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cofactor · 1
The body is doing different work at different points in the cycle: in one small trial, the group that concentrated leg strength training in the follicular phase improved more, though this needs more research to confirm. The uterine lining is rebuilt every month, and eating enough protein day to day supplies the raw material for that repair.
depletes · 1
↮Iron
Blood carries about 0.5 mg of iron per mL, and a period typically loses 30–40 mL, so 15–20 mg of iron leaves with it. That monthly exit is the main reason the iron recommendation for women of reproductive age is nearly double a man's (it is set near the heavy end of the range, not at the average). More than 80 mL counts as heavy menstrual bleeding, and the iron lost more than doubles.
Mechanism · Why the iron ledger is hard to balance
The debit side is simple: 15–20 mg of iron leaves with each period. The credit side is the hard one: iron is not absorbed in proportion to what you eat; it is held back by **absorption**. Plant-source non-heme iron is absorbed poorly and swings hard with the meal: black tea drunk with a meal cut that meal's non-heme iron absorption by 79–94% (Hurrell 1999, against water with the same bread meal); coffee suppresses it too, and calcium and phytate each take a share. Animal-source heme iron is absorbed several times more efficiently and far more steadily, though calcium can lower it as well. That is why the US Recommended Dietary Allowance is **18 mg/day** for women aged 19–50 against 8 mg for men: the near-doubling exists to cover that monthly exit. And that figure is set **near the heavy end of the range, not at the average**.
hurrell-1999-polyphenols-ironClinical · Stored iron runs out first
The body does not wait for hemoglobin to fall before it feels this. The monthly exit draws first on **stored iron**, the part held in ferritin. Clinically, serum ferritin is the most sensitive measure of stored iron: in the absence of inflammation, a level below about 30 µg/L means the stores are empty (lab reports often write it as ng/mL; the number is the same). So someone with a perfectly normal blood count can already have run out of stored iron, and fatigue, hair shedding and falling endurance often arrive before anemia does. So checking iron cannot mean checking hemoglobin alone.
camaschella-2015Red flag · Heavy bleeding needs its cause found
Losing more than **80 mL** meets the definition of heavy menstrual bleeding, and the iron lost more than doubles; at that level, diet cannot catch up. The practical trouble is that nobody can measure 80 mL at home. Useful signs instead: needing to change a pad or tampon every one to two hours, having to get up at night to change, passing large clots, or bleeding for longer than seven days. When these show up, the question to ask is **why is the bleeding this heavy**, not **how much iron should I take**; the second is like pouring water into a tap nobody has turned off. See a doctor to find the cause.
regulates · 6
When the energy spent in training is not eaten back for a long time (low energy availability), the body puts off spending that does not matter for staying alive today, such as reproduction, so cycles first get longer and irregular, then stop. A period that disappears during serious training is therefore not a badge of being lean and disciplined but a sign of too little energy; periods missing for 3 months in a row call for a medical and nutritional assessment.
Endometriosis is driven by estrogen: tissue similar to the uterine lining, growing outside the uterus, thickens, bleeds and becomes inflamed along with the hormonal rise and fall of the menstrual cycle, which is why the pain often follows the cycle and is worst around menstruation.
In the luteal phase the range of temperatures the body finds comfortable narrows, so the same room feels warmer. A group of neurons in the hypothalamus (KNDy neurons) helps control both reproductive hormones and body temperature, and they are also involved in perimenopausal hot flashes, so the two experiences may share part of the same pathway.
The hypothalamic-pituitary-ovarian axis that controls the menstrual cycle is tied to the body clock, so crossing time zones and shift work can disrupt the cycle. This belongs in the same family of causes as stress, eating too little energy and rapid weight change: all of them tell the hypothalamus that now is not a good time.
Period pain is driven mainly by prostaglandins released from the uterine lining, and EPA and DHA shift the prostaglandins the body makes toward the less inflammatory side, acting on the same pathway as painkillers such as ibuprofen, only much more slowly and weakly. A small crossover trial saw period pain ease, but a Cochrane review judges the evidence for fish oil to be very limited.
Vitamin B6 has some evidence for easing premenstrual symptoms, but it is limited. It is also one of the few water-soluble vitamins that harms when overdone: taking more than 100 mg a day long term can cause peripheral neuropathy, meaning numbness and tingling in the hands and feet. Keep both the benefit and the risk in mind.