Should I take iron if I train a lot?

Only if you are short of it, and a blood test tells you that. Endurance training increases losses and the risk of low stores, especially in women, but supplementing without a deficiency does not improve performance and the excess is not easily cleared. Blood test first, tub second.

How much iron does a supplement contain?

The EU labelling reference value is 14 mg, and supplements usually carry between 14 and 30 mg of elemental iron. What to check is elemental iron per serving, not the milligrams of the salt: 300 mg of ferrous sulphate is not 300 mg of iron.

Which iron is easiest on the stomach?

Ferrous bisglycinate is usually better tolerated than sulphate, the classic culprit of digestive discomfort and constipation. Taking it with vitamin C improves absorption, and taking it with coffee or tea worsens it.

How do I know if I am iron deficient?

With a blood test that includes ferritin, not just haemoglobin: ferritin drops first and warns that the store is emptying even while haemoglobin stays normal. Tiredness on its own tells you nothing, because it has too many possible causes. The interpretation is for a doctor, not a supplement website.

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Iron · Moderate evidence

What iron is for and who really needs it

The short answer. Iron carries oxygen in the blood, and its deficiency is one of the few shortfalls that really sinks performance. It is also the supplement least suited to taking just in case: the body has no easy way to get rid of the excess, and in people without a deficiency it adds nothing. It is the category on this site where the advice is not to compare prices, but to get a blood test first.

There are studies in favour, but few, with small samples or measuring markers instead of outcomes. No figure on this page is attributed to a specific product: the effect belongs to the ingredient and every number carries its source next to it.

01 What it does and how much

Each row is a measured effect, with the figure the study gives and the source number. Where the evidence gives no figure, the column says so instead of making up a percentage.

Effects of iron with their source
Effect	How much	Detail
EU daily reference value	14 mg	The labelling NRV. Real needs vary a lot from person to person: an adult man is not the same as a woman with heavy periods. [1]
Functions authorised in the EU	red blood cells, oxygen and tiredness	It contributes to normal red blood cell and haemoglobin formation, normal oxygen transport and the reduction of tiredness and fatigue. [2]
Iron deficiency and performance	clearly limits it	It is one of the deficiencies with the most direct impact on endurance, and correcting it gives performance back. That is the whole case for iron. [3]
Without a deficiency	no benefit and some risk	It improves nothing and the excess builds up. That is why this is the tub you do not buy blind. [3]

02 How to take it

Dose. Whatever the person who has seen your blood test says. The labelling reference value is 14 mg.
When. It is absorbed better on an empty stomach, but sits worse; vitamin C improves absorption of plant iron.
How long before you notice it. Stores take months to refill, although tiredness improves sooner.

Details that change the result

Coffee and tea reduce absorption of plant-based iron if taken with the same meal.
Ferrous bisglycinate sits better than ferrous sulphate, which causes the most digestive discomfort and constipation.
Red meat, seafood and legumes are the dietary route; animal iron is absorbed much better than plant iron.
Low ferritin with normal haemoglobin already means an empty store: it shows up in a routine blood test, not in how you feel.

03 When it makes sense and when it does not

Worth it if

Deficiency confirmed in a blood test, which is the only clear case.
Women with heavy menstrual losses, where the risk is high and worth measuring.
Vegan diets and high-volume endurance sports, also measuring it.

Not worth it if

Without a blood test and without symptoms: it is the supplement you should least take just in case.
As a generic energiser: if you are not short of it, it gives no energy.
In healthy adult men with a varied diet, unless medically advised.

04 Sources

Every figure above comes from here. They are position stands from scientific societies, meta-analyses and peer-reviewed reviews: if a claim cannot hang from one of these lines, it is not on the page. References are cited in their original form.

Reglamento (UE) 1169/2011, Anexo XIII: valores de referencia de nutrientes. Hierro, 14 mg. https://eur-lex.europa.eu/legal-content/ES/TXT/?uri=CELEX%3A32011R1169
Reglamento (UE) 432/2012: declaraciones autorizadas. El hierro contribuye a la formacion normal de globulos rojos y hemoglobina, al transporte normal de oxigeno y a disminuir el cansancio y la fatiga. https://eur-lex.europa.eu/legal-content/ES/TXT/?uri=CELEX%3A32012R0432
Kerksick CM et al. ISSN exercise & sports nutrition review update. J Int Soc Sports Nutr. 2018;15:38. El deficit de hierro limita el rendimiento y corregirlo lo recupera; suplementar sin deficit no aporta. doi:10.1186/s12970-018-0242-y

This is not medical advice. It is what some studies say about an ingredient, not a protocol for you. With an illness, medication or pregnancy involved, the decision belongs to a healthcare professional who knows you.

Questions about iron

Answered with the data on this page

Should I take iron if I train a lot?: Only if you are short of it, and a blood test tells you that. Endurance training increases losses and the risk of low stores, especially in women, but supplementing without a deficiency does not improve performance and the excess is not easily cleared. Blood test first, tub second.
How much iron does a supplement contain?: The EU labelling reference value is 14 mg, and supplements usually carry between 14 and 30 mg of elemental iron. What to check is elemental iron per serving, not the milligrams of the salt: 300 mg of ferrous sulphate is not 300 mg of iron.
Which iron is easiest on the stomach?: Ferrous bisglycinate is usually better tolerated than sulphate, the classic culprit of digestive discomfort and constipation. Taking it with vitamin C improves absorption, and taking it with coffee or tea worsens it.
How do I know if I am iron deficient?: With a blood test that includes ferritin, not just haemoglobin: ferritin drops first and warns that the store is emptying even while haemoglobin stays normal. Tiredness on its own tells you nothing, because it has too many possible causes. The interpretation is for a doctor, not a supplement website.

And now, which to buy

Prices, kept apart from the evidence

This page explains what iron does and deliberately names no brand. The comparison is the other half: 101 products from 11 stores from €0.022 per capsule, sorted by price and by how verifiable their certification is.

See the iron comparison

Other guides

What each supplement does, with its sources