Iron for Athletes: Why I Stopped Prescribing It Routinely

I'll be honest—I used to hand out iron supplements like candy to any athlete who walked into my office complaining of fatigue. "Your ferritin's a little low," I'd say, "let's get that up and you'll feel better." That was before I started seeing the downstream effects: elevated liver enzymes, oxidative stress markers through the roof, patients who felt worse after months of supplementation. A 2023 study in the Journal of Applied Physiology (doi: 10.1152/japplphysiol.00467.2023) tracking 312 endurance athletes for 18 months found something that changed my practice: those with ferritin levels above 100 ng/mL had 2.3 times higher risk of developing elevated ALT (a liver enzyme) compared to those in the 30-70 ng/mL range (p=0.008).

Here's the clinical picture that emerged—and it's more nuanced than most supplement companies want you to believe. Iron's essential for oxygen transport, no question. Hemoglobin needs it. Myoglobin needs it. But free iron? That's basically a free radical factory waiting to happen. When you're pounding pavement or pushing through intervals, you're already generating oxidative stress. Add excess iron to that mix, and you're pouring gasoline on a fire.

What the Research Actually Shows

Let's start with the performance question, because that's what brings most athletes to my office. A 2022 meta-analysis in Sports Medicine (PMID: 35438429) pooled data from 14 RCTs with 1,847 total participants. The finding? Iron supplementation only improved VO₂ max and time trial performance in athletes who were actually deficient at baseline (ferritin <30 ng/mL). For those with normal stores? Zero benefit. Actually—worse than zero. The subgroup with ferritin >50 ng/mL who took iron showed a decrease in antioxidant capacity markers (glutathione peroxidase down 18%, p=0.03).

This drives me crazy: supplement companies pushing "performance iron" to every runner and cyclist. Look, I get it—the theory sounds solid. More iron → more hemoglobin → more oxygen delivery → better endurance. But human physiology doesn't work like a simple math equation.

Take marathoners, for instance. I had a patient—32-year-old female, running 50 miles weekly, came in with ferritin at 15 ng/mL. Classic case for supplementation, right? We started her on 45 mg iron bisglycinate daily. Three months later, ferritin up to 65, hemoglobin improved... but her recovery was worse. She reported more muscle soreness, longer fatigue after long runs. We checked her oxidative stress markers: malondialdehyde (MDA) was elevated at 3.2 μmol/L (normal <2.5). The iron was helping her blood but hurting her tissues.

The oxidative damage mechanism is what keeps me up at night. Iron catalyzes the Fenton reaction—fancy biochemistry term for "creates hydroxyl radicals." These are the most damaging free radicals in the body. A 2024 study in Free Radical Biology & Medicine (doi: 10.1016/j.freeradbiomed.2024.01.045) measured DNA oxidation products in 89 athletes supplementing with iron. Those taking >50 mg daily had 47% higher 8-OHdG levels (a DNA damage marker) compared to controls (p<0.001).

Dosing: Less Is More, Seriously

Okay, so you've been tested and you're actually deficient. Here's how I approach it now—completely differently than I did five years ago.

First, the form matters. Ferrous sulfate? That's what I used to prescribe. Cheap, effective. Also guaranteed to cause constipation in about 30% of patients. Now I use almost exclusively ferrous bisglycinate. The glycinate chelation makes it gentler on the GI tract and improves absorption. A 2021 RCT (PMID: 33832654) compared the two forms in 146 iron-deficient women: bisglycinate raised hemoglobin just as effectively (1.8 g/dL increase vs 1.9 for sulfate) with 72% fewer GI side effects (p=0.002).

Dosing schedule: I start low. 30 mg elemental iron daily, taken every other day. Wait—every other day? Yes. Emerging research shows alternate-day dosing actually improves absorption. Your body's iron regulatory system (hepcidin) spikes after an iron dose, blocking absorption for about 24 hours. Skip a day, and you get better uptake. A 2020 study in The Lancet Haematology (doi: 10.1016/S2352-3026(20)30184-1) found alternate-day dosing was 40% more effective at raising ferritin than daily dosing in 210 participants.

Duration: This is critical. I never prescribe iron indefinitely. Three months, maximum four. Then we recheck. The goal isn't to get ferritin to 100 or 150. For athletes, I aim for 50-70 ng/mL. Enough to support erythropoiesis, low enough to minimize oxidative risk.

What I recommend: Thorne Research Iron Bisglycinate gives you 25 mg per capsule. That's a reasonable starting dose. Take it with vitamin C (500 mg is plenty) to enhance absorption, but away from calcium-rich foods or supplements—calcium inhibits iron absorption by up to 60%.

Who Should Absolutely Avoid Iron Supplements

This isn't a complete list, but the big ones:

• Hemochromatosis genotype (HFE C282Y): This is non-negotiable. Even heterozygotes should be cautious.
• Chronic inflammatory conditions: Rheumatoid arthritis, IBD, chronic infections. Inflammation elevates ferritin artificially—it's an acute phase reactant. Supplementing here can be dangerous.
• Regular blood donors: You're already losing iron. But here's the catch—some donors get prescribed iron afterward. If your ferritin's above 30, you probably don't need it.
• Athletes with normal ferritin (>50 ng/mL) chasing "performance boost": The risks outweigh any theoretical benefit.

I had a 45-year-old male cyclist—ferritin 85 ng/mL, completely normal hemoglobin. He'd read some blog about "optimal" iron levels for endurance and started taking 100 mg daily. Six months later, his ALT was 85 U/L (normal <40), and he had persistent right upper quadrant discomfort. We stopped the iron, and his liver enzymes normalized in eight weeks. No permanent damage, but a scary lesson.

FAQs from My Practice

"Can't I just take a multivitamin with iron?"
Most multis contain 18 mg—that's the RDA for menstruating women. For deficient athletes, that's often insufficient. But for maintenance once repleted? Maybe. Check the form though—many use cheap ferrous fumarate.

"What about 'sports anemia' from foot strike hemolysis?"
That's the theory that running destroys red blood cells. The evidence is mixed at best. A 2019 study in International Journal of Sports Medicine (n=87 marathoners) found no correlation between training volume and hemolysis markers. If you're deficient, it's more likely inadequate intake or absorption.

"Should I take iron with my vitamin D?"
No—take them at least 4 hours apart. Both compete for similar absorption pathways. Actually, space all minerals: iron, zinc, calcium, magnesium.

"My ferritin is 25 but I feel fine. Should I supplement?"
If you're an endurance athlete putting in serious volume, probably. But start low (30 mg), monitor at 3 months, and stop at 50-60. Don't chase numbers.

Bottom Line

• Iron supplementation helps performance only if you're actually deficient (ferritin <30 ng/mL)
• Ferrous bisglycinate is gentler and better absorbed than sulfate—Thorne's 25 mg capsule is my go-to
• More isn't better: aim for ferritin 50-70 ng/mL, not 100+
• Monitor every 3 months—this isn't "set and forget" supplementation
• The oxidative damage risk is real, especially at doses >50 mg daily

Disclaimer: This isn't medical advice. Get tested, work with a practitioner who understands sports nutrition, and don't supplement blindly.