In high school, whenever I was hit by a particularly rough period or particularly bad cramps, my mom would tell me to go for a run. As she liked to remind me, “exercise helps with cramps.” That was true; I always did feel better after I dragged on my sneakers and jogged a mile or so.

When I asked Dr. Nancy Williams, head of the Women’s Health and Exercise Lab at Penn State University, about the relationship between periods and exercise, she started out by clarifying that exercise is, in fact, good for women — which definitely didn’t come as a surprise. She highlighted the physiological and psychological benefits of exercise: apart from helping with cramps, exercise has been show to help with stress levels and blood pressure, among other benefits.

But Dr. Williams added the caveat that, when not balanced properly, exercise can cause women to have problems with their periods. “In a subset of women who exercise and don’t do a good job monitoring their food intake so that they’re replacing the calories that they burn with exercise, they can develop something called the female athlete triad,” said Dr. Williams. The female athlete triad is a syndrome describing the connection between energy deficiency, disruptions in the menstrual cycle, and bone loss.

Here’s what happens: when you don’t replace the calories going out (from exercise) with calories coming in (from eating), the brain tells the body to stop producing the hormones that maintain the menstrual cycle. That causes changes in your period.

While clinical eating disorders or disordered eating behaviors can trigger the triad, Dr. Williams says that exercising women can also develop symptoms of the triad without actively restricting their food intake. “Sometimes an athlete has a high energy output and it’s just difficult logistically to get those calories in,” she said. That’s why high performing athletes try to schedule their meals, or augment them with energy bars and drinks. However, it’s also common for the triad to appear in recreationally active women, or women who diet, or any other population of women. In fact, Dr. Williams said that a majority of the research on the triad is conducted not on casually active women, not professional athletes. All it takes is a disconnect between energy in and energy out.

All it takes is a disconnect between energy in and energy out.

“A lot of metabolic hormones shift at that point and that tells the brain that it’s not a good time to reproduce, and your brain automatically slows down the signals to your ovaries, and eventually women can stop ovulating,” says Dr. Williams. That is a perfectly natural biological adaptation to not enough energy in the body (which is not the best time to be pregnant), but when extended over a long period of time, it can lead to other issues.

For example, shifts in the menstrual cycle means shifts in the body’s production of estrogen — a key hormone involved in heart, blood vessel, breast, hair, and skin health. It’s also critical for bone health. A lack of estrogen can cause a decrease in bone density, which can lead to stress fractures. And low energy availability can cause changes in the regulation of another key hormone, called insulin growth factor, which is also connected to bone health. “When estrogen is that low and you’re not eating enough, you have a double whammy to your bones,” said Dr. Williams.

While a complete halt to periods is a clear sign of menstrual changes, Dr. Williams added that there are other, more subtle indications that there might be problems. “You would see a lengthening or shortening of the cycle, or cycles that are irregular in length,” she says. “You might see that the menses itself can become shorter, bleeding that’s less heavy. Another sign is having cramps… your general cramping is actually a good sign. So if you used to have cramps and all of a sudden you’re having a lighter period and you’re not getting cramps, it might indicate that your cycles are becoming a little less robust.”

The research into the female athlete triad has been ongoing for three or four decades, says Dr. Williams. And awareness of the syndrome is gradually increasing.

“I would say a good third of coaches are aware of the female athlete triad, which means that they’re aware that menstrual health, bone health, and eating and energy balance are all related,” she said. “So you would hope that if a third or so of coaches get it, then their athletes would.”

Dr. Williams chairs the research committee of the Female Athlete Triad Coalition, which aims to educate women and athletes about the risks of the triad. And although more and more women and coaches know about and are on the lookout for the triad, there is still more work to be done.

“There are still coaches who think of the point at which you’re not getting your period is when training is adequate. They almost see it as a sign that you’re training hard enough,” she said. “That’s the kind of myth that we’re looking to dispel.”

Nicole is a science writer based in New York. She tweets @NicoleWetsman, mostly about neurons and women's soccer.