Estrogen is the single biggest difference between you and that guy running next to you in a race. It's a powerful hormone, influencing many aspects of a woman's physiology, from metabolism to glycogen storage and lung function to bone health.
While a man's hormonal environment is pretty stable, your hormonal environment is constantly changing. Any physiological changes resulting from menstrual cycle-induced fluctuations in estrogen and progesterone are exacerbated when you run, especially if the run is intense. Here are five ways estrogen and progesterone affect your running.
Estrogen is the single biggest influencer of bone health in women, so any disruption to the menstrual cycle can cause a decrease in your bone mineral density, increasing your risk for osteoporosis and stress fractures. Estrogen is extremely important in facilitating the absorption of calcium into your bones. Research has shown that female runners with irregular or absent menstruation have significantly lower bone density than those with regular menstruation and even compared to non-athletes. A number of studies have found a significant loss in bone density, particularly at the lumbar spine, in amenorrheic athletes (women who don't have a menstrual cycle). A female runner with irregular menstrual cycles runs the risk of decreasing bone mineral density to such an extent that stress fractures occur with only minimal impact to the bones.
If you have menstrual irregularities, you must take extra care in planning your training so you don't increase your running volume or intensity too quickly, and you may need to increase your dietary intake of calcium and vitamin D to protect your bones.
The surge in progesterone during the luteal phase (second two weeks) of your menstrual cycle increases body temperature. Progesterone acts on your brain's hypothalamus—your temperature control center—increasing your set-point temperature. A higher body temperature means your body must reach a higher temperature before your internal thermostat compensates and begins to cool itself. Not a good thing when you're running on a hot and humid day, as you want to begin the cooling response as soon as you can. Long, intense workouts and races in the heat can be more difficult during the luteal phase of your cycle.
Research on rats has shown that estrogen has the opposite effect on the hypothalamus, decreasing body temperature, which explains why your body temperature is lower during the estrogen-dominant follicular phase of your menstrual cycle.
The progesterone-induced increase in body temperature during the luteal phase can cause your heart rate to rise during your runs even if it's not hot outside. Some studies have shown that women's heart rates are higher when exercising during the luteal phase, while other studies have shown that heart rate is not different between phases of the menstrual cycle. Regardless of whether your heart rate increases, your aerobic power (VO2max) does not change over the course of the menstrual cycle.
If you bleed a lot during menstruation, it's possible that your blood's hemoglobin concentration may decrease, which can negatively impact your ability to transport oxygen in your blood. Since iron is an important component of hemoglobin, you may need to supplement your normal diet with iron. Many female runners exhibit athletic anemia (low blood iron levels due to physical activity), especially if they lose a lot of blood during menstruation.