When rising British tennis star Heather Watson unexpectedly crashed out of the Australian Open in the first round last month, she attributed her poor performance to "girl things."
Even this euphemistic mention of the links between menstrual cycle and athletic performance raised eyebrows, and sparked a wave of discussion about what one commentator called "the last taboo in sport." While there's little doubt that the effects are real, researchers have struggled to nail down exactly what happens and how best to deal with it, in part because everyone responds differently. Solutions that have worked for the world's top marathon runner and a Canadian swim team might not result in similar successes for other athletes.
"Performance just drops off at certain parts of the cycle in some female athletes," says Dr. Andrew Bosch, a researcher at the University of Cape Town and the Sport Science Institute of South Africa. "However, some are never affected, so you can see how individualistic this can become."
While cramping is the most tangible issue, Bosch and others have found that variation in hormone levels throughout the typical 28-day cycle can influence exercise performance in several subtle ways, including changing the body's temperature regulation and how it processes carbohydrates, protein and fat.
The two key hormones are estrogen and progesterone. Following the onset of menstruation, both hormones remain low for most of the first (follicular) menstrual phase, until a sharp spike in estrogen that precedes ovulation. During the second (luteal) menstrual phase, both hormones rise and progesterone takes over from estrogen as the dominant one.
Bosch and his colleague Dr. Tanja Oosthuyse of the University of the Witwatersrand in South Africa have found that these hormone fluctuations hamper pre-exercise carbohydrate storage during the first menstrual phase, and may compromise carbohydrate and protein availability during prolonged exercise during the second phase.
That means that carbohydrate loading the day before exercise is more important for female endurance athletes during the first phase, say Oosthuyse. During the second phase, it becomes more important to refuel during prolonged exercise, primarily with carbohydrates, but perhaps also with protein if you're out for more than a few hours.
The sweet spot? In a cycling time-trial study, Oosthuyse and Bosch found that one to two days before ovulation, when estrogen is highest and progesterone is lowest, may favour endurance performance.
Still, these fuel-burning differences are so subtle that it's not likely worth altering the timing of your cycle: "[The] metabolic effects can mostly be negated with purposeful nutrition," Oosthuyse says.
Severe premenstrual symptoms, on the other hand, can be more disruptive. When asked by the BBC after Watson's flame out, Paula Radcliffe, the British marathon runner and current world record-holder, said she tried taking the drug norethisterone, a synthetic progesterone, to delay menstruation if her period was going to interfere with a competition, but found that it "made things a hundred times worse." Instead, she went on to set her first marathon world record, at the 2002 Chicago Marathon, on the day her period started, not having taken any drugs.
Another option for athletes is to plan their cycles long before major competitions, which is the approach Dr. Greg Wells, a researcher and sports scientist at the University of Toronto and the Hospital for Sick Children, used when working with the national synchronized swimming team. "We actually planned out [and altered, with birth control] when the team would hit parts of their cycle 12 months in advance of the Olympics," he said in an e-mail.
This approach works well for athletes with a single major competitive goal. Radcliffe, for example, would take birth-control pills for three weeks at the start of each season to synchronize her cycle with major championships. Tennis players such as Watson, on the other hand, have to be at their best for weeks at a time, multiple times a year, making such scheduling almost impossible.
Given the widely varying individual responses, Bosch suggests that the first step is to understand how your cycle affects you. "Female athletes should keep a diary of sorts of how they perform at different parts of their cycle, and work out where they perform well and where poorly," he says.
He cites the example of a runner he works with who has a six-week cycle, and performs consistently poorly at one stage in the cycle that doesn't agree with anything suggested by the research. What matters, he says, is that they now know this and can plan around it.
Watson, meanwhile, plans to consult with medical staff to see if there's anything she can do to avoid a recurrence of her Australian Open experiences, which included dizziness, nausea and low energy levels. The solution isn't obvious – but at least she has helped start a discussion about a neglected topic.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com. His latest book is Which Comes First, Cardio or Weights?