Health Reference LibraryWhich form of iron should you buy?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
Iron supplements come as inorganic salts (ferrous sulphate, fumarate, gluconate) and as the chelate iron bisglycinate. The salts are cheap and effective but commonly cause constipation, nausea and cramping, which is the main reason people stop taking them. In a randomised double-blind pregnancy trial, 25mg of iron as bisglycinate matched 50mg as ferrous sulphate on iron status with fewer gastrointestinal complaints. Iron is the one supplement never to take on a guess: test ferritin first, because a level below 30 micrograms per litre confirms deficiency while unneeded iron accumulates. Men and postmenopausal women rarely need it.
How it works
All oral iron is absorbed in the upper small intestine, and unabsorbed iron travelling onward is what irritates the gut, so the less you need to swallow for the same absorbed amount, the fewer the side effects. Bisglycinate wraps the iron ion between two glycine molecules, which keeps it soluble and shielded through the stomach, allowing a lower elemental dose to do the same job as a higher salt dose.
Effective dose
Dose in elemental iron, not compound weight. UK reference intake is 8.7mg daily for men and 14.8mg for women 19 to 49. Correction protocols run far higher under clinical guidance. Absorption per dose falls as doses rise and repeat, which is why alternate-day dosing absorbs more per milligram than daily dosing for many people.
Forms compared
Ferrous sulphate is the reference standard and the cheapest. Ferrous fumarate and gluconate are alternative salts with similar behaviour at matched elemental doses. Iron bisglycinate is the chelated form: in the Milman randomised double-blind trial, 25mg elemental iron as bisglycinate was as effective as 50mg as ferrous sulphate with significantly fewer gastrointestinal complaints (P=0.001).
Timing
Best absorbed away from food, with a vitamin C source, which is the rationale for the paired formulation. Keep tea, coffee, calcium and dairy at least two hours away, since polyphenols and calcium bind iron. If an empty stomach is intolerable, with a small low-calcium snack beats abandoning it.
Safety profile
Unneeded iron is not benign: the body has no route to excrete an excess, and accidental iron overdose is a leading cause of fatal poisoning in young children, so keep any iron supplement out of reach. People with haemochromatosis or unexplained high ferritin should not supplement iron at all.
Special populations
Menstruating women, pregnant women, regular blood donors, endurance athletes and vegans carry the highest deficiency risk. Pregnancy dosing belongs with a midwife or GP. Men and postmenopausal women with low ferritin need the cause investigated by a GP before supplementing, because unexplained iron loss in these groups warrants investigation in UK practice.
Interactions
Iron reduces absorption of levothyroxine and some antibiotics, so separate by four hours. Proton pump inhibitors and other acid suppressants reduce iron absorption. Calcium supplements taken at the same time compete with iron.
Guideline positions
NICE Clinical Knowledge Summary on iron deficiency anaemia: serum ferritin below 30 micrograms per litre confirms iron deficiency in adults, with interpretation caution where inflammation is present. NIH ODS iron fact sheet: elemental dose is what counts across salt forms; UK and US reference intakes differ by sex and life stage. Milman 2014: the randomised dose-matching and tolerability comparison.
Practical framework
Decide in three steps. First, test: ferritin from your GP or a home panel, because supplementing iron without a measured need risks accumulation and masks the real cause of symptoms. Second, if deficiency is confirmed and your GP agrees a supplement is the route, choose by tolerability: salts are fine if your stomach handles them, bisglycinate earns its price if it keeps you taking it. Third, pair with vitamin C and retest ferritin after about three months. The worked example of the paired approach on our shelf is Iron and Vitamin C at nutritailor.co.uk/products/iron-vitamin-c.
Common misconceptions
That more iron is better: absorption saturates and side effects scale with dose, which is why the 25mg bisglycinate arm matched the 50mg sulphate arm. That fatigue means low iron: fatigue has many causes and only a ferritin result settles this one. That black stools mean harm: they are an expected effect of unabsorbed iron at higher doses.
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.