Calculate elemental iron by form. “65 mg ferrous sulfate” delivers different elemental iron than “65 mg ferrous bisglycinate.” Includes RDA + deficiency-correction protocols. New evidence on every-other-day dosing may double your absorption. Math, not medical advice.
Your situation
ferrous sulfate is ~20% elemental iron. To get 100 mg of actual iron you need 500 mg of the compound on the label.
Reference: elemental iron by form
| Form | Elemental Fe % | Best for | Trade-offs |
|---|---|---|---|
| Ferrous sulfate | ~20% | Standard cheap option; most RCT data | GI side effects most common; iron taste; teeth staining |
| Ferrous gluconate | ~12% | Slightly gentler GI than sulfate | Need higher pill burden; lower elemental |
| Ferrous fumarate | ~33% | Highest elemental — smaller pills | GI side effects still common |
| Ferrous bisglycinate | ~20% | Sensitive stomach; minimal constipation; pregnancy | More expensive; absorption similar to sulfate but better tolerated |
| Ferric citrate | ~21% | Kidney disease (phosphate-binding side effect) | Less standard for general IDA |
| Heme iron polypeptide | ~11% | Tolerance failures with ferrous forms | Most expensive; smaller absorption-blunting effect from food |
| Iron protein succinylate | varies (~5-10%) | European Rx (Ferplex, Legofer) | Limited US availability; less hepcidin response |
RDA + UL reference (Institute of Medicine)
| Group | RDA mg/day | UL (supplemental) |
|---|---|---|
| Males 19-50 | 8 | 45 |
| Males 51+ | 8 | 45 |
| Females 19-50 (menstruating) | 18 | 45 |
| Females 51+ (postmenopausal) | 8 | 45 |
| Pregnant | 27 | 45 |
| Lactating 19-50 | 9 | 45 |
| Teens 14-18 female | 15 | 45 |
The 45 mg UL applies to healthy adults. Deficiency-correction protocols (60-200 mg/day) explicitly exceed the UL because acute repletion under clinician supervision is different from chronic over-supplementation.
Absorption boosters and blockers
- Vitamin C (200-500 mg) boosts non-heme iron absorption 2-3×. Take with iron supplement or with iron-rich meal.
- Coffee, tea, milk, dairy, calcium supplements BLOCK absorption. Separate from iron by 2 hours.
- PPI/antacid drugs (omeprazole, etc) reduce stomach acid → reduce iron absorption. Discuss timing with prescriber.
- Bran, soy protein, phytates (whole grains, legumes) bind iron. Avoid with iron supplement.
- Heme iron (from meat or heme-iron supplement) absorbs differently and is less affected by these blockers.
Why every-other-day dosing may work better
Moretti 2015 (Blood) and follow-up trials showed: daily oral iron raises hepcidin (the iron-blocking hormone) for ~24 hours. The next morning’s dose is poorly absorbed. Skipping a day lets hepcidin drop, so the next dose absorbs better.
- Same weekly total dose, higher fraction absorbed. Bonus: fewer GI side effects.
- Best supported in iron-deficient women (the population studied).
- For severe IDA or pregnancy needing rapid repletion, daily may still be standard — discuss with prescriber.
- Modern protocols sometimes use higher single doses (60-120 mg elemental) every other day for IDA correction.
Side effects and red flags
- Constipation, dark stools, GI cramping are common with ferrous salts. Bisglycinate is the gentler option; heme iron has minimal GI burden.
- Iron overload risk: hereditary hemochromatosis affects ~1 in 300; symptoms include fatigue, joint pain, abdominal pain. Get ferritin tested before chronic supplementation.
- Drug interactions: tetracyclines, fluoroquinolones, levothyroxine, levodopa, bisphosphonates — all absorption reduced by iron. Space doses by 2-4 hours.
- Pediatric overdose is a leading cause of accidental poisoning in children under 6. Keep iron supplements out of reach.
- Ferritin over 200 (women) or 300 (men) warrants workup for hemochromatosis, inflammatory disease, or liver disease — not blanket iron supplementation.