end of each 5-day intervention, the participants performed a test of either 1 minute or 15 minutes of maximal cycling, followed by a sprint ability test performed approximately 15 minutes later.

Body mass was measured before the intervention and on the morning of the exercise tests. Muscle biopsies from the vastus lateralis were obtained before and immediately after the cycling tests to assess muscle glycogen content.

Muscle glycogen content prior to the exercise tests was 43% higher (159 mmol per kilogram of dry weight) after the high-carbohydrate diet compared to after the moderate-carbohydrate diet, which resulted in a 0.7 kilogram higher body mass. Body mass decreased in both groups from baseline.

There were no differences between diet interventions in average power output during the 1-minute or 15-minute cycling test. Perceived exertion during the tests and muscle glycogen utilization also did not differ between diet interventions, nor did sprint ability following the cycling tests.

The rate of muscle glycogen utilization during exercise increases as exercise intensity increases, with glycogen becoming the primary fuel source at intensities of at least 75% of VO₂max.^[2][3]

Relative contributions of carbohydrate and fat fuel sources during exercise

Adapted from Hargreaves and Spriet, 2020, Nature Metabolism.^[4]

The importance of muscle glycogen availability during prolonged exercise (more than 1 hour) at moderate to high intensities is well established.^[5] However, there is comparatively little research pertaining to the importance of muscle glycogen availability during shorter-duration exercise, particularly during events of no more than 15 minutes.

Muscle glycogen is a critical fuel source during short-duration/maximal intensity exercise, as evidenced by a high rate of degradation. For example, a 30-second bout of maximal effort cycling reduced muscle glycogen by approximately 89 millimole per kilogram of dry weight (mmol/kg DW),^[6] and 75 seconds of maximal effort cycling reduced muscle glycogen by over 100 mmol/kg DW.^[7] For reference, average resting muscle glycogen levels have been reported to be approximately 485 mmol/kg DW, with levels as high as 700–900 mmol/kg DW in highly-trained athletes.^[8]

Nonetheless, the summarized study reported that higher muscle glycogen levels did not enhance short-duration/high-intensity exercise performance. The results of some prior studies support these findings,^[7][9] while others found better performance when pre-exercise muscle glycogen levels were higher.^[10][11][12]

The difference in pre-exercise muscle glycogen levels between conditions may explain these conflicting findings — specifically, how low muscle glycogen levels were in the lower-carbohydrate diet intervention.

In the summarized study, pre-exercise muscle glycogen levels were 525 in the high-carbohydrate intervention and 367 mmol/kg DW in the moderate-carbohydrate intervention. In 2 other studies that reported that pre-exercise muscle glycogen levels did not affect short-duration/high-intensity exercise performance, pre-exercise muscle glycogen levels in the higher-carbohydrate and lower-carbohydrate diet interventions were approximately 668 and 462 mmol/kg DW^[7] and 765 and 376 mmol/kg DW, respectively.^[9]

In contrast, in studies that reported that pre-exercise muscle glycogen levels influenced short-duration/high-intensity exercise performance, pre-exercise muscle glycogen levels in the higher-carbohydrate and lower-carbohydrate diet interventions were 291 and 175 mmol/kg DW,^[12] 346 and 222 mmol/kg DW,^[10], and 397 and 180 mmol/kg DW, respectively.^[11]

Collectively, these data indicate that when pre-exercise muscle glycogen levels are less than approximately 250 mmol/kg DW, an impairment in short-duration/high-intensity exercise performance is probable.

The leading hypothesis for why short-duration/high-intensity exercise performance is impaired when muscle glycogen levels are too low has to do with sarcoplasmic reticulum function. The sarcoplasmic reticulum is the storage site for calcium within muscle cells, and it releases calcium to trigger muscle contraction.

Evidence suggests that low muscle glycogen content results in reduced calcium release from the sarcoplasmic reticulum,^[13][14][15] which ultimately impairs muscle function and force production.

The available evidence suggests that pre-exercise muscle glycogen content should be at least 300 mmol/kg DW for optimal short-duration (no more than 15 minutes) and high-intensity exercise performance.

Excess nonfunctional weight can impair exercise performance,^[1] which leads many competitive athletes to use strategies to reduce their body weight for the purpose of improving their power-to-weight ratio (i.e., power output during exercise divided by body weight). Although the moderate-carbohydrate diet reduced body mass without adversely affecting exercise performance, there was no significant difference in the amount of power produced normalized to body mass between diet interventions. It’s likely that the difference in body mass between diet interventions was not enough to detect a statistically significant difference for this outcome.