Iron and Ferritin - Supplementing Smart

We have spent two parts of this series on ferritin values and labs, but what does treatment look like when there is some form of iron deficiency?

For most athletes, increasing ferritin and iron levels through diet alone isn’t going to cut it. Or, at least it’s not going to increase levels efficiently.

But the combination of dietary changes + a supplementation strategy. That’s where we can see best results.

I forgot to cover this in Part I

After publishing Part I of this series, I realized I forgot to mention how NSAID medication use can cause iron loss in athletes, so here is a quick one-off discussion because it’s worth mentioning.

Mechanisms in how NSAID meds can impact ferritin/iron levels:

Small-bowel enteropathy

Long-term NSAID use damages the small intestine directly, not just the stomach. Up to 70% of long-term users develop small intestinal inflammation, and around 30% develop erosions or ulcers.

NSAIDs damage the gut lining through two separate pathways:

• The well-known one (COX-mediated): NSAIDs work by blocking an enzyme called COX, which is how NSAIDs reduce pain and inflammation. But COX also produces protective chemicals that keep the stomach and intestinal lining healthy by maintaining its blood flow, stimulating mucus production, and helping it repair itself. When NSAIDs block COX, that protective layer weakens, leaving the gut vulnerable to acid and digestive juices.

• The less recognized one (COX-independent): NSAIDs also directly damage the energy-producing mitochondria of the intestinal cells. They essentially short-circuit the power so the cells lining the intestine can't produce enough energy to maintain themselves. When those cells weaken and die, they form gaps between them, like mortar crumbling between bricks in a wall. This makes the intestinal wall "leaky," allowing bacteria, bile acids, and digestive enzymes to seep into the deeper tissue.

Both pathways cause microscopic bleeding and inflammation along the intestinal wall. The gut is already under stress during intense exercise because blood gets diverted away from the digestive organs to the working muscles. Adding NSAIDs on top of that is essentially a double hit. The gut is already blood-deprived from exercise, and now its cells are also being poisoned from the inside. The result is more tissue damage, more microscopic bleeding, and more iron lost. Often without the athlete ever noticing any symptoms.

Impaired absorption

Beyond blood loss, NSAIDs can interfere directly with intestinal iron absorption.

They can impact your CRP (inflammatory) labs

As mentioned in Part II, when C-reactive protein is high, it can give a falsely higher ferritin lab result.

NSAIDs also lower the CRP inflammatory marker. So an athlete taking NSAIDs regularly may show a deceptively normal inflammatory panel.

Increasing dietary iron

Before we get to supplements, diet deserves its own conversation.

For reference, the Recommended Dietary Allowances are:

• 14-18 years old (male): 11 mg

• 14-18 years old (female): 15 mg

• 19-50 years old (males): 8 mg

• 19-50 years old (females): 18 mg

• 51 years old: 8 mg

• Note: Women during pregnancy require 27 mg iron/day

Heme vs. non-heme iron

Not all dietary iron behaves the same way once it hits your gut.

• Heme iron comes from animal sources, meat, poultry, and seafood, and is absorbed at roughly 15-35% efficiency.

• Non-heme iron comes from plant sources and fortified foods, and is absorbed at only about 2-20% efficiency. It is much more sensitive to what else is on your plate.

Because non-heme iron is so poorly absorbed, it’s recommended that people who eat plant-based diets eat more iron-rich foods than the RDA.

Absorption Enhancers:

• Vitamin C, eaten in the same meal as the iron source. This is the most reliable enhancer. Dosing and timing matter here. If taken hours ahead, it’s far less effective.

• Meat, poultry, or fish added to a plant-based meal, the “meat factor” boosts absorption of non-heme iron from everything else on the plate, not just its own heme iron.

• Cooking acidic, high-moisture foods in cast iron cookware.

Does cooking with cast iron really increase iron?

Yes, and it is one of the more well-documented “kitchen hack” claims out there.

A classic 1986 study tested 20 foods cooked in iron skillets versus non-iron cookware and found that 90% of foods contained more iron after cooking in cast iron. The largest effect was in acidic, high-moisture foods simmered for longer periods. Spaghetti sauce went from under 1 mg of iron to nearly 6 mg per serving when simmered in a cast iron pan.

A more recent systematic review of cast iron cookware interventions confirmed the same pattern: measurable increases in both food iron content and, in several studies, blood hemoglobin levels in populations using iron cookware regularly.

If you don’t want to use a cast iron pan, products like this Lucky Iron Fish (affiliate Amazon link) are an option too.

