If you’re a runner, you’re probably concerned about increasing your endurance, protecting your knees and even keeping your toenails intact. But what you may not realize is that your period can affect how you train.Along with our menstrual cycles come hormonal changes that affect the way our bodies work. For serious runners—who must be especially attuned to their bodies, since they’re pushing them to the limit—knowledge of these changes can help in planning the most efficient, effective workouts.That’s the idea behind Running for Women, a book by Jason Karp, Ph.D., and Carolyn Smith, M.D., that charts out ideal times for women to concentrate their most serious workouts.So what hormonal fluctuations do women experience, exactly, and how do they affect running?

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There are four major hormones involved in menstruation, but progesterone and estrogen are the two main players, and the two that most impact our athletic capacities.

Heat is on:
Progesterone is highest in the middle of the luteal phase, during ovulation, around days 20-22 of a 28-day cycle.Changes in progesterone levels cause our bodies to retain water, which is why you feel bloated starting around the middle of your cycle. The hormone also causes an increase in body temperature. This is helpful knowledge for women trying to figure out the best time to conceive, but for runners, the uptick can be problematic. “Say you’re going to run a marathon and it’s already hot outside,” Karp says. “You wouldn’t want that to coincide with the luteal phase, when your progesterone is highest.”Robert Kraemer, Professor of Exercise Physiology at Southeastern Louisiana University, had the same suggestion. “If you go out and your body temperature is already a bit elevated, and you’re running in the heat as well, then that particular day—around the time of ovulation—you’d have to take extra precaution.”

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But the flip side of our progesterone-based troubles is a batch of estrogen-based benefits. Estrogen levels are highest right before ovulation, on about days 14-15 of a 28-day cycle. According to studies done in 2002 and 2006, the more estrogen we have, the more we tend to break down fat and the less we tend to rely on carbohydrates when we’re exerting ourselves. If we rely on carbohydrates, we more easily run out of fuel—that metaphorical “hitting the wall” sensation—and switching to fat is a great workaround. “There’s a lot less energy in a gram of carbohydrates than a gram of fat, so you’re burning more calories when you burn a gram of fat than a gram of carbohydrates,” Kraemer adds.

In fact, women’s performance in ultramarathons—races longer than marathons, ranging from about 31 to 100 miles (or more!)—is quite similar to men’s, according to Karp. “Because the pace is so much slower on the ultra, you can get by relying on fat,” Karp says. Men do tend to run faster on shorter races—their hearts are, on average, bigger than women’s, plus they have greater blood volume and, as a result, more hemoglobin to carry more oxygen. But owing to the benefits of estrogen—specifically, the shift in fuel source that it causes—performance differences between men and women decrease as race length increases.According to Karp, while women’s 5K and marathon world records are 12.4 and 9.5 percent slower, respectively, than men’s, the women’s 100K world record is only 5.3 percent slower than the men’s world record. Further proof? Pamela Reed, an American runner, won the Badwater Ultramarathon—a 135-mile race from Death Valley to Mt. Whitney, CA—in both 2002 and 2003. Meaning, she beat everyone—men and women both.“When estrogen is highest, estrogen seems to be a very runner-friendly drug,” Karp says. No kidding.

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So by and large, focusing your training on the times when your body is running most efficiently—in other words, right before ovulation, when your estrogen levels are highest—is a good plan. One caveat: if you’re going to have to run while you’re menstruating, it can help to prepare yourself. “If you know the day of your race is going to happen the day you’re right in the middle of your period, then certainly you can train at that time to try to prep yourself both physically as well as psychologically,” Karp says.

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Beyond tuning in to their hormonal changes, women runners should be aware of the tendency to lose iron during menstruation itself. “I’ve coached a lot of females who become anemic,” Karp says. Along with blood loss comes a loss of hemoglobin, which decreases our ability to transport oxygen and can cause fatigue. It’s worth getting your iron levels checked by your doctor, and if you’re low, up your intake of iron-rich foods.An important takeaway here is that women’s bodies work differently from, not worse than, men’s: Women aren’t weaker, and they shouldn’t train less hard. “The individual workouts are the same,” Karp says. “What changes is how those workouts are placed in the program.”He offered an apt analogy. “Remember that commercial for Secret anti-perspirant—‘Strong enough for a man, but made for a woman’? That’s kind of how I look at the training.”

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