Iron forms: bisglycinate, fumarate or sulphate?
Futures Nutrition Editorial Team · 12 August 2026
Iron forms: bisglycinate, fumarate or sulphate?
The front of the pack names the compound, the back carries the number that matters. Between the authorised iron compounds the iron content differs by almost a factor of four: 100 mg of ferrous gluconate contains around 12 mg of iron, 100 mg of ferrous carbonate around 48 mg. Two products are therefore comparable only through the figure mg of iron per daily dose — never through the name of the compound, and not through the milligrams of the compound itself either.
The second point concerns the ranking that product texts like to imply. It does not exist officially. In its 2024 assessment EFSA states that the different iron forms cannot be ranked accurately and consistently with respect to their bioavailability — and that iron from ferrous sulphate is generally better absorbed than from the other authorised forms (EFSA Journal 2024;22(6):8819). What differs between the forms is tolerability rather than yield.
How much iron sits inside the compound
An iron compound consists for the greater part of everything except iron: of sulphate, fumarate, gluconate, glycine or water of crystallisation. How large the iron share is follows from the molar mass and can be calculated. The third column shows how much compound would be needed to deliver 30 mg of iron:
| Compound | Iron content (calculated) | for 30 mg of iron |
|---|---|---|
| Ferrous carbonate | 48 % | 62 mg |
| Ferrous sulphate, anhydrous | 37 % | 82 mg |
| Ferrous fumarate | 33 % | 91 mg |
| Ferric diphosphate (pyrophosphate) | 30 % | 100 mg |
| Ferrous bisglycinate | 27 % | 110 mg |
| Ferrous lactate | 24 % | 126 mg |
| Ferric citrate | 23 % | 132 mg |
| Ferrous sulphate, heptahydrate | 20 % | 149 mg |
| Ferric sodium EDTA | 15 % | 197 mg |
| Ferrous gluconate | 12.5 % | 240 mg |
These values apply to the pure compound. Commercial raw material is often hydrated or blended with carriers — for bisglycinate raw materials the iron content is therefore frequently declared lower than the 27 per cent of the pure substance. That is precisely why the nutrition table on the back is decisive and not the sum one works out oneself. Anyone holding two packs side by side compares, for the same reason, not the milligrams of the compound but what one day of intake costs. The same confusion between compound and element exists with every mineral; for zinc it is set out in Zinc forms compared.
Which forms are permitted in the EU at all
Not every compound that exists worldwide may be used here. Annex II of Directive 2002/46/EC lists exhaustively which iron sources are admissible for food supplements — in the consolidated version of 17 July 2024 there are nineteen:
Ferrous carbonate · ferrous citrate · ferric ammonium citrate · ferrous gluconate · ferrous fumarate · ferric sodium diphosphate · ferrous lactate · ferrous sulphate · ferric diphosphate (ferric pyrophosphate) · ferric saccharate · elemental iron (carbonyl + electrolytic + hydrogen reduced) · ferrous bisglycinate · ferrous L-pidolate · ferrous phosphate · ferrous ammonium phosphate · ferric sodium EDTA · iron hydroxide adipate tartrate (nano) · iron milk caseinate · iron (II) taurate
Two names were only added in 2024 — iron hydroxide adipate tartrate and iron milk caseinate —, both via the Union list of novel foods. And one name is missing that one might expect: iron picolinate is not on the list, although zinc picolinate and chromium picolinate are listed in it. Authorisation attaches to the individual compound, not to the mineral and not to the acid residue. Anyone holding a product from a non-EU shop finds the answer to the question “may this be sold here” in that list, not in the milligram figure.
Divalent or trivalent: what the gut makes of it
In the small intestine iron is absorbed predominantly as divalent iron (Fe²⁺) via the divalent metal transporter DMT1. Trivalent iron (Fe³⁺) — from ferric citrate or ferrous phosphate, for instance — has to be reduced beforehand, enzymatically by DcytB or by reducing agents such as ascorbic acid. That is the factual core behind the rule of thumb that divalent forms are the more reliable ones.
How much actually arrives, however, is decided less by the form than by one's own iron status. EFSA refers to a meta-analysis in which fractional iron absorption fell from an average of 21.9 to 5.8 per cent as serum ferritin rose. This is governed by the hormone hepcidin, which throttles absorption in the gut as soon as the stores are full. An empty store gets more out of a mediocre compound than a full one does out of the best.
Then there is the meal. As inhibitors EFSA names phytates, polyphenols from tea, coffee, cocoa, red wine and vegetables as well as calcium and zinc; as enhancers ascorbic acid and animal muscle protein. A calcium supplement such as Calcium + vitamin D3 therefore does not belong in the same serving as the iron tablet but in another half of the day. Which of these effects has actually been measured and how large the difference is in figures is set out in Iron absorption: vitamin C, coffee and tea.
Tolerability: what has been measured and what has not
The most common reason for stopping an iron supplement again is the gastrointestinal tract. That this is not a matter of imagination is shown by a meta-analysis of 43 randomised trials with 6,831 adults: under ferrous sulphate, gastrointestinal complaints occurred markedly more often than under placebo — odds ratio 2.32 (95 % confidence interval 1.74–3.08), against intravenous iron 3.05 (Tolkien et al., PLoS ONE 2015;10(2):e0117383).
