Carbo-loading:
When carbo-loading works for women
by
Clare Whitehead
for peak
performance
Carbohydrate-loading
before endurance events, which has a
well-established performance-enhancing effect in
men, appears less effective in women. Even when
muscle glycogen levels have increased and
performance times have improved, the changes have
tended to be of a lesser magnitude than those
observed in men.
Carbohydrate
supplementation during prolonged exercise is thought
to enhance time to fatigue by maintaining blood
glucose levels, thus facilitating a high rate of
carbohydrate oxidation during the latter stages of
exercise.
Many
women have drawn encouraging conclusions from
results of male-only trials of carbo-loading and
tried to apply the principles to their own diet,
with a frustrating lack of success.
One
suggested explanation for this apparent discrepancy
is that women have a lower respiratory exchange
ratio (RER) than men, with men tending to use
carbohydrate for energy and women tending to prefer
lipids, possibly on account of hormonal differences.
Another possibility is that women don’t ingest
carbohydrate in sufficient quantities to facilitate
muscle glycogen storage.
A
frustrating lack of research into the effects of
carbo-loading and supplementation on
endurance-trained women was the stimulus behind a
recent study in which US and Chinese researchers
examined the metabolic and performance effects of
augmented carbohydrate intake in a group of female
athletes.
The
study participants were eight well-trained endurance
athletes, who performed three 24.2k treadmill
performance runs under three different trial
conditions, spaced one month apart and performed
5-10 days after the first day of each subject’s
menstrual cycle, to minimise the effects of hormones
on fuel metabolism. The trial conditions were as
follows:
1.
Placebo (P) – no carbohydrate loading and
no supplementation;
2.
Carbohydrate
loading and supplementation (L+S);
3.
Carbohydrate
supplementation only (S).
Supplementation
(conditions 2 and 3) consisted of a 6%
carbohydrate-electrolyte solution, given at
20-minute intervals during the treadmill run.
Carbohydrate loading (condition 2) comprised a diet
in which carbs made up 75% of total energy intake.
In the non-loading conditions (1 and 3), carbs made
up 50% of energy intake, with total energy intake
the same in each condition.
The
results of the study were as predicted by the
researchers: no significant change in performance
time for the 24.2km run, despite an increase in
carbohydrate oxidation in the two active treatment
conditions. Blood glucose and lactate levels were
highest in the loading and supplementation
condition, next highest with supplementation-only
and lowest in the placebo condition, in which blood
glycerol levels were highest.
In
addition to the lack of performance effect with
carbo-loading and/or supplementation, there was no
indication that the runners had to work any harder
in the placebo condition. There were no differences
in heart rate, VO2 or perceived rate of exertion
between the trial conditions, suggesting that no
advantages were gained by loading or
supplementation.
Nevertheless, there was evidence that carbohydrate
utilisation increased and blood glucose levels were
maintained in the carbohydrate trials, suggesting
that when carbohydrate levels are increased, female
athletes will burn it preferentially. The higher
glycerol levels observed with placebo indicate that
fat was employed to a greater extent than in the
other two conditions.
Interestingly,
the combination of carbo-loading and supplementation
was not much better than supplementation alone. The
performance time for the run was no faster for L+S,
and there was no difference between the two
conditions for any other measurements except blood
lactate, which was significantly higher in the L+S
trial, suggesting a greater use of muscle glycogen
following carbo-loading.
The
carbo-loading women consumed on average 335g
(5.5g/kg body mass) of carbohydrate for four days
before the trial run, compared with 238g (3.9g/kg
BM) and 214g (3.5g/kg BM) during supplementation
only and placebo respectively. And the point is that
this may not be an adequate amount to raise muscle
glycogen levels sufficiently to improve performance.
Failure
by women to consume as high an absolute amount of
carbohydrate as men has been put forward as one of
the reasons why women do not manage to improve their
performance by means of carbo-loading. Previous
studies on carbo-loading have shown that
carbohydrate intake must reach 500g per day to
optimally fill the muscle and liver glycogen stores
in men, and it could be that the same principle
holds for women. In most successful carbo-loading
studies, men have been consuming more than 8g/kg BM
per day.
The
theory that it is the absolute amount – rather
than the proportion – of carbohydrate in the diet
that is key to performance improvement was borne out
by the results of a previous trial comparing
carbo-loading and the relationship to energy intake
in both men and women. The researchers measured
muscle glycogen content following a
high-carbohydrate diet (75% of total normal energy
intake) and a high-carbohydrate diet plus 34% extra
total energy, which increased both energy and
carbohydrate intake by comparison with their normal
diets (comprising 58% carbs).
While
the men increased their total glycogen concentration
following both the high-carb and the high-carb-plus
diets, women failed to increase muscle glycogen
simply by boosting the proportion of carbohydrate in
their diets. Only by raising their overall energy
intake by 34% and maintaining a 75% carbohydrate
intake did they manage to increase their muscle
glycogen concentration.
The
results of both these studies suggest that for women
to successfully increase their muscle glycogen
levels prior to an endurance event, they must
consume at least 8g/kg/day. For an average 60kg
female to achieve this on a total energy intake of
2000kcal/day, carbs would need to account for more
than 90% of total energy intake. A more practical
solution would be to increase not just the
proportion of carbs in the diet but also the total
energy intake.
Strong
evidence links increased muscle glycogen stores with
improved endurance capacity, and efforts to boost
carbohydrate intake should be considered an
important part of pre-race preparation for any
serious female endurance athlete.
Given
that carbo-loading is a strategy likely to be
employed only once or twice a year, this should not
pose major problems for athletes who are dedicated
to improving their race times.
However,
since carbohydrate supplementation during exercise
failed to make up for lack of adequate
carbo-loading, it should not be considered a viable
alternative.
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