Absorption Inhibitors:

• Calcium from dairy, calcium-fortified foods, or supplements. The only one that blocks both heme and non-heme absorption

• Phytates, found in plant-based foods, mostly in whole grains, legumes, nuts, and bran. This is dose-dependent. The largest inhibition effect is at higher amounts.

• Polyphenols, in tea, coffee, cocoa, and red wine. Again, dose-dependent. Black tea alone can cut absorption 56-72%.

• Tannins and other compounds in tea/coffee generally, best kept an hour or more away from iron-rich meals

Iron-rich foods, and how much iron they actually provide

Here is a practical list of standard portions and iron content per USDA data.

Heme sources:

• Oysters (3 oysters): 6.9 mg

• Mussels (3 oz): 5.7 mg

• Beef liver (3 oz): ~5 mg (organ meats range widely, 1.8 to 19 mg depending on the cut)

• Sardines, canned (3 oz): 2.5 mg

• Beef (3 oz): 2.5 mg

• Clams (3 oz): 2.4 mg

• Turkey leg (3 oz): 2.0 mg

• Lamb (3 oz): 2.0 mg

• Shrimp (3 oz): 1.8 mg

Non-heme sources:

• Fortified whole grain cereal (1/2 cup): 16.2 mg

• Fortified hot wheat cereal (1 cup): 12.8 mg

• Fortified toasted oat cereal (1 cup): 9.0 mg

• Fortified bran flakes (3/4 cup): 8.4 mg

• Spinach, cooked (1 cup): 6.4 mg

• Lima beans, cooked, 1 cup: 4.9 mg

• Soybeans, cooked, 1/2 cup: 4.4 mg

• Swiss chard, cooked, 1 cup: 4.0 mg

• White beans or lentils, cooked, 1/2 cup: 3.3 mg

• Chickpeas, cooked, 1/2 cup: 2.4 mg

• Kidney beans, cooked, 1/2 cup: 2.0 mg

• Sesame seeds, 1/2 oz: 2.1 mg

• Cashews, 1 oz: 1.9 mg

• Prune juice, 1 cup: 3.0 mg

Dietitian thoughts: those fortified cereal numbers look significant next to a cup of cooked spinach. Fortification is one of the most effective yet underrated ways non-heme iron gets into the average diet.

Does supplementation actually work?

Good news: once someone is identified as iron deficient, the research on supplementation is fairly consistent.

A few caveats on dose and duration matter, though.

A 2024 meta-analysis (Šmid and colleagues, Sports Medicine) pooled 13 randomized trials and found:

• Oral iron reliably increases serum ferritin, but only in people who started deficient.

• Athletes with a starting ferritin at or below 12 ng/mL saw meaningful increases.

• Athletes starting above that saw only minimal changes, regardless of dose.

• No significant effect on VO2max overall.

• Doses from 16 to 100 mg of elemental iron daily worked, but only when given for at least 6 to 8 weeks. Shorter supplementation protocols, even at high doses, did not move the needle.

A separate 2026 meta-analysis focused specifically on active women (McCarthy and colleagues) backed this up using ferrous sulfate specifically: a significant average increase of 12.61 ng/mL in ferritin and 0.42 g/dL in hemoglobin.

The biggest gains were seen in women who started more depleted (iron-deficient, non-anemic stage 2). Duration mattered more than dose, and longer durations brought smaller additional gains, basically a diminishing returns curve as stores refill.

Pengelly’s review of female athletes zeroed in on performance specifically, not just biomarkers. The real performance benefits showed up at higher doses:

• Around 100 mg of elemental iron daily, or every other day, taken for up to 8 weeks orally.

• Or parenteral (injected) administration over 8 to 10 days for more severe cases.

Lower doses in the 16-60 mg range still improved secondary markers like energy efficiency and lactate handling. They just did not move the performance needle on straight-line endurance time trials.

Key takeaway: Iron supplementation works, but on a timeline of weeks, not days. It works best in people who are meaningfully depleted rather than borderline, and the dose needs to be adequate if the goal is a performance change rather than just a biomarker change.

How much iron should you supplement with?

For athletes with low ferritin (<12 µm/L), 60–100 mg of elemental iron daily (or alternate-day) for a minimum of 6–8 weeks is the general recommendation, though the optimal dose depends on the severity of depletion and the clinical scenario.

For ferritin between 12-30 µm/L, the same 60 mg elemental iron once daily or alternate-day dosing is appropriate.

For ferritin 30–50 µg/L (suboptimal for athletes), lower doses (e.g., 30 mg/day) with dietary optimization may be adequate. But if you’re looking to speed up ferritin levels to improve athletic performance for an event, I’d go with the 60 mg elemental iron once daily.

