You've been training consistently. Your mileage is up, your long runs are ticking over — but something isn't clicking. Your easy paces feel harder than they should. You're hitting target heart rates at efforts that should be comfortable. Your legs feel oddly heavy, even after rest days.
Before you blame your training plan, your sleep, or your fitness, there's one physiological variable that's frequently overlooked: your iron levels. Specifically, your ferritin — and the connection between ferritin and VO2 max is far more significant than most runners realise.
What Is VO2 Max, and Why Does It Matter for Runners?
VO2 max is the maximum rate at which your body can consume oxygen during intense exercise. It's measured in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min), and it's widely regarded as one of the best indicators of aerobic fitness and endurance capacity.
In practical terms, a higher VO2 max means your body can deliver and use more oxygen during exercise — which translates directly to better pace at a given effort, the ability to sustain faster speeds for longer, and faster recovery between hard efforts.
For runners, VO2 max isn't just a lab number. It underpins everything from parkrun to ultramarathons. And it's acutely sensitive to one key variable: the oxygen-carrying capacity of your blood.
The Iron-Oxygen Connection
Iron is the functional core of haemoglobin — the protein in red blood cells that binds oxygen in the lungs and carries it to working muscles. Without adequate iron, your body cannot produce haemoglobin effectively, and your blood simply cannot carry as much oxygen per unit volume.
Beyond haemoglobin, iron is also central to myoglobin — a similar protein that stores oxygen directly within muscle tissue and is critical for sustained aerobic effort. It also plays a key role in the mitochondrial enzymes responsible for energy production itself.
The chain of events when iron is insufficient looks like this:
- Iron stores fall → haemoglobin synthesis is compromised
- Reduced haemoglobin → less oxygen delivered to muscles per heartbeat
- Reduced oxygen delivery → aerobic capacity falls
- VO2 max drops → paces that were comfortable now feel hard
This mechanism is well-established in sports science literature. What's less well understood by runners is that this process begins long before clinical anaemia develops.
Ferritin and VO2 Max: What the Research Shows
Here's where it gets important for performance-focused runners. Clinical anaemia — defined as haemoglobin below 120 g/L in women and 130 g/L in men — is relatively straightforward to diagnose. But by the time haemoglobin drops to those levels, your iron stores have already been depleted for some time.
Ferritin is the primary storage form of iron in the body. It's a more sensitive early marker of iron status than haemoglobin, because stores deplete first — before circulating haemoglobin starts to fall.
Research published in the British Journal of Sports Medicine and elsewhere has shown that runners with low-normal ferritin (below 30–40 μg/L) demonstrate measurably reduced aerobic capacity compared to those with adequate stores — even when their haemoglobin levels remain within the normal clinical range. One frequently cited threshold for performance impact in endurance athletes is ferritin below 40 μg/L, though some sports medicine practitioners use an even higher target of 50–75 μg/L for trained runners.
A study by Hinton et al. (2000) in the Journal of Applied Physiology found that iron-depleted, non-anaemic women showed significant improvements in endurance performance after iron supplementation — despite having haemoglobin levels that were normal throughout. The implication: ferritin depletion alone was impairing performance, independently of haemoglobin status.
This is the distinction that matters. You don't have to be anaemic for low iron to be undermining your VO2 max.
Sports Anaemia vs Iron Depletion: Not the Same Thing
There's a further complication for runners. A common phenomenon called "sports anaemia" — or dilutional pseudoanaemia — occurs when endurance training increases plasma volume faster than red blood cell mass. This can make haemoglobin appear low on a standard blood test even when iron status and red cell production are perfectly fine. It's a training adaptation, not a deficiency.
This means a standard haemoglobin test in a trained runner can be misleading in both directions:
- It may look low (sports anaemia) when iron is actually adequate
- It may look normal when iron stores (ferritin) are already depleted and affecting performance
This is why sports medicine specialists almost universally recommend that runners get both haemoglobin and ferritin tested — not haemoglobin alone. Ferritin gives you the early warning signal that a standard FBC (full blood count) will miss.
Why Runners Are at Higher Risk of Iron Depletion
Iron depletion is significantly more common in runners than in the general population, for several compounding reasons:
Foot-Strike Haemolysis
Repeated impact on hard surfaces physically destroys red blood cells in the capillaries of the feet — a process called foot-strike haemolysis. This releases haemoglobin into the bloodstream, which is then filtered out by the kidneys, taking iron with it. High-mileage runners on hard surfaces are particularly affected.
Sweat Losses
Iron is lost through sweat, and runners produce a lot of it. In hot conditions and long sessions, sweat-related iron loss becomes a meaningful contributor to overall depletion.
GI Losses
Intense endurance exercise can cause microscopic GI bleeding — a well-documented but often overlooked source of iron loss in distance runners. Post-marathon blood in stool (not always visible) has been found in a significant proportion of runners in research studies.
Increased Demand
Training drives red blood cell production, which requires more iron. A runner building mileage has higher iron demands than a sedentary person — but often isn't eating significantly more iron-rich food.
Hepcidin Response
Intense exercise triggers a temporary rise in hepcidin, a hormone that suppresses iron absorption in the gut. This means that even if you're eating adequate iron, your body may absorb less of it in the hours after a hard session.
