Every month, your body does a significant amount of work during menstruation, and it costs more than most people account for. The fatigue, the cravings, the low mood: a lot of it traces back to specific nutrients your body is actively losing or burning through at an accelerated rate.
Iron is the biggest loss, but it's not the only one.
During menstruation, the average woman loses around 25 mg of iron per cycle according to a 2023 review in the International Journal of Gynecology and Obstetrics (MacLean et al.), based on average menstrual blood loss of 40 to 50 mL. Magnesium and zinc also decline measurably across the cycle. Understanding what's actually shifting, and when, is the foundation for supporting your body in a way that makes a real difference.
How much iron do you actually lose during your period?
More than most supplement labels account for. The International Journal of Gynecology and Obstetrics review (MacLean et al., 2023) found that average menstrual flow results in a loss of around 25 mg of iron per cycle, with women who have heavy periods losing considerably more. That monthly loss directly depletes ferritin, your iron storage protein, even when your hemoglobin looks normal on a blood test.
This is why iron deficiency without anemia is so common and so commonly missed. A study in the American Journal of Clinical Nutrition (Murray-Kolb & Beard, 2007) found that iron status was a significant factor in cognitive performance in women aged 18 to 35, with iron-deficient but non-anemic women performing at levels between iron-sufficient and anemic women. By the time you feel it, the deficit has usually been building for months.
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Does menstruation affect magnesium and zinc levels too?
Yes, and the pattern is consistent across the cycle. A prospective cohort study in the Journal of the American College of Nutrition (Gernand et al., 2022) found that both magnesium and zinc concentrations decline from the early follicular phase through to the mid-luteal phase, by 4.6% and 6.6% respectively, and that the prevalence of magnesium deficiency was significantly higher by mid-luteal. These aren't dramatic swings, but they are consistent.
They also compound on top of existing deficits. Most women aren't hitting the RDA for magnesium from food alone to begin with, so a predictable monthly decline on an already low baseline is what drives the fatigue, irritability, and poor sleep that many women associate with the second half of their cycle.
Why does the luteal phase drive up nutrient demand?
The luteal phase appears to increase your body's energy demands, even at rest. A 2020 systematic review and meta-analysis in PLOS ONE (Benton, Hutchins & Dawes) pooled 26 studies and found a small but statistically significant increase in resting metabolic rate during the luteal phase compared to the follicular phase. That increased metabolic load, combined with the known drops in magnesium and B6 during this time, helps explain why the week before your period can feel so much harder.
It's because your body is working more and running lower on the nutrients that support energy, mood, and sleep. When magnesium and B6 are both declining and your baseline energy expenditure is higher, the deficit becomes harder to ignore.
Does nutrition during ovulation matter as much as during menstruation?
It does, though it gets far less attention. A comprehensive review published in Nutrients (Kapper et al., 2024) found that zinc is particularly critical around ovulation, playing a direct role in follicle development, LH and FSH hormone synthesis, and protecting the oocyte from oxidative damage. The body actively draws on zinc reserves during this window, which contributes to the overall decline in zinc across the cycle.
Iron also plays a role in ovulatory function beyond just red blood cell production. The same review noted that iron supports endometrial cell function during the luteal phase, and that deficiency at this stage can interfere with implantation and cycle regularity. Consistently low iron doesn't just affect how you feel during your period; it affects the whole system.
How Cycle Routine is built around this.
Most supplements treat every day of the month the same, even though your body's nutrient demands clearly don't. Iron is depleting during menstruation, magnesium and zinc decline through the cycle, and the luteal phase drives up demand for mood and sleep support. Cycle Routine addresses this with two formulas: Replenish, which prioritizes iron bisglycinate and zinc for the first half of your cycle, and Balance, which focuses on magnesium bisglycinate and B6 for the second. Your nutrition shifts with your hormones instead of ignoring them.
FAQ
How do I know if I'm losing too much iron during my period?
The symptoms of iron depletion, including fatigue, brain fog, and reduced physical stamina, can show up well before a blood test flags anything as clinically abnormal. The most useful test is a serum ferritin level, which measures your iron stores rather than circulating iron alone. A 2023 review in the International Journal of Gynecology and Obstetrics (MacLean et al.) noted that even women with normal menstrual flow can develop gradually depleted iron stores if dietary intake doesn't consistently replace what's lost each cycle. If your fatigue is worst in the days after your period, it's worth asking your doctor to check ferritin specifically.
Can diet alone replace the nutrients lost during menstruation?
For some women, yes, but it requires consistent effort and the margin is narrow. The average dietary iron intake for women in the US falls below the RDA of 18 mg/day, which is already calibrated to account for menstrual losses. A prospective cohort study in the Journal of the American College of Nutrition (Gernand et al., 2022) found that 28% of healthy women of reproductive age had low ferritin at the start of their cycle, before any menstrual loss had occurred. Getting adequate iron from food requires eating red meat, legumes, or leafy greens consistently and pairing them with vitamin C for absorption, which most women aren't doing at therapeutic levels every day.
Why do I crave certain foods during my period?
Cravings during menstruation are partly hormonal and partly a response to real nutritional shifts. The drop in progesterone and estrogen at the start of menstruation affects serotonin production, which drives cravings for carbohydrates and sugar. Iron depletion can independently trigger cravings for red meat or, in more severe cases, non-food items. A study published in Appetite (Yen et al., 2018) found that premenstrual food cravings, particularly for energy-dense foods, were consistent and measurable across the luteal phase. The cravings are your body's imprecise attempt to address a deficit.
Is it normal for period symptoms to get worse over time?
It can be a sign that nutritional deficits are accumulating rather than being addressed each cycle. Because iron stores take eight to twelve weeks to meaningfully rebuild, and magnesium deficiency compounds over time, symptoms can worsen gradually without a single dramatic cause. It is also worth ruling out conditions like endometriosis, fibroids, or PCOS with a doctor, as each can increase blood loss and therefore accelerate nutrient depletion. Worsening symptoms over multiple cycles are a legitimate reason to get bloodwork done, not something to normalize.
Your cycle is asking for something specific every month, not just generally "good nutrition," but the right nutrients at the right time. The more you understand the pattern, the easier it is to actually meet it. We're here to help you figure that out, every phase of the month.