Quick Answer: Heavy menstrual bleeding is the leading cause of iron deficiency in women of reproductive age, and endurance training adds several smaller losses on top. The result is frequently low ferritin with normal haemoglobin - so a standard blood count comes back fine while you feel exhausted. Ask specifically for ferritin, not just a full blood count, and treat persistently heavy bleeding as a medical issue rather than a fact of life.

This is one of the most common and most missed explanations for training fatigue in women.

How much iron does a heavy period actually cost?

Enough to outrun normal dietary intake, which is the crux of the problem.

A typical menstrual period loses around 30 to 40ml of blood, costing roughly 15 to 20mg of iron across the cycle. Heavy menstrual bleeding is defined as losing more than 80ml, which is closer to 40mg or more - and plenty of people lose considerably more than that.

For context, the body absorbs only about 1 to 2mg of iron per day from a typical diet - absorption from food is genuinely inefficient, in the range of 5 to 15 percent for haem iron and lower still for plant sources. Recommended intake for menstruating women is around 18mg daily specifically to account for these losses.

So the arithmetic is unforgiving: heavy monthly losses can exceed what the diet replaces, month after month, and the deficit compounds. Iron stores deplete gradually over months to years, which is why the onset is so insidious - there is no single moment where something changed.

Training adds several smaller drains on top:

• Sweat losses, small but real over high volumes
• Gastrointestinal microbleeding, documented particularly in distance runners
• Foot-strike haemolysis - red blood cells damaged by repeated impact, most relevant in running
• Exercise-induced hepcidin elevation, a hormone that reduces iron absorption for several hours after hard sessions

That last one deserves attention because it is actionable and almost nobody knows about it. Hepcidin rises for roughly three to six hours after intense exercise and blocks iron absorption during that window - which means the post-workout meal, the one most people take an iron supplement with, is one of the worst times to absorb it.

Why do blood tests keep coming back normal?

Because the test most commonly run is not the one that detects this.

A standard full blood count measures haemoglobin and red cell indices. Haemoglobin is the last thing to fall. Your body prioritises circulating haemoglobin over stored iron, so it will empty the stores completely before the count starts dropping.

The progression runs like this:

• Stage 1 - depleted stores. Ferritin falls. Haemoglobin normal. Symptoms frequently present: fatigue, poor training response, breathlessness, cold intolerance.
• Stage 2 - iron-deficient erythropoiesis. Ferritin low, transferrin saturation falling, haemoglobin still technically normal. Symptoms clearer.
• Stage 3 - iron deficiency anaemia. Haemoglobin finally drops. This is what a standard blood count detects.

Many people spend months or years in stages 1 and 2 being told their bloods are fine.

What to ask for specifically: ferritin, transferrin saturation, and a full blood count together. Ferritin is the marker of stored iron and the one that moves first.

A complication worth knowing: ferritin is also an acute phase reactant, meaning it rises with inflammation, infection, and after hard training. A ferritin drawn the day after a hard session, or during a cold, can look falsely reassuring. Have it drawn rested, ideally 48 hours after hard training and not during illness.

On interpretation: laboratory reference ranges for ferritin often extend down to around 15 micrograms per litre, but many sports medicine practitioners consider levels below 30 likely to affect performance in athletes, and some use higher thresholds. A ferritin of 18 may be flagged as normal and still be the reason you feel terrible. That is a conversation to have with a doctor, ideally one familiar with athletes. This is general information, not medical advice.

What does low iron actually feel like?

Frustratingly like several other things, which is why it gets attributed elsewhere.

The training-specific picture:

• Sessions that used to be routine feel disproportionately hard
• Heart rate higher than usual at familiar paces
• Recovery between intervals taking longer
• Legs feeling heavy from the start rather than fatiguing normally
• Performance declining despite consistent training - the classic sign
• Breathlessness out of proportion to the effort

The general picture:

• Fatigue that sleep does not resolve
• Feeling cold, particularly hands and feet
• Hair shedding more than usual
• Brittle nails, sometimes spooning of the nail bed
• Difficulty concentrating
• Restless legs at night, which is strongly associated with low iron and frequently overlooked
• Unusual cravings, including for ice - a genuinely characteristic sign

The most common misattribution is overtraining. The presentations overlap almost completely, and the standard response to suspected overtraining is to rest - which does not fix iron deficiency and delays the diagnosis by weeks. If you have heavy periods and you are being told you are overtrained, ask for ferritin before you rearrange your training year.

The second most common misattribution is simply life - stress, work, age, being busy. Which is exactly why the objective marker matters.

What actually helps?

In two parts, because the bleeding and the iron are separate problems and both need addressing.

Address the bleeding, medically. This is the part most often skipped, and it is the more important half. Heavy menstrual bleeding is common but not something to simply accept - it has identifiable causes including fibroids, polyps, adenomyosis, thyroid dysfunction, and bleeding disorders, and it has effective treatments including hormonal options, tranexamic acid, and the hormonal IUD, which substantially reduces menstrual blood loss for many people. If you are soaking through protection hourly, passing large clots, bleeding more than seven days, or needing to plan your life around your period, that is a medical conversation. Replacing iron while the drain continues is bailing without fixing the leak.

Improve absorption, which is largely about timing.

• Take iron with vitamin C. Ascorbic acid substantially increases absorption of non-haem iron. Orange juice, a kiwi, or a vitamin C tablet alongside.
• Avoid tea, coffee, and calcium within two hours. Tannins and calcium both inhibit absorption considerably, and a cup of tea with an iron-rich meal can cut absorption dramatically.
• Take supplements in the morning, away from training. Because of the hepcidin effect, morning dosing before exercise absorbs better than post-workout dosing.
• Consider alternate-day dosing. Emerging evidence suggests every-other-day dosing produces better total absorption than daily dosing, because daily doses keep hepcidin elevated. Worth raising with your doctor.

Food sources, remembering that food alone rarely corrects an established deficiency: red meat, liver, shellfish - particularly mussels and oysters - and for plant sources, lentils, beans, tofu, fortified cereals, and dark leafy greens, always paired with vitamin C.

On supplementation: do not start high-dose iron without testing. Iron overload is genuinely harmful, and some people carry haemochromatosis genes without knowing. Test, then treat, then retest in about three months. Repletion of stores typically takes three to six months even with good adherence - the fatigue may lift within weeks, but the stores take much longer, and stopping early is why people end up back in the same position a year later.

Myth vs Fact: Iron, Periods, and Training

Myth: If your haemoglobin is normal, your iron is fine. Fact: Haemoglobin falls last. Ferritin can be at rock bottom with a completely normal blood count, and that stage is where most symptomatic people actually sit.

Myth: Heavy periods are just what some people get. Fact: Common, yes. Something to accept, no. There are identifiable causes and effective treatments, and persistent heavy bleeding warrants investigation regardless of iron status.

Myth: You can fix a deficiency with spinach and red meat. Fact: Diet maintains iron status; it rarely corrects an established deficiency, because absorption is limited and the monthly losses continue. Established deficiency generally needs supplementation under supervision.

Myth: Taking iron with your post-workout meal makes sense. Fact: One of the worst windows, because hepcidin is elevated for hours after hard exercise and blocks absorption. Morning, away from training, with vitamin C, is better.

Myth: More iron is better if you are low. Fact: Higher single doses do not absorb proportionally and cause more gastrointestinal side effects, which is the main reason people abandon supplementation. Lower doses, or alternate-day dosing, frequently deliver more total iron with fewer problems.

FAQ: Iron and Heavy Periods Questions, Answered

How do I know if my period is actually heavy?

Useful markers: soaking through a pad or tampon every hour or two, needing double protection, passing clots larger than a 10p coin, bleeding beyond seven days, or waking at night to change protection. Any of those meets the threshold for a medical conversation.

When in my cycle should I get tested?

Ideally not during or immediately after your period, and rested from hard training for 48 hours. Consistency matters most if you are tracking over time - test at the same point in your cycle each time.

How long until I feel better?

Energy often improves within two to four weeks of starting supplementation, well before stores are replete. That improvement is exactly why people stop early. Continue for three to six months and retest.

Do I need iron if I am vegetarian and have heavy periods?

You are in the highest-risk group - plant iron is absorbed less efficiently and your losses are higher. That is a reason to test rather than a reason to supplement blindly, but the case for testing is strong.

Can low iron affect my mood as well as my training?

Yes. Iron is involved in neurotransmitter synthesis, and low iron is associated with low mood, irritability, and poor concentration independent of anaemia. People frequently report the mental change more strongly than the physical one once it resolves.

TL;DR:

• Heavy periods are the leading cause of iron deficiency in menstruating women, and training adds sweat, gut, and impact losses on top.
• Haemoglobin falls last, so a normal full blood count does not rule this out. Ask for ferritin and transferrin saturation specifically.
• Have ferritin drawn rested and not during illness - it rises with inflammation and can look falsely reassuring.
• Symptoms mimic overtraining almost exactly. Performance declining despite consistent training is the classic presentation.
• Address the bleeding medically as well as the iron. Take supplements in the morning with vitamin C, away from training and away from tea and coffee.

If you have heavy periods, train regularly, and feel worse than your training justifies, the ferritin test is the cheapest question you can ask.