Iron

Iron exists in food in two forms that behave very differently. Haem iron, found in meat, fish and poultry, is well absorbed and relatively unaffected by whatever else is on the plate. Non-haem iron, found in plants, fortified cereals and supplements, is poorly absorbed and heavily influenced by other foods.

That difference is the single most useful thing to understand about iron. Someone eating red meat regularly and someone eating lentils are not getting comparable iron even when the numbers on paper look similar.

Supplement forms vary in tolerability. Ferrous sulphate is cheap, effective and notorious for digestive upset. Ferrous bisglycinate is gentler and better absorbed per milligram, which is why it costs more and why it is worth the difference for many people.

Most of the body's iron sits in haemoglobin, the protein that carries oxygen from the lungs to the tissues. A smaller amount is in myoglobin, which holds oxygen within muscle, and in enzymes involved in energy production.

Absorption is tightly regulated because the body has no mechanism for excreting excess iron. Uptake is dialled up or down in the gut according to need, which is an elegant system and also the reason supplementing unnecessarily is a bad idea — what goes in largely stays in.

Vitamin C converts non-haem iron into a more absorbable form and substantially improves uptake. Tea, coffee, calcium and the phytates in wholegrains and legumes all reduce it. These interactions are large enough to matter more than the dose on the label.

Iron's essentiality is beyond dispute, and correcting genuinely low iron status produces clear, well-documented improvement in energy and exercise capacity.

Low iron status is genuinely common in menstruating women, endurance athletes, and people eating plant-based diets without attention to it. Endurance athletes are affected by several mechanisms at once, including losses through sweat and the mechanical impact of running.

For anyone whose iron status is normal, supplementation offers no performance or energy benefit and carries real downside. This is the mineral where the difference between correcting a shortfall and topping up sufficiency is starkest, and it is the reason a blood test should come before a supplement.

Iron supplements commonly cause constipation, nausea, stomach pain and dark stools. These are dose-related and often solved by switching form or dosing on alternate days.

Because the body cannot excrete excess iron, sustained unnecessary supplementation causes it to accumulate, which is genuinely harmful over time. Anyone who has been told they retain too much iron should avoid supplements entirely.

Iron tablets are a leading cause of accidental poisoning in young children, and a small number of tablets can be enough to cause serious harm. Keep them where children cannot reach them, in child-resistant packaging. This is the most important safety point on this page.