Symptoms That Look Like Poor Fitness (But Might Be Low Iron)
The frustrating thing about iron depletion is that its symptoms closely mimic the feeling of being unfit — or overtrained. Common presentations include:
- Heavy, leaden legs — particularly in the first few miles of a run
- Struggling to hit target paces at efforts that previously felt comfortable
- Elevated heart rate at easy effort — your cardiovascular system is working harder to compensate for reduced oxygen delivery
- Persistent fatigue — not just post-run tiredness but background fatigue that doesn't resolve with rest
- Breathlessness earlier than expected during tempo efforts or interval sessions
- Poor recovery between sessions
- Cognitive fog — iron is also important for brain function and concentration
If several of these sound familiar, it's worth investigating your iron status before adding more training volume or intensity. Training through iron depletion doesn't work — and it risks compounding the problem.
What to Do: Testing, Targets, and Supplementation
Get the Right Test
Ask your GP for a full iron panel including ferritin, not just a standard FBC. In the UK, GPs will often run only haemoglobin unless you specifically request ferritin — so ask explicitly. Some runners use private blood testing services (e.g. Thriva, Medichecks) for more comprehensive panels.
Know Your Targets
Standard NHS laboratory reference ranges for ferritin are typically 15–300 μg/L for women and 30–400 μg/L for men. These are population-wide ranges, not performance ranges for endurance athletes.
For runners who want to optimise aerobic capacity, most sports medicine practitioners suggest:
- Minimum threshold for performance: >30 μg/L
- Optimal range for endurance athletes: 40–75 μg/L
- Alert for intervention: below 30 μg/L, even if haemoglobin is normal
Food First
Dietary iron comes in two forms: haem iron (from red meat, poultry, fish — highly bioavailable) and non-haem iron (from plant sources — less well absorbed but still valuable). Key sources include:
- Red meat and liver (the most concentrated haem iron sources)
- Fortified cereals
- Legumes, lentils, tofu
- Dark leafy greens (spinach, kale)
- Pumpkin seeds, dried apricots
Pairing non-haem iron sources with vitamin C improves absorption. Conversely, tea, coffee, and calcium consumed around the same time can inhibit absorption — worth bearing in mind if you're a runner who has a coffee before breakfast and tea after every meal.
Supplementation: Choosing the Right Form
If dietary iron isn't sufficient to maintain ferritin at performance levels — and for many runners it isn't — supplementation is an effective option. The form of iron matters considerably. Traditional ferrous sulphate (the most commonly prescribed form) is effective but frequently causes GI side effects: constipation, nausea, and stomach discomfort.
Iron bisglycinate is a chelated form that has been shown in clinical research to be better tolerated and to have superior bioavailability compared to ferrous sulphate — with significantly fewer GI complaints. It's the form used in RunStrong, at 5mg per serving (25% of the UK NRV), which is designed as a daily maintenance dose — keeping ferritin topped up consistently rather than correcting a severe deficiency (which would require medical-grade doses under GP supervision).
If your ferritin is severely depleted (below 20 μg/L), speak to your GP — you may need a therapeutic dose prescribed. RunStrong's iron is a maintenance supplement, not a clinical treatment.
For more detail on why runners are specifically vulnerable to iron loss, see our article: Why Runners Are at Risk of Low Iron Levels. And if you want to understand the clinical picture of runner's anaemia more deeply: Runner's Anaemia: The Hidden Performance Thief.
The Bottom Line
Low iron — specifically, low ferritin — measurably reduces VO2 max and running performance, often before clinical anaemia develops. It's one of the most common and most overlooked causes of stalled performance in distance runners, particularly women and those training at high mileage.
The key actions:
- Get tested — ask for ferritin, not just haemoglobin
- Know your number — aim for >40 μg/L for performance
- Support iron through diet — prioritise haem iron and pair plant sources with vitamin C
- Consider supplementation if diet alone isn't keeping ferritin at performance levels — preferably in a bioavailable, well-tolerated form like iron bisglycinate
If you've been training hard and something still doesn't feel right, iron is worth ruling out before you change anything else.
References
- Hinton PS, Giordano C, Brownlie T, Haas JD. (2000). Iron supplementation improves endurance after training in iron-depleted, nonanemic women. Journal of Applied Physiology, 88(3), 1103–1111. https://doi.org/10.1152/jappl.2000.88.3.1103
- Brownlie T, Utermohlen V, Hinton PS, Haas JD. (2004). Tissue iron deficiency without anemia impairs adaptation in endurance capacity after aerobic training in previously untrained women. American Journal of Clinical Nutrition, 79(3), 437–443. https://doi.org/10.1093/ajcn/79.3.437
- Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. (2015). Is iron treatment beneficial in iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. British Journal of Sports Medicine, 49(21), 1389–1397. https://doi.org/10.1136/bjsports-2014-093624
- Telford RD, Sly GJ, Hahn AG, Cunningham RB, Bryant C, Smith JA. (2003). Footstrike is the major cause of hemolysis during running. Journal of Applied Physiology, 94(1), 38–42. https://doi.org/10.1152/japplphysiol.00631.2001
- Peeling P, Dawson B, Goodman C, Landers G, Trinder D. (2008). Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. European Journal of Applied Physiology, 103(4), 381–391. https://doi.org/10.1007/s00421-008-0726-6
- Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, Peeling P. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology, 119(7), 1463–1478. https://doi.org/10.1007/s00421-019-04157-y
- Clénin G, Cordes M, Huber A, Schumacher YO, Noack P, Scales J, Kriemler S. (2015). Iron deficiency in sports — definition, influence on performance and therapy. Swiss Medical Weekly, 145, w14196. https://doi.org/10.4414/smw.2015.14196
- DeRuisseau KC, Cheuvront SN, Haymes EM, Sharp RG. (2002). Sweat iron and zinc losses during prolonged exercise. International Journal of Sport Nutrition and Exercise Metabolism, 12(4), 428–437. https://doi.org/10.1123/ijsnem.12.4.428