Intense running and other forms of endurance exercise can delay or stop a menstrual period. This disruption results from the body interpreting the combined physical and energy demands of intense training as a state of physiological stress. The menstrual cycle requires significant energy reserves, and when the body senses a threat to its overall energy balance, it temporarily shuts down non-survival functions, including reproduction, to conserve energy.
The Biological Link Between Running and Menstrual Changes
The connection between intense physical activity and menstrual changes is governed by the Hypothalamic-Pituitary-Ovarian (HPO) axis, which acts as the central command system for the female reproductive cycle. Intense exercise, particularly when combined with insufficient calorie intake, signals to the brain that the body is under duress. This signal causes the hypothalamus, the brain’s control center, to suppress its regular output.
The hypothalamus normally releases Gonadotropin-releasing hormone (GnRH) in a rhythmic, pulsatile manner. This GnRH then stimulates the pituitary gland to release Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), which in turn signal the ovaries to produce estrogen and progesterone. When the body is stressed by exercise, the GnRH pulses slow down or stop entirely, a condition known as functional hypothalamic amenorrhea.
This suppression is mediated by the body’s energy sensing mechanisms and stress hormones. Low energy availability increases the production of stress hormones like cortisol, which interfere with the hypothalamus’s ability to release GnRH. The resulting drop in LH and FSH means the ovaries do not receive the signal to mature an egg or produce sufficient estrogen and progesterone. This hormonal cascade leads directly to a delayed or completely missed period, known medically as oligomenorrhea or amenorrhea.
Training and Dietary Factors That Increase Risk
The primary scientific driver behind exercise-related menstrual disruption is a state called Relative Energy Deficiency in Sport (RED-S). This is a sustained mismatch between the energy consumed through food and the energy expended through both daily life and exercise. This chronic caloric deficit causes the body to enter a protective low-energy state.
For many runners, this deficit occurs unintentionally because they significantly increase their training volume or intensity without adjusting their food intake accordingly. Studies indicate that a consistent daily energy deficit as small as 250 to 400 calories over several days can begin to contribute to hormonal dysfunction. Endurance athletes, such as distance runners, are especially susceptible because of their high-volume training schedules that burn a large number of calories.
A sudden increase in weekly mileage combined with inadequate recovery time acts as a physical stressor that contributes to HPO axis suppression. While a low body mass index (BMI) or low body fat percentage is often seen in affected runners, it is a symptom of the energy deficit, not the underlying cause. Chronic psychological stress from training pressure or life events can compound the physical stress. This further signals to the body that it is not a safe time to maintain reproductive function.
Health Consequences of Cycle Disruption and When to Seek Medical Advice
The immediate consequence of an absent or irregular period is the loss of estrogen’s protective effects. Estrogen plays a central role in maintaining bone mineral density by helping the body absorb calcium and build bone tissue. Without sufficient estrogen, bone breakdown outpaces bone formation, leading to reduced bone density, which can manifest as osteopenia or osteoporosis.
For athletes, this loss of bone density increases the risk of stress fractures. Research shows that female athletes with exercise-induced amenorrhea have a risk of low bone mineral density up to 23 times higher than those with regular cycles. Prolonged menstrual disruption can also lead to fertility issues, including luteal phase defects and anovulation. However, the reproductive system often recovers once the underlying energy imbalance is corrected.
If a period is delayed by only a few days, it may be due to temporary training stress, but persistent changes require medical attention. You should consult a doctor or gynecologist if you have missed three consecutive menstrual cycles, a condition known as secondary amenorrhea. Medical evaluation is necessary to rule out other causes of menstrual irregularity, such as pregnancy, thyroid disorders, or Polycystic Ovary Syndrome (PCOS). Treatment often involves working with a sports medicine physician and a registered dietitian specializing in sports nutrition to safely increase energy availability and reduce training load, which is the most effective path toward restoring a healthy cycle.