The ‘supercompensation’ of muscle glycogen stores that is known to occur as a result of carboloading following exhaustive exercise cannot be relied on after successive bouts, according to a fascinating new study from Australia.

The research team set out to discover whether it is possible to repeatedly supercompensate muscle glycogen stores after repeated exercise bouts undertaken within several days.

Six well-trained subjects completed an intermittent exhaustive cycling protocol on three occasions separated by 48 hours (days 1, 3 and 5) in a five-day period. Twenty-four hours before day 1, they consumed a moderate carbohydrate diet (6g per kg of body weight) followed by five days of a high-carb diet (12g per kg). Biopsies to measure muscle glycogen levels were taken at rest, immediately after exercise on days 1, 3 and 5 and after three hours of recovery on days 1 and 3.

The researchers, who had hypothesised that these highly trained subjects would be able to supercompensate their muscle glycogen stores more than once, were surprised to find themselves wrong. Compared with day 1, resting muscle glycogen was elevated on day 3 but not on day 5.

‘We feel confident,’ the researchers note, ‘that our high-CHO diet would have provided ample substrate for glycogen resynthesis: subjects consumed [12g per kg of body weight] for four successive days: such an amount is 20-35% more than typical glycogen-loading protocols.

‘Accordingly, the failure of muscle glycogen stores to reach supercompensated values on day 5 compared with day 3 of the experimental protocol strongly suggests an impairment in one or more of the mechanisms responsible for muscle glycogen storage, possibly as a direct consequence of the cumulative effect of repeated exhaustive exercise.’

The good news was that, despite this failure to supercompensate, exercise capacity, which was improved on day 3 by comparison with day 1, was maintained on day 5. The researchers find it difficult to explain this phenomenon but suggest it is due to a substantial increase in the contribution to total energy requirements from lipid (fat) oxidation.

Whatever the reason for this continued increase in exercise capacity, it suggests, they conclude, ‘that muscle glycogen supercompensation may not be required in the trained athlete during successive days of competition’.

Med Sci Sports Exerc, vol 37, no 3, pp404-411, 2005