Verdict: Test, don't guess. Correcting iron deficiency can restore aerobic capacity, but extra iron does nothing for people with normal stores.
Safety: Constipation and nausea are common; iron overload is harmful, so never supplement without a blood test
Key takeaways
Iron is needed to make haemoglobin, so iron-deficiency anaemia directly lowers oxygen delivery and VO2max
A 2015 meta-analysis found iron treatment improved iron status and aerobic capacity in iron-deficient non-anaemic endurance athletes
Women and endurance athletes are at higher risk, so a ferritin test before supplementing is the sensible first step
Iron is the one supplement on this site with a straightforward link to VO2max — but only in one direction. If you are low in iron, your oxygen delivery suffers and fixing it can bring your fitness back. If your iron is normal, extra iron adds nothing and can cause harm. That makes iron a "test first" nutrient, not a performance booster.
What it is
Iron is a mineral the body uses to make haemoglobin, the protein in red blood cells that carries oxygen from the lungs to working muscles, and myoglobin, which stores oxygen inside muscle. It is also part of enzymes in the mitochondria that turn oxygen into energy. Iron comes from food (red meat, fish, legumes, leafy greens and fortified cereals) and is stored mainly as ferritin.
Does it raise VO2max?
VO2max depends heavily on how much oxygen the blood can carry (see what limits VO2max). When iron runs low enough to cause anaemia, haemoglobin falls and so does VO2max. Correcting the anaemia restores oxygen-carrying capacity.
The grey zone is iron deficiency without anaemia — low ferritin but normal haemoglobin. A systematic review and meta-analysis by Burden and colleagues (2015) pooled 17 studies of iron-deficient non-anaemic endurance athletes. Iron treatment had a large effect on ferritin and other iron markers, and a moderate effect on both haemoglobin and VO2max (Hedges' g of about 0.6 for VO2max). Treatment lasting beyond about 80 days appeared to have the least effect on ferritin.
The key word is deficient. No good evidence shows that iron raises VO2max in people whose stores are already normal.
Does it improve endurance performance?
In deficient athletes, restoring iron can improve how training feels and how well you adapt, because oxygen delivery and energy production are no longer limited. In iron-replete athletes, there is no expected performance benefit. The IOC consensus on supplements (Maughan 2018) groups iron among products used to manage micronutrient deficiencies, not as a direct performance aid, and stresses a full nutritional assessment before supplementing.
How it is used in studies
Studies use oral iron tablets or, in some cases, intravenous iron given by a doctor, usually for several weeks to a few months, with blood tests before and after. Doses and schedules differ widely, and treatment is guided by the starting ferritin level. That is why there is no sensible "standard dose" to copy: iron should be prescribed after a blood test, not picked off a shelf.
A narrative review by Sim and colleagues (2019) notes that exercise raises hepcidin, the hormone that controls iron absorption, for several hours afterwards — one reason endurance athletes can struggle to absorb iron. Timing of doses around training is one of the practical issues a doctor or sports dietitian can help with.
Side effects, safety and anti-doping
- Common side effects: constipation, nausea, stomach pain and dark stools.
- Iron overload: the body has no efficient way to get rid of excess iron. Over time it can damage the liver, heart and pancreas, and people with hereditary haemochromatosis are especially at risk without knowing it.
- Interactions: iron can reduce absorption of some medications (for example certain thyroid drugs and antibiotics). Talk to a doctor or pharmacist if you take any regular medication.
- Children: accidental iron overdose is dangerous for children; store tablets safely.
- Anti-doping: oral iron is not prohibited, but intravenous infusions above a set volume can fall under anti-doping rules for athletes. Competitive athletes should check current rules and use third-party tested products.
This page is not medical advice. Always test before supplementing iron.
Who might benefit
Sim and colleagues (2019) report that iron deficiency is most common in female athletes (about 15–35% of athlete cohorts) but also affects about 5–11% of male athlete cohorts. Groups at higher risk include:
- Women, especially with heavy periods, and anyone pregnant
- Endurance athletes with high training volumes, particularly runners
- Vegetarians and vegans, and people who restrict energy intake
- Frequent blood donors
- Athletes training at altitude, where iron needs rise (see altitude training and VO2max)
If you feel unusually tired, your heart rate is higher than normal at easy paces, or your VO2max trends keep slipping despite consistent training, ask for a blood test including ferritin, haemoglobin and transferrin saturation. Women may also find VO2max, the menstrual cycle and menopause useful.
Bottom line
Iron matters for VO2max because haemoglobin carries oxygen. If you are deficient, treatment can restore aerobic capacity, and the evidence for this in endurance athletes is reasonable. If you are not deficient, iron does nothing for performance and can cause harm. Get tested, supplement only under medical guidance, and retest to check it worked. For everything else, training remains the main lever — see how to improve VO2max, compare other options on the supplements page, and track progress with the VO2max calculator.
Frequently asked questions
Does low iron affect VO2max?
Yes. Iron is needed to make haemoglobin, which carries oxygen in the blood, so iron-deficiency anaemia lowers VO2max. Even low iron stores without anaemia have been linked to lower aerobic capacity in athletes.
Do iron supplements increase VO2max?
Only if you are deficient. In iron-deficient endurance athletes, treatment improved iron markers and VO2max in a meta-analysis, but there is no evidence that extra iron helps people with normal iron stores.
How do I know if I have iron deficiency?
Ask for a blood test that includes ferritin, haemoglobin and transferrin saturation. Fatigue and poor training response are common but not specific, so a test is the only reliable way to know.
Can taking too much iron be harmful?
Yes. Iron is not easily excreted, and excess iron can damage organs over time, especially in people with genetic iron overload. Never take iron without a blood test and medical advice.
Sources
- Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. Br J Sports Med. 2015. PMID 25361786.
- Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, Peeling P. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol. 2019;119(7):1463–1478.
- Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Int J Sport Nutr Exerc Metab. 2018. PMID 29589768.