Iron

What it is

Iron is central to hemoglobin, the protein that carries oxygen in red blood cells. Low iron is a common cause of fatigue, especially in menstruating people, pregnancy, and plant-based diets. It's the one supplement where testing first really matters.

How it works

Iron is the core of hemoglobin, the protein in red blood cells that carries oxygen from your lungs to the rest of the body, and of myoglobin in muscle. When iron runs low, your blood carries less oxygen — which shows up as fatigue, breathlessness, pale skin, and poor concentration long before it becomes full anemia. Your body has no active way to get rid of excess iron, which is exactly why supplementing only when needed matters.

What it's been studied for

Used to correct low iron and iron-deficiency fatigue, and as part of prenatal care.

What the evidence shows

A Cochrane review found daily iron improved iron status and reduced fatigue in menstruating women, and iron in pregnancy supports healthy iron levels. The catch: benefits apply when you're actually low — supplementing without a need offers no upside and real downside.

Who should consider it

The people who benefit are those who are genuinely low — and iron is the one supplement where testing first is essential. Higher risk includes menstruating people (especially with heavy periods), pregnant people, endurance athletes, frequent blood donors, vegetarians and vegans (plant iron is less absorbable), and anyone with a digestive condition causing blood loss. Unexplained fatigue is a common reason to get iron checked.

Amounts commonly studied

Doses depend on the deficiency and are best set with a provider; every-other-day dosing is increasingly used to improve absorption and tolerance.

How to take it

Doses depend on the degree of deficiency and are best set with a provider. Taking iron every other day, rather than daily, is increasingly recommended because it can improve absorption and reduce side effects. Vitamin C or a citrus food boosts absorption; coffee, tea, calcium, and dairy reduce it, so separate them. Ferrous bisglycinate tends to be gentler on the stomach than ferrous sulfate.

Forms & absorption

Ferrous bisglycinate is gentler on the stomach than ferrous sulfate. Taking it with vitamin C aids absorption; calcium, coffee, and tea reduce it.

Food sources

Heme iron (well absorbed) comes from red meat, poultry, and seafood. Non-heme iron (less absorbed) is in beans, lentils, tofu, spinach, and fortified cereals — pair these with vitamin C to boost absorption. Cooking in cast iron adds a little.

Safety & interactions

Don't supplement iron without knowing your levels — excess iron is harmful and can be dangerous, especially for children (a leading cause of poisoning). Constipation and stomach upset are common. Keep away from kids.

Common questions

Why does iron upset my stomach?

Constipation and nausea are common, especially with ferrous sulfate. Try ferrous bisglycinate, take it with food, or use every-other-day dosing, which is often just as effective and better tolerated.

How long until my levels recover?

Symptoms often improve within a few weeks, but rebuilding iron stores can take several months. Your provider will usually recheck bloodwork to confirm.

Can I just take iron to be safe?

No — because the body can't excrete excess iron, supplementing without a confirmed need can cause harm. It's also a leading cause of poisoning in children, so keep it away from kids.