Should I supplement daily or every other day?

I’ve seen recommendations on the internet about increase absorption by supplementing with iron every other day instead of daily. So let’s look at the research.

The physiological rationale: hepcidin rebound

The key mechanism is hepcidin, the hormone that controls iron absorption.

When a dose of ≥60 mg elemental iron is taken, hepcidin peaks at ~8 hours, remains elevated at 24 hours, but returns to baseline by 48 hours.

This elevated hepcidin effectively shutting down iron absorption for up to 48 hours after a dose. By dosing every other day, the next dose arrives after hepcidin has normalized, allowing 35–50% higher fractional absorption per dose compared with consecutive-day dosing.

This is the rationale, but what do the studies say?

Clinical Outcome Data

• A masked, placebo-controlled randomized control trial (RCT) of 150 premenopausal women with ferritin ≤30 ng/mL found parallel ferritin repletion between daily dosing for 90 days and alternate-day dosing for 180 days, with significantly fewer GI side effects in the alternate-day group.

• An RCT in 200 adults with IDA showed no significant difference in hemoglobin change between daily and alternate-day dosing at 8 weeks.

Athlete-specific evidence

An 8-week study in 31 endurance-trained runners with ferritin <50 µg/L found that daily and alternate-day oral iron produced parallel increases in serum ferritin (~19.7 µg/L increase in both groups), but the alternate-day group had far fewer severe GI complaints.

The takeaway: For athletes who tolerate daily iron without GI issues, daily dosing remains reasonable for faster repletion.Even though alternate-day dosing takes longer to replete stores, it’s the best option for athletes with gut sensitivity.

Choosing your iron supplement

This is where a lot of athletes get stuck. The supplement that works best is also, unfortunately, the one most likely to upset your stomach.

Ferrous sulfate: the gold standard

Cheap, well studied, and effective.

A 2024 clinical practice update from the American Gastroenterological Association supported ferrous sulfate as the preferred first-line oral iron formulation. Mostly because none of the fancier alternatives are quite as effective.

Their guidance:

• Dose once daily at most. More frequent dosing does not improve absorption. It just adds side effects, such as nausea, constipation, and GI discomfort.

• Consider every other day dosing if tolerance is an issue.

• Many iron supplements include Vitamin C within the supplement, so check the label. If it doesn’t, make sure to pair the supplemental iron with vitamin C to help absorption.

Dietitian Note: The ALN team has found with our athletes that NatureMade Iron (ferrous sulfate) has produced the best results raising ferritin levels. Taking 3-4x/week instead of daily has supported better gut tolerability.

A 325 mg tablet is 65 mg of elemental iron.

Ferrous bisglycinate: what an athlete-specific dosing study found

Ferrous bisglycinate is a chelated iron form (iron bound to the amino acid glycine) marketed as gentler on the gut than ferrous salts.

Most of the supporting research comes from non-athlete populations, but a 2024 study published in PM&R looked specifically at NCAA Division 1 female athletes, worth knowing about if you’re considering this formulation.

The study followed 33 Division 1 athletes (soccer and track and field) with confirmed non-anemic iron deficiency (ferritin <35 µg/L and a normal hemoglobin). Athletes self-selected into one of two dosing schedules for an 8-week course of ferrous bisglycinate at 25 mg elemental iron per dose: daily, or three times a week.

Both schedules worked. Daily dosing raised ferritin by an average of 12.88 µg/L, 3x-weekly dosing raised it by 5.17 µg/L, and statistically that gap wasn’t significant given the study’s small size.

But a more practical marker told a clearer story: 60% of the daily group crossed the 35 µg/L threshold by the end of the study, compared with only 17% of the 3x-weekly group, a difference that was statistically significant.

Side effects moved in the opposite direction. Athletes on the daily schedule reported significantly more nausea and constipation than those dosing three times a week.

The study’s limitations:

This was not a randomized trial. Athletes chose their own schedule rather than being assigned one, the sample was small (33 people at a single university), and there was no direct comparison to ferrous sulfate, just a reference to prior literature suggesting bisglycinate tends to be gentler generally. The study also found that athletes starting with ferritin <15 µg/L were unlikely to reach the 35 µg/L threshold within 8 weeks regardless of schedule. This suggests that a more severe deficiency needs a longer time frame or a higher dose than either regimen tested here.

Key takeaway: In this small athlete-specific study, daily ferrous bisglycinate got more athletes to a normal ferritin level faster, but at the cost of more GI side effects than dosing 3x/week. If tolerance is the limiting factor, 3x-weekly dosing is a reasonable compromise, just budget more time before rechecking labs.

