From Running for Women by Jason R. Karp, PhD
Running Around the Menstrual Cycle
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 the four hormonal markers of the menstrual cycle—estrogen, progesterone, follicle-stimulating hormone, and luteinizing hormone—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. Indeed, many physiological aspects are affected by the phase of the menstrual cycle, including oxygen consumption, body temperature, hydration, and metabolism, as the sex hormones rise and fall, suggesting that the menstrual cycle affects how women will respond and adapt to training.
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 last 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 your athletes feel bloated during their periods, they can blame progesterone. The high concentration of progesterone during the luteal phase affects fluid balance, causing them to lose water and electrolytes. The rapid drop in progesterone as they transition from the luteal phase back to the follicular phase results in excess premenstrual water and electrolyte retention, causing them to feel bloated.
Premenstrual syndrome (PMS) is a variety of physical and/or psychological and emotional symptoms that occur toward the end of the luteal phase in the days leading up to menses. Many women of reproductive age experience PMS. Interestingly, the incidence of PMS is higher among separated and divorced women than among married or single women. Hormonally, PMS is characterized by a rapid drop in both estrogen and progesterone. The most common symptoms are headache, breast swelling and tenderness, cramping, bloating, fatigue, depression, and irritability. The specific cause of PMS is not known, although a number of theories exist, including progesterone deficiency, progesterone withdrawal, excessive amounts of estrogen, estrogen withdrawal, and changes in the estrogen-to-progesterone ratio, changes in prolactin levels, a drop in the level of endorphins, and psychological issues. Cramping, one of the more difficult menstrual issues for the female runner, is thought to be due to an increase in prostaglandin, a hormone produced by the uterus that causes the uterus to contract. Birth control pills and over-the-counter anti-inflammatory drugs, such as ibuprofen or naproxen sodium, can reduce the severity of cramps by inhibiting the release of prostaglandin. Premenstrual irritability may be related to a high estrogen-to-progesterone ratio and premenstrual depression to a low ratio, however, research has been unable to document specific changes in the hormone levels in relation to the appearance of PMS symptoms. Breast tenderness may be due to an increase in prolactin, a hormone secreted by the pituitary glands. PMS symptoms may worsen when runners experience major hormonal changes, such that occur during pregnancy, immediately following childbirth, miscarriage, or when taking oral contraceptives.
While there’s not a lot of research on the effects of exercise on PMS, the research that has been done has shown that exercise reduces its symptoms. Research examining the effects of exercise on mood has generally found that intense exercise performed on a regular basis, especially running, is psychologically beneficial for women, as it reduces tension and increases psychological well-being. However, there will always be some women for whom exercise is ineffective at alleviating the effects of premenstrual mood swings.
Many female runners who train hard and train a lot who have a low body fat percentage often experience irregular or even absent menstrual cycles, which reduces estrogen levels. Women who start training before menarche delay their menstruation for almost a year, compared to women who already have menstrual periods when they start training. In other words, training, especially intense training, can cause a delay in menarche for up to a year. Once menstrual activity commences, its continued occurrence is also sensitive to training. In response to heavy training, the first change in menstrual cycle activity is a shortening of the luteal phase, followed by cycles without ovulation and, finally, cessation of menses called amenorrhea. Amenorrhea (defined as 0 to 3 periods per year) results in constant low levels of estrogen and progesterone. A female runner with amenorrhea has about one-third the estrogen concentration and about 10 to 20 percent the progesterone concentration of a normally menstruating woman. Thus, endocrinologically, the amenorrheic female runner experiences an estrogen-deficient state similar to that of a post-menopausal woman.
The incidence of menstrual irregularity or amenorrhea is variable—some female runners can train with high volumes and never disrupt or lose their menstrual cycle activity, while some women notice changes in cycle activity with relatively little training. High training volumes, low body weight, and endurance sports like distance running increase the incidence of menstrual irregularities. Long-distance runners in particular are at an increased risk for menstrual irregularity or amenorrhea. Inadequate caloric intake to match caloric expenditure, rather than the stress of exercise, is responsible for the loss of menstrual activity. Consuming more calories to compensate for the large caloric expenditure from running can prevent amenorrhea. Therefore, if your athletes run a lot, they need to increase how many calories they consume throughout the day to keep up with a large number of calories they expend by running.
One of the biggest ramifications of menstrual irregularity or amenorrhea is its effect on your athletes’ bones. Any disruption to the menstrual cycle can cause a decrease in bone mineral density, increasing the risk for osteoporosis and stress fractures. Estrogen is extremely important in facilitating the absorption of calcium into bones. Female distance runners with irregular or absent menstruation have significantly lower bone density than those with regular menstruation and even compared to non-athletes. Furthermore, there is a significant loss in bone density, particularly at the lumbar spine, in amenorrheic athletes. 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.
