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Nutrition: carbo-loading for female athletes
When carbo-loading works for women
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 (1).
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:
- Placebo (P) – no carbohydrate loading and no supplementation;
- Carbohydrate loading and supplementation (L+S);
- 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(2). 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.
- Journal of Applied Physiology, vol 95, pp584-590, 2003
- Journal of Applied Physiology, vol 91, pp225-230, 2001