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Losing your period was once seen as a normal sign of hard training, or even a badge of honor. Now, medical experts understand it’s a red flag you aren’t taking in enough fuel to support your running—a problem that could lead to poor performance, injuries, and other health problems over time.
Missed months are one thing, but an interesting new study in soccer players suggests even minor shifts in your cycle could signal problems, lending credence to tracking your period. The study also offers some early data on whether injury risk fluctuates based on where you are in your cycle.
For the study, published in the journal Frontiers in Sports and Active Living, British researchers tracked almost 4,000 high-level soccer players for four years. When the women got hurt, they’d report their injury, along with the typical length of their menstrual cycle and how long it’d been since their last period.
In that time, 115 players developed 156 injuries. About one in five injuries occurred when they were “overdue”—in other words, when the women were expecting their periods, but hadn’t yet gotten them. “For example, if an athlete normally has a menstrual cycle length of 28 days but the injury occurred when it had been 32 days since their last period, we classified this as ‘overdue,’” study author Dr. Daniel Martin, lecturer in sport & exercise physiology at the University of Lincoln in Lincolnshire, U.K., writes in an email to Women’s Running.
These disruptions may signal the start of relative energy deficiency in sport, or RED-S, which stems from not taking in enough energy to cover what you expend during training, In some athletes, this happens intentionally (to lose weight, or because of disordered eating); in others, it’s accidental (say, because they got too busy to snack, or just didn’t realize they needed to eat more).
Your body then begins shutting down functions it deems non-essential, like preparing to have a baby, to keep vital operations humming. You might have a harder time recovering from training, or from minor twinges and niggles that can turn into bigger injuries. And if RED-S isn’t addressed, it can lead to low bone density, stress fractures, and many other problems.
“Lighter periods, shorter durations, and delays in the next period—more days in between each period—all are potential signs of low energy availability,” says Dr. Emily Kraus, who studies and treats running injuries as a clinical assistant professor at Stanford Children’s Orthopedic and Sports Medicine Center. “We think about that as kind of a check-engine light: ‘OK, let’s sit down and reflect and see what’s going on.’”
In addition to the effect of delayed periods, the study also aimed to explore injury rates by phase in the menstrual cycle, at least based on women’s reporting from tracking their own periods (they’d need blood tests to tell for sure, something that’s tough to do in larger study). The results were compelling, Martin says: Muscle and tendon injuries were 88 percent more likely to occur in the late follicular phase, when levels of estrogen are highest in preparation for your period, compared to the early follicular or luteal phases.
The study wasn’t designed to pinpoint why, but there are several possibilities. One is that hormonal shifts affect your coordination. If that’s the issue, the results might be different in runners, Martin says. After all, we’re more prone to overuse injuries that develop over time. Meanwhile, soccer players might fall or collide while leaping to block a goal or cutting around an opposing player, activities where coordination plays a bigger role.
However, high estrogen levels could also influence the structure of muscles, tendons, and ligaments. In fact, a few previous studies have shown women are more prone to ACL injuries at this phase, perhaps because their ligaments are looser and knee joints less stable. If that’s the case, Martin says, the study findings might indeed have implications for endurance athletes.
Right now, neither Martin nor Dr. Kraus recommend altering your training based on your cycle to avoid getting hurt. While it’d be great to have these types of guidelines eventually, more research is needed, including in runners specifically. “This study does, however, support many other studies which have consistently highlighted the health and performance benefits of regular menstruation, which should be a priority for exercising women,” Martin says.
Dr. Kraus agrees, and suggests tracking your period using an app or a paper log. Don’t just look for skipped cycles; drill into the details to spot potential issues even earlier. “If there are inconsistencies from month to month, that’s still a sign that something’s off,” she says. “Over time, that could be an injury that’s brewing, and could also lead to performance impairments.”
When irregularities arise, take a look at your training log (if you keep one) for clues, Dr. Kraus says. Did you recently bump up your mileage, and if so, are you eating more to cover it? Also consider if you have other sources of stress in your life, which can boost your energy needs.
In some cases, simple tweaks to your training or fueling—such as eating more or increasing the amount of recovery time between workouts—are easy to make on your own. But if problems persist or you aren’t sure how to solve them, consider talking with a sports-medicine specialist or booking a visit or two with a sports dietitian. Blood tests and a review of your training and nutritional habits can spot any mismatch between the energy you take in and what you’re putting out, so you can get back to running healthier—and faster.