It is Sunday morning. You have just finished a twenty-five-kilometer long run and are sitting at a drinks stall by the lake, opening your phone to look at next week’s training plan. Your period-tracking app sits right next to it and shows a line in red: 46 days late. The second time this training season. A friend from your running group glances over and laughs: “Be glad, it means the training is working. No worrying about your period on race day.” For months now you have been running six days a week, heading out while it is still dark. You get home late from work, and many evenings there is only time for a bowl of soup with a few vegetables before bed.
Your friend’s words sound like praise. A sports doctor reads that figure of 46 days very differently. The consensus statement (the shared position of an expert panel) of the International Olympic Committee (IOC) lists infrequent and absent periods among the signs of not eating enough for the amount of training, and places them right next to another sign: bones that break easily.
What is left after the run
Sports scientists think about this quite simply. Each day the body takes in a certain amount of energy from food, and the run uses up part of it. Only what remains is available for everything else the body does. Mountjoy and colleagues (2023), in the IOC consensus on Relative Energy Deficiency in Sport (REDs), call this remainder energy availability: energy intake minus the energy spent in training, divided by fat-free mass (body weight not counting fat), calculated day by day.
This remainder can fall short even if you are not dieting at all. Your training plan adds distance each week, while your meals stay the same as when you first started running, and on some days you even miss a meal because you are busy. The part taken away grows, and the part left over shrinks. So low energy is not about anyone eating carelessly. Usually the running has gone up and the meals have not yet caught up.
You will often see a figure online of 30 kcal per kg of fat-free mass, with the claim that anything below it is REDs. The 2023 document itself states that this cut-off is still debated and is not to be used as a general diagnostic threshold for everyone. It comes from short experiments in women who did little exercise. Around 45 kcal/kg is considered adequate. To calculate it properly you need to know your fat-free mass and the real energy cost of each session. You do not need to sit and do the sums and then draw your own conclusion, because the body gives clearer signs.
Thirty-five days, three cycles
There are two markers for periods. A cycle longer than 35 days is called oligomenorrhea (infrequent periods). Having had periods and then missing them for 3 cycles or more is called secondary amenorrhea.
Doctors do not diagnose from a single number. They use the IOC REDs assessment tool (called REDs CAT2), which combines primary and secondary indicators and places the person in one of four color-coded levels. Infrequent periods also have many causes, so the job of the consultation is to find out why this period is late. Not every late period in a runner is caused by running.
Your friend at the stall sees infrequent periods as a natural price of hard training, even a convenience. In the consensus, infrequent periods in someone who trains a lot are a reason to see a doctor. If you are trying to conceive, there is all the more reason not to wait.
Periods and bones on the same chart
Periods and bones sit on the same chart of indicators. For bone, the primary indicator is a stress fracture (bone that cracks gradually under repeated loading) in bones with a lot of spongy bone inside, such as the femoral neck, the pelvis and the sacrum, or repeated stress fractures elsewhere; in cross-country runners, Joachim and colleagues (2024) found this association at a moderate level of evidence.
Colebatch and colleagues (2025) pooled studies in female runners. Those who were injured ate about 303 kcal less and about 20 g less fat per day than those who were not. These are observational data: they show that eating less and injury tend to go together, but they do not prove that one causes the other. Even so, if you are cutting back on oil and fat to “feel lighter” this training season, that figure of 20 g of fat is worth a second thought.
Some people look for a shortcut: take the contraceptive pill to make periods regular again and the problem is solved. The IOC consensus, the sports rehabilitation and injury prevention book by Comfort and Abrahamson, and the Williams endocrinology textbook all state the same thing. In exercise-related amenorrhea, the contraceptive pill does not reverse bone loss. According to all three, what needs fixing is the missing energy, not the level of estrogen (the female hormone). Periods return through eating enough and reducing the training load.
A mother’s quiet question
When a daughter was growing up, the mother of the house would often ask one quiet question, while clearing the dinner tray or brushing her daughter’s hair: “Has it been regular this month, dear?” The older generation had never heard the word REDs. Yet that question still treated a regular period as part of being healthy, and on this point it meets the sports doctors. Today the question needs one more part, about meals, especially when you are running six days a week.
The fix goes through meals and the training plan at the same time. In heavy training weeks, meals have to grow with the distance: no skipped meals, extra snacks, and no cutting out oil and fat entirely. Distance and intensity are also reduced for a while. How much to cut back, and for how long, the doctor discusses with you after the examination. For people with low energy availability or menstrual disorders, Maughan and colleagues (2018), also in an IOC consensus, recommend a specific level of calcium and vitamin D, which the doctor adjusts according to test results.
