Iron
Preliminary· human dataVital for oxygen transport — supplement only to correct a tested deficiency.
What it is
Iron is essential for carrying oxygen in the blood. It is one of the clearest "test, don't guess" supplements: correcting a genuine deficiency clearly helps, but supplementing without a shortfall is pointless and carries real risks.
What the evidence shows
Correcting deficiency. Well-supported· human data When a blood test shows you are low, correcting iron deficiency clearly improves energy and endurance.
Supplementing when replete. Marketing claim Taking iron when your levels are already normal offers no performance benefit and risks harmful build-up.
Who it suits
People with a tested deficiency. Those most at risk include people who menstruate, vegetarians and vegans, and endurance athletes, whose turnover and losses are higher.
Practical notes
The body has no easy way to excrete excess iron, so over-supplementing can cause harmful accumulation, and some people carry a genetic tendency to overload — which is why you should not supplement without testing. Iron tablets are a leading cause of poisoning in young children, so store them out of reach. Supplements also commonly cause constipation.
Sources
Frequently asked questions
- Should I take iron for energy or endurance?
- Only if a blood test shows you're low. Correcting genuine iron deficiency clearly improves energy and endurance, but taking iron when your levels are normal offers no benefit and carries real risks.
- Who is most at risk of deficiency?
- People who menstruate, vegetarians and vegans, and endurance athletes (who can lose iron and have higher turnover) are more prone to low iron. Testing beats guessing.
- Why not just take it to be safe?
- Because the body has no easy way to excrete excess iron, over-supplementing can cause harmful build-up. It's one of the clearest 'test, don't guess' supplements.
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Written and reviewed by Mathew Beale — MSc Biotechnology, University of Reading.
Last reviewed: