Posted: September 20, 2005
Science of Sport: Muscle Glycogen Stores - Supercompensation Only Works Once
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
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