Iron: What It Actually Does, How Much to Take, and When It's Pointless

Iron is one of the few supplements that can genuinely change how you feel in training — but only if you're actually short on it. If you're not, the pill does nothing, and at higher doses it can do harm. This is a topic where you get bloodwork first and buy second.

What iron actually does

About 70% of your body's iron sits in hemoglobin, the protein that carries oxygen from your lungs to your muscles. Some sits in myoglobin inside the muscle itself, and some in the mitochondrial enzymes that produce ATP. In practice that's three things: how much oxygen you can deliver, how much the muscle can hold onto, and how fast you turn fuel into energy.

When stores drop, endurance goes first. People describe it as heavy legs: heart rate runs 10-15 beats higher at the same pace, rest between sets stretches from 90 to 150 seconds, and a 5 km run you used to do without thinking suddenly hurts. Your one-rep max usually holds on longer, but a set of 12 becomes a fight. Anemia — an actual drop in hemoglobin — is the last stage, not the first.

The numbers that matter on your labs

Ask for three things: ferritin, hemoglobin, and transferrin saturation. A standard complete blood count on its own isn't enough, because ferritin falls for months before hemoglobin budges.

• Ferritin under 15 ng/mL — stores are empty, essentially confirmed.
• Ferritin 15-30 ng/mL — deficiency without anemia; usually treated in people training hard.
• Ferritin 30-50 ng/mL — grey zone; the call depends on your symptoms and training volume.
• Ferritin above 100 ng/mL — extra iron buys you nothing.
• Transferrin saturation below 16% supports the diagnosis.

One trap: ferritin spikes with any inflammation and for 2-3 days after a very hard session, a long run, or a half marathon. Get blood drawn in the morning, fasted, and at least 3 days after your last hard session — otherwise you'll get a falsely reassuring number. That's why CRP is usually ordered alongside it.

Who is actually at risk

• Women who menstruate — average loss is roughly 0.5-1 mg of iron per day, considerably more with heavy cycles.
• Long-distance runners — foot strike destroys red blood cells, plus sweat losses and small gut bleeds.
• Vegetarians and vegans — plant iron absorbs 2-3 times worse.
• Recent blood donors — one donation removes roughly 200-250 mg of iron.
• Anyone in a long calorie deficit, simply because total food intake is low.

The average guy training 4 times a week who eats meat regularly is probably not at risk, and a supplement is wasted money.

Food before pills

The recommended daily intake is about 8 mg for men and 18 mg for women of reproductive age; endurance athletes realistically need 30-70% more. Here's what 100 g of food gives you:

• Beef: about 2.5 mg, and it's heme iron, absorbed at 15-35%.
• Liver: 6-7 mg, the densest source there is.
• Cooked lentils: about 3.3 mg, but absorption is only 2-10%.
• Pumpkin seeds (30 g): about 2.5 mg.
• Spinach: about 3.6 mg on paper, far less in reality because of oxalates.

One trick that works without any pill: eat 100-250 mg of vitamin C alongside a plant iron source — half a raw bell pepper, a kiwi, or lemon squeezed over beans. That can double absorption from that meal.

Dosing: how much and how

If labs confirmed a deficiency, typical treatment is 40-100 mg of elemental iron. Watch the numbers: 325 mg of ferrous sulfate contains about 65 mg of elemental iron, and the box usually advertises the bigger figure.

1. Take it every other day, not daily. Hepcidin, the hormone that shuts down absorption, stays elevated for about 24 hours after a dose, so a day off gives you more total iron with fewer side effects.
2. One dose in the morning, on an empty stomach, with a glass of water and a vitamin C source.
3. No coffee, tea, or milk 1 hour before or after — polyphenols and calcium cut absorption by 40-60%.
4. Don't take it 3-6 hours after a hard session; hepcidin peaks there.
5. Recheck ferritin at 8-12 weeks; fully refilling stores takes 3-6 months.

If nausea or constipation is a problem — it happens to roughly a third of people — take it with a small meal. Absorption drops somewhat, but the pill you actually swallow beats the one sitting in a drawer. Black stool while on iron is normal and not a reason to panic.

When it makes no sense

If your ferritin is above 50 ng/mL, extra iron doesn't improve VO2max or endurance, and that finding is fairly consistent across studies. The upper limit without medical supervision is 45 mg per day. In hemochromatosis, a hereditary condition affecting roughly 1 in 200-300 people, taking iron preventively directly loads the liver, heart, and pancreas.

Iron isn't a stimulant. It restores what's missing; it doesn't add anything beyond that.

When to see a doctor

Don't take iron blind. See a doctor if:

• Fatigue lasts longer than 3-4 weeks with no clear reason.
• You're a man or a postmenopausal woman with confirmed anemia — the source of bleeding is always investigated, most often in the digestive tract.
• You see blood in your stool, or your stool is black and you're not taking iron.
• Your period lasts more than 7 days or you change protection more often than every 2 hours.
• You get heart palpitations, run out of breath on stairs, or have restless legs at night.
• You've taken iron for 3 months and ferritin isn't rising — that means it's being lost somewhere or not absorbed (celiac disease, gastritis, inflammation).

The short version

• Iron only helps if you're deficient — ferritin, hemoglobin, and transferrin saturation first, purchase second.
• Ferritin under 30 ng/mL with symptoms is worth a doctor's visit; above 100 ng/mL there's nothing to gain.
• The treatment dose is 40-100 mg of elemental iron, every other day, in the morning, with vitamin C.
• Coffee, tea, milk, and a hard session in the previous 6 hours all kill absorption.
• Recheck at 8-12 weeks; stores refill over 3-6 months, so don't expect a change in a week.