Which form of iron is best?

Iron bisglycinate (ferrous bisglycinate) is best-absorbed with the fewest GI side effects. Ferrous sulfate is the most studied and cheapest but causes constipation and nausea in 20-30% of users. Heme iron (from animal sources) has the highest absorption but costs significantly more. For most people: bisglycinate 25-50 mg/day with vitamin C is the best balance of absorption, tolerability, and cost.

How do I know if I need iron?

Get blood work — do not supplement based on symptoms alone. Ferritin below 30 ng/mL indicates iron deficiency. Hemoglobin below 12 g/dL (women) or 13 g/dL (men) indicates anemia. Symptoms of deficiency — fatigue, pallor, shortness of breath, restless legs — are nonspecific and common to many conditions. Iron overload (hemochromatosis) is dangerous and underdiagnosed. Testing before supplementing is essential.

Why does iron cause constipation?

Unabsorbed iron is pro-oxidative in the gut, irritating the intestinal lining and slowing motility. Bisglycinate is better tolerated because the glycine chelation improves absorption, leaving less unabsorbed iron in the gut. Taking iron with vitamin C (200 mg) increases absorption. Taking iron every other day (rather than daily) may improve absorption and reduce side effects through the hepcidin mechanism.

Can men take iron supplements?

Rarely. Men do not menstruate and are at much higher risk of iron overload (hemochromatosis affects 1 in 200 people of Northern European descent). Do not supplement iron without a confirmed deficiency on blood work. Excess iron is pro-oxidative and associated with increased cardiovascular and cancer risk. The exception: male endurance athletes (foot strike hemolysis) and vegetarians may have higher needs.

What blocks iron absorption?

Calcium (dairy, supplements), tannins (tea, coffee), phytates (whole grains, legumes), and PPIs (reduce stomach acid needed for absorption). Take iron 2 hours apart from these. Conversely, vitamin C (200 mg) and heme iron (meat) enhance absorption. The best time: morning on an empty stomach with vitamin C, but if this causes nausea, take with a small non-dairy meal.

Evidence Review · 3 References

Iron: The Supplement You Should Not Take Without a Blood Test

Iron deficiency is the most common nutritional deficiency worldwide — affecting 25% of the global population. But iron overload is also dangerous, and supplementing without testing can cause harm. Here is how to supplement safely, which form to choose, and when iron is actually necessary.

Iron — do not supplement without a blood test.

Quick answer

Test ferritin before supplementing. If ferritin is below 30 ng/mL: iron bisglycinate 25-50 mg/day + vitamin C 200 mg. Take on an empty stomach if tolerated, every other day to reduce GI effects. Re-test in 3 months. Do not supplement without a confirmed deficiency — iron overload (hemochromatosis) is underdiagnosed and dangerous. Women with heavy periods, vegetarians, and endurance athletes are at highest risk for deficiency [1,3]. Ferrous sulfate is the cheapest but causes constipation in 20-30% — bisglycinate is worth the small premium [2].

At a Glance · Iron Form Comparison

Form	Absorption	GI Tolerance	Cost/mo	Best For
Iron Bisglycinate	Excellent	Good	$8-15	Best all-purpose choice
Ferrous Sulfate	Good	Poor (constipation)	$3-5	Budget, short-term correction
Heme Iron	Best	Excellent	$20-30	Severe deficiency, poor responders

Bottom line

Iron is not a general wellness supplement — it is a deficiency correction tool. Test before supplementing. Bisglycinate at 25-50 mg every other day + vitamin C is the best-tolerated, best-absorbed approach for most people [1,2]. Men and postmenopausal women should rarely supplement iron without confirmed deficiency. The risks of iron overload (oxidative damage, organ dysfunction) outweigh the benefits in iron-replete individuals [3].

References

[1] Stoffel NU, et al. (2017). Iron bisglycinate vs ferrous sulfate. Lancet Haematol, 4(11): e524-e533. PubMed →
[2] Moretti D, et al. (2015). Iron bioavailability from supplements. Blood, 126(17): 1981-1989. PubMed →
[3] Camaschella C. (2015). Iron-deficiency anemia. N Engl J Med, 372(19): 1832-1843. PubMed →

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How to read Iron: The Supplement You Should Not Take Without a Blood Test

Iron bisglycinate, sulfate, and heme compared. Evidence-based dosing for deficiency, anemia, and who should not supplement without blood work. This guide is intended to help readers make sense of evidence, safety, and practical fit without turning supplement research into a one-size-fits-all checklist. Use it alongside the linked herb and compound profiles for deeper mechanism and safety details.

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