Women are routinely underrepresented in clinical trials, and studies around sport and exercise are no exception. One major reason for the oversight: periods.
In an editorial published on Monday in the British Journal of Sports Medicine, a group of UK and US researchers calls urgently for research into athletic performance to include more women—and to actually take their menstrual cycles into account. After all, menstruation is a recurrent feature in most women's lives, and female athletes don't stop being athletes when they get their period.
"There isn't really a gender gap in exercise participation levels between men and women, but actually, a lot of research is focused on men," Georgie Bruinvels, a sports health researcher and lead author of the editorial, told me in a phone interview.
If research does include women, it tends to be in the early follicular phase of their menstrual cycle—the first few days, when bleeding occurs. "At that point, your hormone levels are really low and actually most similar to those of a man," Bruinvels explained.
"There's still like a taboo around the menstrual cycle—people don't like talking about it."
As a result, we don't know much about how menstruation and related hormone changes could affect athletic performance, or what kind of training might benefit women at different points in their cycle. At the most fundamental level, we don't know as much about female physiology as we probably should.
In sport and exercise research, the authors found that 39 percent of participants in studies from 2011 to 2013 were women.
This gender bias isn't unique to sport and exercise studies, though the importance of including both sexes is now clear. The authors give the example of women responding differently to drugs. "Evidence suggests that women are almost twice more likely to have an adverse reaction to a drug than their male counterparts, and 80 percent of drugs withdrawn from the market are due to unacceptable side effects in women," they report.
Historically, women have not been included in trials owing to concerns over unborn foetuses but also, ironically, because of the variation that menstrual cycles add, which researchers fear could affect their results by adding more variables. But this is precisely the reason Bruinvels and her colleagues argue we need more studies on women, and to look at them throughout their cycles.
It seems pretty obvious: Women are obviously different from men in this respect, so it's not possible to just pretend men are an effective proxy for women.
41.7 percent of exercising women surveyed said their menstrual cycle had a negative impact on their performance
Why, then, are scientists scared of periods? "I think it's because it's so complex," said Bruinvels. "I think the biggest problem is that, ideally, you'd need a group of females you can get into the lab every day and test for so many different areas—function, strength, injury susceptibility, stress levels—there's so many different areas that need to be appreciated."
This would require a lot of time, funding, and womanpower, and would likely mean taking a few steps back to answer quite basic questions about the menstrual cycle.
But it's necessary work. In a recent study, the same team found that 41.7 percent of exercising women they surveyed said their menstrual cycle had a negative impact on their performance—but, scientifically, we don't really know why. Bruinvels herself is looking into heavy menstrual bleeding and how that might affect performance.
Overall, she wants to open people's eyes to the issue and encourage a focus on female athletes.
"I think that is happening and things are progressing, but there's still like a taboo around the menstrual cycle—people don't like talking about it. There are lots of male coaches particularly out there and I can totally see it's really hard to understand."
She added that there's obviously also a lot of variation among women, and that findings could differ for women from month to month. "But at the moment we're just really clueless about the whole thing."
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