The menstrual cycle, which occurs monthly from menarche (age 11-14) until menopause (age 45-50), is the defining physiological characteristic of females. The levels of estrogen and progesterone change continuously throughout the cycle as a complex interaction of positive and negative feedback mechanisms regulate the timing and amount of hormone secretion. With the large fluctuations in the concentrations of these hormones, the phase of the menstrual cycle significantly affects the female runner’s hormonal environment and therefore her physiology.
Phases of the Menstrual Cycle
The menstrual cycle is usually 28 days and is divided in half by ovulation on day 14, as the ovum is released from the ovary. The first half of the cycle is the follicular phase and the second half is the luteal phase. The exact length of the menstrual cycle can vary from woman to woman, cycle to cycle, and year to year. Changes in hormone levels can affect the length of the cycle. Teenagers tend to have low or changing progesterone levels, which can alter cycle length. Birth control pills, low body fat, weight loss, being overweight, stress, or intense exercise can also change menstrual cycle length.
The follicular phase of the menstrual cycle, which begins with the onset of menses (the “period”), typically lasts 14 days (but can last 11-21 days). Following menses, which typically lasts three to five days, estrogen rises, peaking on day 14, right before ovulation. The burst of estrogen toward the end of the follicular phase causes a surge in luteinizing hormone on day 15 to initiate ovulation. During the follicular phase, progesterone level remains low.
During the luteal phase of the menstrual cycle, which always lasts 14 days, progesterone rises. Estrogen drops after ovulation before rising again toward the middle of the phase. The increase in progesterone causes body temperature to increase to prepare for the fertilization of an egg. If fertilization does not occur, both estrogen and progesterone levels decrease abruptly. The luteal phase ends with the onset of menses, and the cycle starts all over again.
When you feel bloated during your period, you can blame progesterone. The high concentration of progesterone during the luteal phase affects fluid balance, causing you to lose water and electrolytes. The rapid drop in progesterone as you transition from the luteal phase back to the follicular phase results in excess premenstrual water and electrolyte retention, causing you to feel bloated.
Physiological Effects and Performance Implications of the Menstrual Cycle
While a man’s hormonal environment is pretty stable, a woman’s hormonal environment is constantly changing. Any physiological changes resulting from menstrual cycle-induced fluctuations in estrogen and progesterone are exacerbated during exercise, especially if it’s intense. When you go for a hard run, the concentrations of estrogen and progesterone in your blood increase during both the follicular and luteal phases. Low-intensity exercise, however, does not alter the concentrations of these hormones.
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, iron loss often accompanies a lot of bleeding. If this happens, 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. Athletic anemia is very common among female runners, especially those training at altitude.
Body temperature changes rhythmically throughout the menstrual cycle, peaking during the luteal phase in response to the surge in progesterone. Progesterone acts on the brain’s hypothalamus (the temperature control center), which increases set-point temperature. A higher body temperature increases the threshold for dissipation of heat. In other words, a woman’s body must reach a higher temperature before her 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. Estrogen has the opposite effect on the hypothalamus, decreasing body temperature, which explains why body temperature is lower during the estrogen-dominant follicular phase.
The increased body temperature during the luteal phase remains elevated during exercise and when exercising in the heat. A higher body temperature during the luteal phase makes it harder to run in the heat during this phase, as you don’t begin sweating to dissipate heat until you have reached a higher body temperature. You also have a decreased ability to dilate the small blood vessels under the skin, which compromises your ability to release heat to the environment. Hyperthermia—an increased body temperature—is one of the factors that cause fatigue during prolonged exercise. Thus, long, intense workouts and races in the heat can be more difficult during the luteal phase of the menstrual cycle. The increased body temperature during the luteal phase can also put you at an increased risk of developing heat illnesses like heat exhaustion and heat stroke. Training improves your ability to regulate body temperature.
Metabolism and Muscle Glycogen
Menstrual phase variations in running performance may largely be a consequence of changes to exercise metabolism stimulated by the fluctuations in estrogen and progesterone concentrations. The magnitude of increase in these hormones between menstrual phases and the ratio of estrogen to progesterone concentration appear to be important factors determining an effect on metabolism. Estrogen may improve endurance performance by altering carbohydrate, fat, and protein metabolism, with progesterone often acting antagonistically to estrogen. Estrogen promotes both the availability of glucose and uptake of glucose into slow-twitch muscle fibers, providing the fuel of choice during short duration exercise.
The ability to run for a long time is greatly influenced by the amount of glycogen stored in your skeletal muscles, with fatigue coinciding with glycogen depletion. Research comparing the amount of muscle glycogen in women eating either a normal diet (2.4 grams of carbohydrate per pound of body weight per day) for three days or a high carbohydrate diet (3.8 grams of carbohydrate per pound of body weight per day) for three days has shown that muscle glycogen content is greatest during the mid-luteal phase after both normal and high carbohydrate diets. Muscle glycogen is lowest during the mid-follicular phase. However, a female runner can increase the amount of muscle glycogen in the follicular phase by eating a high carbohydrate diet. There is also a glycogen-sparing effect to the luteal phase, with a greater reliance on fat during submaximal exercise.
Theoretically, with less reliance on carbohydrate for energy, less lactate (and therefore other metabolic byproducts) is produced. Some studies have documented that less lactate is indeed produced during exercise in the mid-luteal phase, while other studies have not. Interestingly, when men are given a synthetic version of progesterone, they produce less lactate during maximal exercise, suggesting that progesterone, which is elevated during the luteal phase, may lower lactate levels.
As a female runner, you can expect to perform better during times of the menstrual cycle when estrogen is the dominant hormone and perform the worst when progesterone is the dominant hormone. Many of the female runners I’ve coached have experienced their worst training days in the few days leading up to and including menstruation. You may find that, while harder workouts may be more challenging during your period, easy running may actually improve your mood and alleviate physical symptoms associated with your period.
Avoid challenging workouts around menses, especially if you don’t feel well at that time or if you feel bloated due to the rapid drop in progesterone as you transition from the luteal phase to the follicular phase. For example, if you have a 28-day cycle starting on Monday, and menses occurs on days 1 to 3 (Monday to Wednesday), plan your hard workout on Thursday or Friday that week. If you have two workouts planned, schedule them on Thursday and Saturday, or schedule just one workout the week of menses and two workouts during the other three weeks of your cycle. If menses lasts five days (Monday to Friday), schedule one workout the week of menses and two workouts during the other three weeks of your cycle.