More tired than any season before: the iron question
Female runners with long-lasting fatigue often ask whether they should take iron. Iron deficiency is a different matter from low energy, but it also holds you back. In women, iron deficiency lowers endurance by about 3–4 %. Those who are truly deficient and receive supplements may improve by 2 % to 20 %. Pengelly and colleagues (2024) still concluded that the evidence is not yet firm, so these figures only tell you that iron deficiency is worth checking for.
The only way to know whether you are deficient is a blood test: ferritin (the body’s stored iron), transferrin saturation and serum iron (the iron currently in the blood), together with the red cell indices Hb, Hct and MCV printed on the complete blood count report. Ferritin is lower in the early follicular phase, that is, the first days of the cycle, so on the day of the blood draw tell the doctor which day of your cycle you are on. There is no agreement yet on what ferritin level counts as low, 30 or 40 µg/L. Once the doctor has prescribed iron, taking it every other day is absorbed better than taking it daily; these data come from iron-deficient women in general, not athletes (Stoffel and colleagues, 2017).
Iron is also linked to injury. The book by Comfort and Abrahamson cites a study of female high school cross-country runners: the 34 girls with the lowest ferritin had twice as many injuries as the 34 girls with the highest ferritin. We have not been able to read the original of this study. Iron tablets should only be taken after testing. The US National Institutes of Health (NIH) sets the highest intake considered safe for adults at 45 mg of iron per day.
Tired, but not to exhaustion
Traditional medicine has a saying, “劳则气耗” (勞則氣耗, láo zé qì hào): overexertion depletes qi, with rapid breathing, sweating and qi being used up. The textbook of basic theory of Chinese medicine adds that heavy sweating depletes blood and damages body fluids (the water and fluids that nourish the body). The same book records the advice “劳而不极” (勞而不極, láo ér bù jí): work hard, but not to exhaustion. People of earlier generations still let their children work hard; they only warned them not to let the tiredness go beyond what the body could make up for.
For women who run a lot, that depletion adds to the blood lost with each period. The doctor first orders a complete blood count and ferritin. If fatigue and slow recovery come with a pale complexion and poor appetite, traditional medicine may classify this as the pattern of qi and blood deficiency (气血虚 / 氣血虛, qì xuè xū). Scanty periods, pale nails and lips, and frequent calf cramps at night make the doctor think more of the pattern of Liver blood deficiency (肝血虚 / 肝血虛, gān xuè xū; in traditional medicine, the Liver is the organ that stores blood, and in this pattern the blood the Liver stores runs short). Depending on the pattern, a decoction in ready-to-drink 50 ml pouches is prescribed to tonify qi and blood or to nourish Liver blood. If you are trying to conceive, you get a separate version that leaves out the herbs that are not suitable. If an upcoming race has doping control, the doctor needs to know from the day of the consultation, before choosing the herbs.
The evidence for the herbal part is still thin. The only trial of a traditional herbal formula in runners was done in 36 men, measured running time, and has not been repeated (Chang and colleagues, 2018). For periods in female runners, there has been no trial of any decoction. Whether periods return still depends on meals and the training plan. The pouches cannot make up for meals that fall short, nor replace iron tablets when tests show a deficiency.
The hips tire along with the cycle
The clinic also looks at infrequent periods through Applied Kinesiology (AK, a method of testing the strength of each muscle by hand), starting with a few muscles around the hip. In Walther’s table of muscle–gland relationships, the gluteus medius, gluteus maximus, piriformis and the adductor group are all grouped with the reproductive glands (Synopsis, pp. 309, 314–315, 320–321). This same group keeps the pelvis level every time you stand on one leg, which means throughout your run. Garten (Muscle Test Handbook, pp. 102, 176) notes that when the gluteus medius is working below threshold, meaning weaker than its true strength, the pelvis cannot stay level when you stand on that leg, and the gait sways; the piriformis working below threshold on both sides often suggests a disorder of the organs in the lesser pelvis. In the clinic’s experience, women whose cycles are getting longer and whose hips sway and buttocks ache at the end of a long run often have both problems together.
