Iron Bisglycinate vs Ferrous Sulfate: Absorption and Tolerance

Why ferrous sulfate is the reference point

Ferrous sulfate is the most commonly used ferrous iron salt, named in most clinical guidelines and the cheapest thing on the shelf. That's why trials of newer iron compounds use it as the active control.

The label arithmetic matters more than the form name. A 325 mg ferrous sulfate tablet delivers 65 mg of elemental iron, not 325 mg, and different salts carry different elemental fractions.

The tolerability problem is why anyone looks past it. Gut effects from iron salts rise with dose and are a common reason people quit. Case reports at around 130 mg of iron describe gastritis and gastric lesions.

What the chelate is supposed to do differently

Ferrous bisglycinate is a chelate: one iron atom bound to two glycine molecules, forming a stable ring.

The argument is partly chemical. Bound iron is less available to bind with phytates, fibre, and polyphenols in a meal, the compounds that make non-heme iron poorly absorbed. The second argument is about the gut lining. Less free iron in the lumen means less direct contact with the intestinal mucosa.

Both mechanisms are plausible. Neither is proof, and the trials testing them have mixed results.

What the trials found

The largest summary is a 2023 systematic review in Nutrition Reviews covering 17 randomized trials of at least four weeks, nine of them in pregnant women and four in children.

In pregnant women, bisglycinate produced higher haemoglobin than other iron supplements, a standardized mean difference of 0.54 g/dL with a 95% confidence interval of 0.15 to 0.94. Gastrointestinal adverse events were reported less often, an incidence rate ratio of 0.36 with an interval of 0.17 to 0.76. Ferritin trended higher without reaching significance.

In children, the pooled estimates found no significant difference in haemoglobin or ferritin.

An older tolerability comparison at matched 50 mg doses reported side effects in 37% of the ferrous sulfate group against 21% of the bisglycinate group.

Where the evidence disagrees

A 2023 trial in the Journal of Nutrition ran the comparison the other way.

It randomized 480 non-pregnant women in Cambodia to 60 mg of ferrous sulfate, 18 mg of ferrous bisglycinate, or placebo for 12 weeks. Ferritin at 12 weeks came in at 99 µg/L for ferrous sulfate, 84 µg/L for bisglycinate, and 78 µg/L for placebo. The adjusted difference between iron groups was 14.6 µg/L with a confidence interval of 7.6 to 21.6, and the upper bound crossed the 20 µg/L noninferiority margin set in advance. The lower dose of the chelate didn't match the higher dose of the salt.

No differences showed up in faecal calprotectin or enteropathogen detection.

Both findings can stand together. The pooled result comes from pregnant women, a group with high iron demands. The Cambodian trial enrolled a mostly iron-replete population, with baseline anemia at 17% and iron deficiency at 6%.

How strong is this evidence

Moderate at best, and worth reading with the funding in view.

The trials are small, the populations narrow, and the comparators inconsistent. The authors called for more trials outside pregnancy.

Funding is the part that gets skipped. Several trials used a branded bisglycinate chelate made by Albion, now part of Balchem. Balchem cites the 2023 analysis in its own sustainability reporting and started a pregnancy trial of its own in 2023. An earlier pregnancy trial comparing 25 mg of the chelate against 50 mg of ferrous sulfate drew commentary from the supplier's sales director in trade coverage.

None of that invalidates a randomized trial. It does mean independent replication is thin next to how confidently the form is marketed.

The safety side of iron

This applies to every form, chelate included.

The tolerable upper intake level is 45 mg per day for adults and 40 mg per day for children through age 13, set against gastrointestinal effects from iron salts.

Acute toxicity is the serious risk. Intakes above 20 mg/kg, roughly 1,365 mg for a 150 lb adult, can cause corrosive necrosis of the intestine. Severe ingestions around 60 mg/kg, roughly 4,090 mg, can lead to multisystem organ failure, coma, convulsions, and death.

Children are the concern. Between 1983 and 2000 at least 43 US children died from ingesting iron supplements, and accidental iron ingestion accounted for about a third of poisoning deaths among children reported between 1983 and 1991. FDA required unit-dose packaging and warning labels for products containing more than 30 mg of elemental iron per dose in 1997, and reported childhood deaths fell to one between 1998 and 2002. A court decision removed that packaging requirement in 2003. The warning label requirement stands.

Every iron supplement sold in solid form in the US carries this statement: accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Keep it out of reach, and call a doctor or poison control centre immediately after an accidental overdose.

Two more cautions. People with hemochromatosis should avoid iron supplements. Iron also interacts with levodopa and levothyroxine, and proton pump inhibitors can reduce its absorption.

Frequently asked questions

Is bisglycinate better absorbed?

The trials point that way in pregnant women. In children the pooled data found no difference, and one head-to-head trial found a lower dose of bisglycinate didn't match a higher dose of ferrous sulfate.

Why does ferrous sulfate upset the stomach?

Free iron in the gut lumen irritates the intestinal lining, and the effect rises with dose. The chelate is designed to reduce that exposure.

Do I need to worry about the elemental iron number?

Yes. A 325 mg ferrous sulfate tablet gives 65 mg of elemental iron, and different salts give different amounts from the same tablet weight.

Is the chelate safer for children?

No. The accidental overdose risk is identical, and iron tablets can look like sweets to a toddler.

What dose should I take for anemia?

That's a prescription decision, not a label decision, and it depends on bloodwork this page can't see.

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This article is for general information only and does not constitute medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration and are not intended to diagnose, treat, cure, or prevent any disease. Talk with your health care provider before using iron supplements, especially if you take prescription medication or have a child in the household.

Sources

• Fischer JAJ et al., Oral ferrous bisglycinate and hemoglobin and ferritin concentrations, Nutr Rev 2023
• Hou K et al., 18-mg ferrous bisglycinate vs 60-mg ferrous sulfate, noninferiority trial, J Nutr 2023
• NIH Office of Dietary Supplements, Iron Fact Sheet for Health Professionals
• NIH Dietary Supplement Label Database, ferrous sulfate 325 mg, 65 mg elemental iron
• DailyMed, ferrous sulfate 325 mg tablet labeling and accidental overdose warning
• MedlinePlus, Iron Supplements drug information and overdose warning