Most athletes are aware of the concept of 'supercompensation', whereby muscle glycogen stores and endurance performance are boosted in response to a regime of exercise tapering and carbohydrate loading. But do women benefit to the same extent as men?
Studies of these regimes to date have mostly been performed on men. And those which have included women have tended to produce negative or marginal results. But now a small but important Canadian trial has demonstrated that well-trained women do boost their muscle glycogen stores and their endurance performance in response to carbo-loading and exercise tapering - albeit to a lesser extent than men.

The study involved six female athletes - three triathletes, two cyclists and a runner - who had their endurance and their muscle glycogen content tested after two different week-long dietary regimes combined with exercise tapering. On days 1-6 of each trial the women followed an exercise-tapering programme, with 90 minutes of exercise (either cycling or running at their normal intensity) on day one, 45 minutes on days two and three, 20-30 minutes on days four and five, and rest on day six.

For the dietary aspect of the experiment, the women were randomly assigned to one of two regimes: a 7-day moderate-carbohydrate (MD) diet, with about 48% of total daily energy derived from carbohydrate or a 3-day MD diet followed by four days of a high-carbohydrate diet (HCD), in which about 78% of total daily energy came from carbohydrate.

On the seventh day all the women cycled to voluntary exhaustion at about 80% of their VO2 max. Glycogen content was measured by biopsy of their vastus lateralis leg muscles before and after this exercise bout. In a subsequent week-long trial the women switched dietary regimes so that all six followed both types of diet.

In terms of performance, the women were able to cycle for about nine minutes longer after the HCD than the MD (an average of 115.31 minutes compared with 106.35). In terms of muscle glycogen, pre-exercise glycogen content was on average 13% higher after HCD than after MD. However, post-exercise muscle glycogen contents were not significantly different suggesting greater net utilisation by muscles after HCD.

Why did these results differ from those of previous studies involving women? One key difference is that the female athletes in previous studies were tested during the follicular (early) phase of their menstrual cycles, when circulating reproductive hormones are low. Other research has suggested that glycogen synthesis may be increased during the luteal (late) stage of the cycle, when hormone levels are much higher. And the current study, carried out exclusively during the luteal phase of the women's cycles, seems to bear out that conclusion.

The researchers conclude that 'the women were able to supercompensate glycogen - but not to the same magnitude as generally reported in men'. The women's performance and muscle glycogen increases (8% and 13% respectively) were much smaller than those previously reported in men (20% and 40%).

One reason for this disparity may be the simple fact that women eat less than men. The researchers point out that it may be difficult for women whose habitual calorie intakes are less than 2,400 kcal/day - which was true of all the women in the trial - to achieve higher carbohydrate-intake values. By contrast, male athletes are known to consume more than 3,000 and often more than 5,000 kcal/day during carbo-loading.

Supercompensation may also be influenced by gender differences related to the uptake and storage of glucose. But the team concludes that: 'Little information presently exists to explain this gender difference in the ability to store muscle glycogen.'

J Appl Physiol 88:2151-2158, 2000