For ferrous bisglycinate, which is marketed with exactly this argument, there is a systematic review of 17 studies with 2,191 participants (Fischer et al., Nutrition Reviews 2023;81(8):904–920). Its result, sorted by what it actually supports:
| Group | Result |
|---|---|
| Pregnant women, haemoglobin | higher than under the comparator preparations (SMD 0.54 g/dl; 95 % CI 0.15–0.94) |
| Pregnant women, gastrointestinal complaints | less frequent (IRR 0.36; 95 % CI 0.17–0.76) |
| Pregnant women, ferritin | no statistically confirmed difference |
| Children, haemoglobin and ferritin | no statistically confirmed difference |
The authors expressly rate the overall certainty of this evidence as very low: high risk of bias, indications of publication bias, strong heterogeneity between the studies. The advantage is thus dependable for pregnant women, and even there only with reservations; for all other groups it has not been shown. “Better tolerated” is therefore not a property of the form as such but a finding from a particular body of evidence.
One observation that regularly causes concern belongs here as well: black stools are not iron that has been absorbed in the gut. EFSA based its derivation on precisely this endpoint and does not classify it as harm as such; it marks the amount from which iron passes through the gut unused. The safe level of intake derived from it is 40 mg per day for adults including pregnant and lactating women.
What is in our preparation
Our Iron 30 mg + inulin uses ferrous fumarate, a divalent form with an iron content of around 33 per cent. In arithmetic terms the declared 30 mg of iron therefore correspond to about 91 mg of ferrous fumarate per tablet; the label states the iron amount, the list of ingredients the compound. 30 mg are 214 % of the nutrient reference value of 14 mg (Regulation (EU) No 1169/2011, Annex XIII). The usage advice is one tablet daily with a glass of water 15 to 30 minutes before a meal — that is, away from the inhibitors in the meal; why the morning is the better slot for this and what an every-other-day rhythm changes is set out in Taking iron: fasting, in the morning, every other day. Further strengths and combinations stand side by side in the Iron category.
To put the amount in context: the German Federal Institute for Risk Assessment (BfR) proposes 6 mg per daily portion for food supplements, EFSA names 40 mg per day as the safe level of intake from all sources together. An EU-wide maximum level for minerals in food supplements does not exist to this day; the entire market moves between these two figures. Whether one should supplement at all is therefore not a question of the form but of the status established — the German Nutrition Society (DGE) advises supplementation only after a deficiency has been established and under medical supervision.
Among the claims authorised for iron are these (Regulation (EU) No 432/2012): Iron contributes to normal formation of red blood cells and haemoglobin. Iron contributes to normal oxygen transport in the body. Iron contributes to the reduction of tiredness and fatigue. Iron contributes to the normal function of the immune system. They apply to the nutrient iron, irrespective of the authorised compound it comes from. Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.
Frequently asked questions
How do I recognise which compound is in it? Not from the nutrition table — that only says “iron” with the milligram figure. The compound appears in the list of ingredients, usually in the form “ferrous fumarate (iron)”. If none of the nineteen authorised designations appears there, the origin of the product is worth a second look.
Is bisglycinate better tolerated than sulphate? In pregnancy an advantage is documented, though at very low certainty of the evidence; in children the same review found no difference in haemoglobin and ferritin. At the same time EFSA regards ferrous sulphate as the generally best absorbed form. So anyone with complaints may change something by switching the form — what they certainly change is the milligram figure, provided they also look at the iron amount.
“Chelate”, “bisglycinate”, “amino acid chelate” — the same thing? Bisglycinate is a chelate of one iron ion and two glycine molecules, and glycine is an amino acid. The three terms therefore describe the same construction principle. In the EU, however, only the specific designation ferrous bisglycinate is capable of authorisation; “amino acid chelate” on its own is an advertising term without defined content.
Why do black stools not count as a side effect? Because they are not damage but an indication: they show iron that has passed through the gut unused. EFSA drew on them for precisely that reason as a measurable endpoint for its safe level of intake of 40 mg per day. Nausea, abdominal pain, vomiting and diarrhoea are a different matter — those are the complaints counted in the studies.
Sources: Directive 2002/46/EC, Annex II (consolidated version of 17/07/2024) · EFSA, Scientific opinion on the tolerable upper intake level for iron, EFSA Journal 2024;22(6):8819 · Tolkien Z, Stecher L, Mander AP, Pereira DIA, Powell JJ, Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults, PLoS ONE 2015;10(2):e0117383 (PubMed 25700159) · Fischer JAJ, Cherian AM, Bone JN, Karakochuk CD, The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children, Nutrition Reviews 2023;81(8):904–920 · BfR, maximum level proposals for iron in foods including food supplements · DGE, selected questions and answers on iron · Regulation (EU) No 1169/2011, Annex XIII · Regulation (EU) No 432/2012.
