One thing is certain– there are a lot of iron food supplements out there and they often cause different side effects, mainly constipation and nausea. The main difference between iron food supplements are the iron salt and the amount of iron used in the supplement. Sulphate, fumarate, gluconate, citrate, and bisglycinate – the average person often can’t tell the difference. So what to do?

All the iron salts mentioned above contain different amounts of elemental iron. More importantly, the bioavailability of iron salts, i.e. the available iron to the body, is also different. Therefore, 100 mg of iron in a pill does not mean that 100 mg of iron is actually absorbed by the body. For example, of 100 mg of elemental iron in iron sulphate and fumarate form, bioavailability is around 27-28 mg. In case of iron gluconate it is about 32 mg. What happens to the rest of the iron that is not absorbed? That iron exits the body but before that, it has time to cause side effects – constipation, stool coloration, nausea, abdominal pain and metal-flavored ‘burps’. The side-effects can differ and really depend on the person. The individual characteristics of the particular person’s gut and different diseases can also weaken iron absorption. For certain iron salts, the absorption is weaker when consumed at the same time with foods rich in fiber, calcium and phytates (found mainly in grain and legumes).


Iron bisglycinate 90
Iron sulphate 27
Iron ascorbate 40
Iron fumarate 28
Iron gluconate 32


  • iron amino acid chelate – iron reacts with two amino acid glycine molecules
  • high, approximately 90%, bioavailability
  • absorbed better by the gut compared to other iron forms
  • studies have found that iron bisglycinate is 2–4 times better absorbed by the body than widely-used iron sulphate and fumarate, whose bioavailability is around 27-28% [1] [2] [3]
  • does not usually cause gastrointestinal side effects

Senior researcher and associate professor of chemistry at the University of Tallinn, Rando Tuvikene: ”Divalent iron has high reactivity and can thus cause oxidative stress and damage cells in its free form. Ferrous ion charge in iron bisglycinate is neutralized by two ligands (glycine), meaning this kind of iron complex´s (i.e. chelate) reactivity is lower, oxidative effect smaller and the effect on the gastric mucosa milder (less likely to irritate the gut). Strong connections between glycine and iron ions avoid iron´s reaction with many substances found in food that would considerably weaken iron absorption when using other iron salts [4]. Such substances are, for example, polyphenols, phytates and oxalic acid.

The absorption of non-heme iron takes place in the duodenum. The solubility of many iron compounds decreases in the alkaline environment of the small intestine. Iron compounds precipitated in the gut (mainly iron hydroxides) are not absorbed by the body. For example, iron in iron fumarate and iron sulphate precipitates in the small intestine, whereas iron bisglycinate maintains good solubility also in alkaline environments [5]. Dissolved iron bisglycinate hydrolyzes in the mucosa of the small intestine, released iron is absorbed and another dissolution substance, glycine, is used by the body´s normal metabolic processes (biosynthesis of nucleic acids and proteins). Therefore, no new substances are produced by the body during the decomposition of iron bisglycinate.

The absorption of iron bisglycinate and other iron salts also depends on the person´s iron level – excess iron is not absorbed and toxic iron overdose does not occur in a normal healthy individual. Still, iron food supplements should only be used in case of physiological need.”


[1] Rossana B. et al. Efficacy of Supplementation with Iron Sulfate Compared to Iron Bisglycinate Chelate in Preterm Infants. Current Pediatric Reviews, 2018, 14, 00-00.
[2] Szarfarc S. et al. Relative effectiveness of iron bis-glycinate chelate (Ferrochel) and ferrous sulfate in the control of iron deficiency in pregnant women. Archivos Latinoamericanos de Nutricion, 2001, No. 51, p. 42-47.
[3] Milman N, et al. 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. J Perinat Med. 2014 Mar;42(2):197-206. doi: 10.1515/jpm-2013-0153.
[4] Bovell-Benjamin, A.C., et al. Iron absorption from ferrous bisglycinate and ferric trisglycinate in whole maize is regulated by iron status. The American Journal of Clinical Nutrition, 2000, 71, 1563–1569.
[5] García-Casal, M.N., et al. Vitamin A and beta-carotene can improve nonheme iron absorption from rice, wheat and corn by humans. Journal of Nutrition, 1997, 128, 646–650.

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