Along with the other two characteristics of the female athlete triad—osteoporosis and disordered eating—menstrual irregularities greatly increase a female runner’s risk for stress fractures. Therefore, if you coach a team of female runners who are at risk for menstrual irregularities, the runners’ bone density should be checked on a regular basis and you must take extra care in planning their training program so they do not increase their running volume or intensity too quickly, and they may need to increase their dietary intake of calcium and vitamin D to protect their bones.
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 your athletes go for a hard run, the concentrations of estrogen and progesterone in their blood increase during both the follicular and luteal phases of the menstrual cycle. Low-intensity exercise, however, does not alter the concentrations of these hormones.
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 the 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 your athletes are running on a hot and humid day, as they want to begin the cooling response as soon as they 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 runners don’t begin sweating to dissipate heat until they have reached a higher body temperature. They also have a decreased ability to dilate the small blood vessels under the skin, which compromises their 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, such as 10,000 meters on the track (and half-marathons and marathons for the general public), can be more difficult during the luteal phase of the menstrual cycle. The increased body temperature during the luteal phase can also put a runner at an increased risk of developing heat illnesses like heat exhaustion and heat stroke. Training improves a runner’s 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. The research suggests that 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 on the luteal phase, with a greater reliance on fat during submaximal exercise.
Another ramification of the altered metabolism is the possible delay of fatigue during submaximal exercise. Theoretically, with less reliance on carbohydrates 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.
Progesterone stimulates ventilation independent of the intensity of a run, which can increase a runner’s perception of effort since runners typically link their perception of effort to how much they’re breathing. Thus, breathing is greater during the luteal phase, when progesterone concentration is highest. Thus, a female runner may feel more winded during her luteal phase workouts compared to her follicular phase workouts.
The increased breathing during the luteal phase may also increase the oxygen demand of breathing itself since the muscles responsible for breathing need oxygen to work just like the leg muscles do. More oxygen being used by the breathing muscles means less oxygen available to the leg muscles. The increased breathing could hypothetically reduce the running economy since a runner will consume more oxygen to support the extra breathing. Most research, however, has not documented a change in the running economy across the menstrual cycle.
Lung function after exercise is also affected by the phase of the menstrual cycle, with women having more trouble breathing during the luteal phase. This has huge implications for runners with asthma since exercise is a powerful trigger of asthma symptoms. Thus, the declining lung function in the luteal phase can negatively impact training and competition strategies in an asthmatic runner. Females with asthma experience a worsening of asthma symptoms and increased bronchodilator use during the mid-luteal phase. Interestingly, lung function and asthma symptoms seem to vary cyclically. Thirty-three to 52 percent of asthmatic women report a premenstrual worsening of asthma symptoms, and an additional 22 percent report that their asthma is worse during their periods.
If your athletes bleed a lot during menstruation, it’s possible that their blood’s hemoglobin concentration may decrease, which can negatively impact their ability to transport oxygen in their blood. Since iron is an important component of hemoglobin, iron loss often accompanies a lot of bleeding. If this happens, your athletes may need to supplement their 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.
The documented effects of the menstrual cycle on physiological characteristics is one thing; how they influence your running performance on Saturday is quite another. As with most of the research on the menstrual cycle, the research on how it affects endurance performance is not totally clear. Survey-based research has shown that many female athletes do not report any noticeable detriment in performance between phases of the menstrual cycle. However, many others report an improvement in performance during menstruation. The best performances have generally been reported to occur in the immediate post-menstrual days with the worst performances occurring during the pre-menstrual interval and the first few days of menstruation. However, this type of survey-based research needs to be interpreted with caution, since there are many confounding variables surrounding the menstrual cycle, the perception of exercise effort, and women’s inherent bias about the menstrual cycle, especially the premenstrual days.
Research that has actually measured performance in women is also conflicting, with some studies showing that performance is influenced by the menstrual cycle and other studies showing that it is not. While theoretically and often anecdotally endurance performance may be better in the mid-luteal phase compared to the early follicular phase, it may only be so when the ratio of estrogen to progesterone is high in the mid-luteal phase (remember that both estrogen and progesterone are elevated in the mid-luteal phase). Improved performance also tends to occur in the late follicular phase, which is characterized by the pre-ovulatory surge in estrogen and suppressed progesterone. It seems that a female runner 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.
Anecdotally, many of the female runners I’ve coached have experienced their worst training days in the few days leading up to and including menstruation. How your athletes’ workouts and races are affected is highly individual. They may find that, while harder workouts may be more challenging during their periods, easy running may actually improve their moods and alleviate physical symptoms associated with their periods.
