Health Reference LibraryIron bisglycinate vs ferrous fumarate: which should you take?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
If your GP has found iron deficiency, the form and dose they give you is the right one, and ferrous fumarate is usually it: 210 mg of fumarate delivers about 68 mg of iron in one tablet and it is the NHS form. Iron bisglycinate is the gentler alternative, iron bound to two glycine molecules so it passes the stomach and upper gut without the constipation, nausea and black stools that make people stop taking fumarate. It is absorbed well enough that 25 mg of bisglycinate matched 50 mg of ferrous sulphate for raising haemoglobin in a 2014 trial. Use bisglycinate for a low normal ferritin or when fumarate upsets you, and tell your GP if you switch.
How it works
Iron absorption is controlled by hepcidin, a hormone that rises after each iron dose and shuts the gate for the rest of the day. That is why the recent trials found alternate day dosing absorbs more per tablet than daily dosing, whatever the form. Bisglycinate appears to trigger a smaller hepcidin rise for the iron delivered. The 2014 Danish trial in pregnant women found 25 mg of iron as bisglycinate kept haemoglobin and ferritin as well as 50 mg as ferrous sulphate, with fewer gut complaints.
Effective dose
The UK reference intake is 8.7 mg a day for men and 14.8 mg for women aged 19 to 50. For diagnosed deficiency, NICE describes a course of a ferrous salt from the GP, usually one tablet a day, continued for three months after haemoglobin recovers to refill stores; the dose is the GP’s decision. For a low normal ferritin without anaemia, 15 to 25 mg a day of bisglycinate for eight to twelve weeks, then a retest, is the usual approach.
Forms compared
Ferrous fumarate, ferrous sulphate and ferrous gluconate are iron salts. They dissolve in the stomach and release free iron, which is absorbed in the first part of the small intestine and which also irritates the gut lining and feeds bacteria further down, hence the side effects. Ferrous bisglycinate, sold as gentle iron, is a chelate: the iron is held between two glycine molecules and stays bound until it reaches the absorbing cells. Less free iron in the gut means fewer side effects and a higher fraction absorbed, which is why the effective dose is lower.
Timing
Iron is best absorbed away from food, but bisglycinate is gentle enough to take with a light meal. Keep tea and coffee an hour clear on the far side, and take iron at a different time from calcium supplements. Alternate day dosing is worth trying if a daily dose upsets you.
Safety profile
Iron is the one common supplement where more is dangerous. Never take iron for tiredness without a blood test, because the same symptoms occur in iron overload. Keep every iron product out of reach of children; accidental overdose is a leading cause of poisoning in under fives. Black stools are expected with fumarate and unusual with bisglycinate; stomach pain that does not settle, or any bleeding, needs a GP.
Guideline positions
NICE Clinical Knowledge Summary: anaemia, iron deficiency (2024). NIH Office of Dietary Supplements: iron fact sheet (2025). Ferrous bisglycinate 25 mg iron is as effective as ferrous sulphate 50 mg in pregnancy, randomised trial (2014).
Practical framework
Get the number first. If ferritin is under 30, that is iron deficiency and your GP decides the form and dose. If ferritin sits in the low normal band, or fumarate has defeated you, Nutri Tailor stocks [iron bisglycinate with vitamin C](/products/iron-vitamin-c), 15 mg of iron as bisglycinate with 400 mg of vitamin C per capsule, made in the UK. Retest after eight weeks. To read your own result against the UK ranges, use the free blood test result interpreter. This is a summary of published research, not personal health advice. Talk to your GP or a registered nutritional therapist before changing what you take, especially if you are pregnant, have a diagnosed condition or are under ongoing care.
Common misconceptions
That gentle iron is weak iron: it is absorbed better, which is why the dose is lower. That black stools mean the iron is working: they mean unabsorbed iron is passing through. That more iron corrects a deficiency faster: hepcidin closes the gate, and alternate day dosing often does better than double doses. That you can diagnose iron deficiency from symptoms: you cannot, and the test is cheap.
Forms compared
| Iron Bisglycinate | Ferrous Fumarate | |
|---|---|---|
Sources
- National Institute for Health and Care Excellence (NICE, UK government) 2024. Anaemia - iron deficiency: Clinical Knowledge Summary. NICE Clinical Knowledge Summaries.
- NIH Office of Dietary Supplements 2025. Iron: Fact Sheet for Health Professionals. National Institutes of Health, Office of Dietary Supplements.
- Milman N, Jønsson L, Dyre P, Pedersen PL, Larsen LG 2014. Ferrous bisglycinate 25 mg iron is as effective as ferrous sulfate 50 mg iron in the prophylaxis of iron deficiency and anemia during pregnancy in a randomized trial. Journal of Perinatal Medicine 42(2):197-206, DOI 10.1515/jpm-2013-0153, PMID 24152889.