Sucrosomial iron: the newer alternative

About a year ago, I had an athlete client (who was also a physician) tell me about a new form of iron supplement, sucrosomial iron.

Sucrosomial iron (SI) is a novel oral iron formulation where the iron particle is wrapped in a protective coating, essentially a tiny two-layer fatty membrane (similar to what surrounds a living cell), held together by a sugar-fat binder that forms an outer shell around it.

This gives unique absorption properties and excellent GI tolerability compared to conventional iron salts. The clinical trials have been promising, and it’s been studied successfully in populations often managed with IV iron: chronic kidney disease, cancer, bariatric surgery, and inflammatory bowel disease patients.

In theory, that makes absorption less dependent on hepcidin, relevant given everything in Part 2 about post-exercise hepcidin spikes blocking absorption.

There are no head-to-head comparisons to ferrous sulfate, and no dedicated trials measuring athletic performance outcomes.

Sucrosomial iron is widely available across Europe. It’s not available in the U.S. unless you order it online. It’s pricey, ~$2.40 per capsule (compared to ~$0.07/capsule of ferrous sulfate).

Typical doses range from 28–30 mg elemental iron once daily for mild deficiency, with higher doses (60 mg/day) used for moderate-to-severe IDA.

IV Iron

If oral iron genuinely is not tolerated, or ferritin has not budged after a solid trial (generally 4 to 8 weeks of consistent use), intravenous iron is a legitimate next step.

It’s increasingly used earlier in athlete populations for exactly this reason.

Competitive athletes should loop in a sports medicine practitioner here, both to rule out other causes of poor absorption and to make sure any IV protocol clears anti-doping requirements.

Iron itself is not a prohibited substance under WADA rules, but the route of administration is regulated. Intravenous infusions or injections are capped at 100 mL within any 12-hour period. Anything beyond that is prohibited unless it is part of a legitimate hospital treatment, surgical procedure, or clinical investigation.

Key takeaway: A standard IV iron infusion for repletion typically falls within that 100 mL limit, but confirm this with a sports medicine practitioner before scheduling one.

Don’t chase a high number

More is not better here.

Pushing ferritin above a sufficiency threshold does not translate into additional performance benefit. Too high a ferritin carries risks. Iron overload has been linked to oxidative stress and, in more severe cases, organ damage. The goal is repletion, not maximization.

The general upper limit of ferritin is >200 µg/L in women and >300 µg/L in men.

Where this leaves you

If you are coaching yourself or your athletes through this, here is the practical version:

• Get ferritin tested rather than guessing from symptoms alone, and read the number in the context of training load rather than the lab’s general population range.

• Treat anything under 30 as worth a conversation, especially with fatigue or flatlining performance alongside it.

• If supplementation is warranted, expect it to take 6 to 8 weeks minimum to show up in bloodwork.

• Use a dose in the 65 to 100 mg elemental iron range if the goal is a performance change rather than just a biomarker change.

• Build in the absorption basics: away from calcium and polyphenols, with vitamin C, at a frequency you can actually stick with for two months straight.

And if your ferritin comes back and the lab calls it normal while three other papers would have called it borderline, you are not imagining things. It really is the value nobody agrees on. That just means the number needs a coach’s judgment attached to it, not a rubber stamp.

TL;DR:

Diet is the first lever worth pulling. Heme iron (meat, poultry, seafood) absorbs far more reliably than non-heme iron (plants, fortified foods), and pairing non-heme sources with vitamin C matters. Cooking acidic, high-moisture foods in cast iron genuinely adds meaningful non-heme iron, too. Beyond diet, oral iron supplementation reliably raises ferritin, but the benefit depends on how depleted someone was to begin with. It takes 6 to 8 weeks minimum to show up, and performance gains specifically tend to need higher doses (around 100 mg elemental iron daily) sustained for weeks. Ferrous sulfate remains the most effective option but is also the hardest on the stomach. Sucrosomial iron and iron polymaltose complex are gentler alternatives with meaningfully weaker efficacy data, useful mainly for people who cannot tolerate ferrous sulfate at all. IV iron is a legitimate option when oral iron fails, but WADA caps IV infusions and injections at 100 mL per 12 hours. And more ferritin is not automatically better. Chasing a high number past sufficiency has no proven upside and carries real downside.

Let’s hear your thoughts!

What resonated with you? What questions do you still have? Whether you're an athlete, a coach, or a fellow RD, I want to hear your perspective. Leave a comment below and let's keep the conversation going.

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