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A Strong evidence · Health & Micronutrients

Iron (Only If Deficient)

Also known as: iron supplements, ferrous sulfate, ferrous bisglycinate, ferritin

Iron supplements can correct deficiency, which impairs endurance and causes fatigue. Use only after a blood test confirms low iron, and follow medical advice on dose and duration.

Why grade A?

Graded A for the specific use of treating diagnosed iron deficiency, with or without anemia, under medical supervision. Correcting deficiency can restore endurance capacity and reduce fatigue. Don't take iron without a confirmed need: excess iron can be harmful and gives no performance advantage.

At a Glance

Iron (Only If Deficient)
EvidenceA
RatingStrong evidence
CategoryHealth & Micronutrients
Food sourcesRed meat, Liver, Shellfish such as mussels and clams
Side effects listed2

Grades follow the style of the Australian Institute of Sport Sports Supplement Framework. Summary of published research, not advice.

How it works

Iron is a core component of hemoglobin, which carries oxygen in blood, and of myoglobin and enzymes involved in muscle energy production. Athletes, especially women, endurance runners, vegetarians and frequent blood donors, are at higher risk of deficiency due to menstrual losses, sweat, gut losses, foot-strike hemolysis and low intake. Hard exercise also temporarily raises hepcidin, a hormone that reduces iron absorption. Low ferritin (stored iron), even before anemia develops, can impair training adaptation and cause fatigue.

What research supports

  • Restores iron stores and hemoglobin in people with diagnosed deficiency
  • Can improve endurance capacity and reduce fatigue in iron-deficient individuals
  • No performance benefit in people with normal iron status

Dosage

Dose and duration should be set by a doctor based on blood tests (ferritin, hemoglobin and related markers). Medical guidance commonly involves elemental iron doses well above dietary recommendations for a limited time, with follow-up testing. Recommended dietary intake for adults is 8 mg (men) and 18 mg (pre-menopausal women) per day; the adult upper limit for routine intake is 45 mg per day.

Timing

Iron is best absorbed on an empty stomach or with vitamin C, and absorption may be better in the morning or soon after exercise, before hepcidin rises. Taking iron every other day may improve absorption and tolerance for some people. Avoid taking it with tea, coffee, calcium or dairy, which reduce absorption.

Side effects

  • Constipation, nausea, stomach pain and dark stools are common
  • Excess iron intake can damage the liver and other organs over time

Cautions

  • Only supplement after testing confirms deficiency; unsupervised use can cause iron overload, especially in people with hemochromatosis
  • Accidental iron overdose is a leading cause of poisoning in young children; keep supplements out of reach
  • Iron interacts with thyroid medication, some antibiotics and Parkinson's medications
  • Fatigue can have many causes; see a doctor rather than self-diagnosing
  • Athletes subject to anti-doping rules should only use products certified by an independent third-party testing program for banned substances; certification lowers, but does not eliminate, the risk of contamination.

Food sources

Red meat, Liver, Shellfish such as mussels and clams, Lentils, beans and tofu, Spinach, Fortified cereals, Pumpkin seeds.

Frequently asked questions

Should athletes take iron supplements?

Only if a blood test shows deficiency. Many athletes, particularly female endurance athletes, should have iron status checked regularly, but supplementing without a need is not recommended.

What ferritin level is too low for athletes?

Cut-offs vary by laboratory and guideline. Your doctor can interpret ferritin alongside hemoglobin and other markers.

How can I improve iron absorption from food?

Combine plant iron with vitamin C sources like citrus or peppers, include some heme iron from meat or fish if you eat it, and avoid tea or coffee with iron-rich meals.

How long does it take to correct iron deficiency?

It typically takes several weeks to a few months of supplementation to rebuild iron stores. Follow-up blood tests guide how long to continue.

Sources & further reading

  1. NIH Office of Dietary Supplements (2024). Iron: Fact Sheet for Health Professionals . National Institutes of Health
  2. Sim M, Garvican-Lewis LA, Cox GR, et al. (2019). Iron considerations for the athlete: a narrative review . Eur J Appl Physiol
  3. Maughan RJ, Burke LM, Dvorak J, et al. (2018). IOC consensus statement: dietary supplements and the high-performance athlete . Br J Sports Med · doi:10.1136/bjsports-2018-099027
  4. Australian Institute of Sport (2021). AIS Sports Supplement Framework . Australian Sports Commission