Oral contraceptives, which supply a woman with synthetic sex hormones, are the most common form of birth control for women. Oral contraceptives mimic the normal female menstrual cycle by increasing and then subsequently decreasing the concentrations of estrogen and progesterone on a set 28-day schedule leading up to menses. The three different types of oral contraceptives are monophasic, biphasic, and triphasic. Monophasic pills, the most commonly used, provide fixed doses of estrogen and progesterone over 21 days, followed by seven days of placebo. These pills regulate the hormonal environment, decreasing hormonal fluctuations across the cycle, which can provide a controlled environment for the runner and minimize potential variations in physiological variables. Biphasic pills switch the dosage of the hormones once during the 21-day cycle. Triphasic pills supply three different doses of estrogen that are increased throughout the cycle. Oral contraceptives reduce the natural production of estrogen, progesterone, luteinizing hormone, and follicle-stimulating hormone, which inhibits ovulation and prevents pregnancy.
In addition to preventing pregnancy, oral contraceptives force a regular 28-day cycle, which makes it easier to plan your athletes’ training and races. Some research has shown that runners consume less oxygen while running at submaximal speeds (i.e., their running economy is improved) when taking oral contraceptives. However, both the maximum ability to consume oxygen (VO2max) and running performance do not seem to be affected.
Because oral contraceptives supply estrogen, it’s possible, at least theoretically, that they can reduce the risk for bone injuries associated with menstrual irregularities by increasing bone mineral density. However, research examining the effects of supplemental estrogen provided by birth control medication on bone mineral density has shown mixed results. Some studies have shown that it has no effect, some studies have shown an increased bone mineral density, and still other studies have shown a decreased bone mineral density, especially when contraceptives are taken during late adolescence or early adulthood.
In women with normal menstrual cycle activity, oral contraceptive use does not seem to confer any benefit to bones. In other words, if your athletes have normal menstrual cycles, their estrogen level is already adequate to protect their bones; supplying more estrogen from a pill is not going to make their bones any stronger. Any benefit to bones seems to be specific to active women with menstrual irregularities who have compromised skeletal health.
One of the possible side effects of oral contraceptives that can affect your athletes’ running is the potential to gain weight. Studies on physically active women have found that oral contraceptives, when taken either as a single, fixed-dose of estrogen or as multiple doses over the menstrual cycle, increase body mass and percent body fat. But the weight gain doesn’t seem to be permanent and can return to what it was once they stop taking the pill.
Another side effect of oral contraceptives is a progesterone-mediated increase in body temperature, much like that which occurs during the luteal phase of the menstrual cycle. Since temperature regulation is an important factor in long races, the increased set-point body temperature from oral contraceptives can affect a runner’s ability to run in the heat.
Since estrogen has such a big effect on bone health, one thing to consider during the aerobic base-building phase of your athletes’ training is the time of the month that they increase their mileage. Try not to increase their weekly mileage during menses or the early part of the follicular phase and the latter part of the luteal phase of the menstrual cycle, as those are times of the month when estrogen concentration is low. Conversely, good times of the month to increase weekly mileage are during the latter part of the follicular phase and the mid-luteal phase, when estrogen concentration is high.
Avoid challenging workouts around menses, especially if your athletes don’t feel well at that time or if they feel bloated due to the rapid drop in progesterone as they transition from the luteal phase to the follicular phase. For example, if a runner has a 28-day cycle starting on Monday, and menses occur on days 1 to 3 (Monday to Wednesday), plan their 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 their cycle. If menses last five days (Monday to Friday), schedule one workout during the week of menses and two workouts during the other three weeks of their cycle. For those lucky runners who are not adversely affected by their periods and don’t experience much discomfort, it’s okay to do the workouts and see how they respond.
Racing During the Menstrual Cycle
Racing across the menstrual cycle is a complicated matter. Although a number of studies have found endurance performance to vary between phases of the menstrual cycle, there is an equal number of studies that have shown no difference in endurance performance between phases. Menstrual phase variations in endurance performance may largely be a consequence of changes to exercise metabolism that is stimulated by the fluctuations in the concentrations of estrogen and progesterone. Anecdotally, many women claim that they don’t run well in the few days surrounding their periods. If any of your athletes have ever run a race during “that time of the month,” they know how bad of an experience that can be. It’s pretty clear that they should try to avoid racing during their periods. The amount of menstrual flow, and therefore the amount of blood and iron they lose, also affects how they feel the week following their periods. Women who bleed a lot may feel sluggish following their periods, which would make that a difficult time to race.
If endurance performance is indeed better at certain times of the month, it seems that, in general, it is better during the late follicular phase of the menstrual cycle prior to ovulation, which is characterized by the pre-ovulatory surge in estrogen and suppressed progesterone concentrations. Performance may also be better during the middle part of the luteal phase (a week after ovulation), which is also characterized by rising estrogen accompanying a high level of progesterone. Since progesterone exerts some negative influences on body temperature, fluid balance, and breathing, endurance performance may only be improved in the mid-luteal phase compared with the follicular phase when the ratio of estrogen to progesterone is high (i.e., the increase in estrogen concentration is high relative to the increase in progesterone concentration so the effects of the rising estrogen outweigh the effects of the rising progesterone).
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