Cuthbert (2013, p. 287) writes that eating too little, overtraining and severe emotional events are the kinds of stress that commonly lead to amenorrhea through the hypothalamus (the brain region that controls the endocrine glands). He advises also checking the pituitary, thyroid and adrenal glands, iron deficiency, and essential fatty acid deficiency (fats the body cannot make on its own and must get from food).
The doctor follows exactly these leads. First comes testing each muscle around the hip to find the ones that cannot hold. For the emotional part, the doctor tests while asking about work and family, to see which topic changes the muscle’s strength (Walther, Synopsis, p. 427). For the nutritional part, you taste a few drops of an oil containing essential fatty acids or a little of an iron preparation, and the muscle is retested. For iron specifically, Walther also tests one muscle about ten times in ten seconds in a row; when only one muscle weakens partway through, he considers that the body is usually in need of iron (pp. 188–189). Every lead is checked against ferritin and hormone tests, and only then does the doctor mobilize the sacroiliac joint (where the sacrum at the base of the spine joins the pelvic bone) in the direction shown by challenge, that is, the direction that changes the muscle’s strength on retesting, and choose the supplements.
When to come in for an examination
Being 46 days late, and for the second time this season, is already past the 35-day marker. You do not need to wait for a third cycle, nor for pain in a bone. Come in for an examination if:
- Your cycle is longer than 35 days, or your periods have stopped for 3 cycles, during a heavy training season;
- You have a second stress fracture, or a stress fracture of the femoral neck, pelvis or sacrum;
- You have hip or groin pain when you put your weight onto one leg;
- You have long-lasting fatigue, slow recovery or a pale complexion after a training block;
- You are trying to conceive and your periods have become infrequent with heavy training.
At the consultation, Dr. Pham Tran Dan Tam (MD, Specialist Level I in Traditional Medicine) looks at the menstrual side: your cycles over the past few months, other causes that need to be ruled out, and whether you are trying to conceive. Dr. Dinh Ngoc Duc (MD, MSc in Traditional Medicine) looks at the running side: how your weekly distance has increased, whether any bone needs imaging, whether an iron test is needed, and how your muscles work in motion. Either way, the doctor will ask what day your last period started, and what you eat on an ordinary day, from breakfast to dinner.
You may still be able to run this year’s race, or you may have to postpone. When your meals keep up with your legs, one Sunday morning at the stall by the lake, the app on your phone may stop showing that line in red. Book a consultation.
Worth keeping
- A cycle longer than 35 days, or periods missing for 3 cycles or more, during a heavy training season is a reason to see a doctor, not a sign that training is going well;
- When your distance goes up, your meals go up too; do not skip meals in heavy training weeks;
- Do not cut out oil and fat entirely during the training season;
- With a second stress fracture, or a fracture of the femoral neck or pelvis, ask the doctor about meals and periods as well, not just about imaging;
- Do not take iron on your own before a ferritin test; on the day of the blood draw, tell the doctor which day of your cycle you are on;
- The contraceptive pill does not replace eating enough and reducing the training load when periods stop because of training;
- If you are trying to conceive and your periods have become infrequent with heavy training, see a doctor early.
References
Sources in English
- Alfaro-Magallanes VM, Barba-Moreno L, Romero-Parra N, Rael B, Benito PJ, Swinkels DW, et al. 2022. Menstrual cycle affects iron homeostasis and hepcidin following interval running exercise in endurance-trained women. Eur J Appl Physiol. 122(12):2683-2694. doi:10.1007/s00421-022-05048-5. PMID: 36129579.
- Chang CW, Chen CY, Yen CC, Wu YT, Hsu MC. 2018. Repressed Exercise-Induced Hepcidin Levels after Danggui Buxue Tang Supplementation in Male Recreational Runners. Nutrients. 10(9). doi:10.3390/nu10091318. PMID: 30231484.
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- Maughan RJ, Burke LM, Dvorak J, Larson-Meyer DE, Peeling P, Phillips SM, et al. 2018. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med. 52(7):439-455. doi:10.1136/bjsports-2018-099027. PMID: 29540367.
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- NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals; Calcium: Fact Sheet for Health Professionals; Vitamin D: Fact Sheet for Health Professionals [Internet].
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- Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, et al. 2017. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 4(11):e524-e533. doi:10.1016/S2352-3026(17)30182-5. PMID: 29032957.
- Walther DS. 2000. Applied Kinesiology: Synopsis. 2nd ed. Pueblo (CO): Systems DC. p. 188–189, 309, 314–315, 320–321, 427.
Sources in Chinese
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- 中医内科学. 第 34、38–39 页.
