Iron and calcium labels: forms, doses and digestive side effects

Constipation. A heavy, unsettled stomach. More gas than usual. When digestion changes shortly after starting a new supplement, iron and calcium are two of the first ingredients worth looking at.

Both are widely used, and both have measurable digestive side effects in clinical trials. The form on the label, the dose and even the schedule can make a difference. Here's what the research shows.

Iron: the form on the label matters

Iron supplements come in several forms, listed in parentheses on the Supplement Facts panel: ferrous sulfate, ferrous fumarate, ferrous gluconate, ferrous bisglycinate and others. The milligram number shown should be the elemental iron, the iron itself.

Ferrous sulfate is the standard, inexpensive form, and it has a reputation for upsetting the stomach. A meta-analysis of 43 trials with 6,831 adults confirmed it: ferrous sulfate raised the odds of gastrointestinal side effects compared with placebo (odds ratio 2.32) and compared with intravenous iron (odds ratio 3.05). The analysis didn't find a clear relationship between the dose and the side effects (Tolkien et al., 2015).

Ferrous bisglycinate is iron bound to the amino acid glycine, and it's often marketed as gentler. A systematic review of 17 randomized trials found that, compared with other iron supplements taken for 4 to 20 weeks, it led to higher hemoglobin and fewer reported gastrointestinal side effects in pregnant women (incidence rate ratio 0.36). But most of the trials were in pregnant women and children, and the authors said more trials are needed in other groups, including non-pregnant women (Fischer et al., 2023). So the "gentle" claim has some support, just not much in women in their forties and fifties specifically.

Iron: when and how often

How you take iron may matter as much as which form you take. In the first of two randomized trials in young women with low iron stores, 60 mg of iron taken on alternate days for 28 days was absorbed more efficiently than the same dose taken daily for 14 days. Cumulative fractional absorption was 21.8% versus 16.3%, and total iron absorbed was 175.3 mg versus 131.0 mg. In the second trial, splitting the dose into two a day didn't improve absorption and raised levels of hepcidin, a hormone that limits iron uptake (Stoffel et al., 2017).

The researchers called for confirmation in people with anemia, and these women were 18 to 40. Still, it's a useful question to bring to your doctor if daily iron is hard on your stomach. Don't change a prescribed regimen on your own.

Calcium: carbonate or citrate

The two most common forms are calcium carbonate and calcium citrate. Carbonate packs more calcium into each tablet, so you often need fewer or smaller pills. Citrate provides less calcium per tablet.

Absorption may matter less than marketing suggests. In a randomized crossover study of 24 postmenopausal women, single doses of marketed calcium carbonate, encapsulated calcium carbonate and marketed calcium citrate produced identical 24-hour rises in blood calcium. The authors concluded that the two marketed products were equally absorbed and that the cheaper carbonate was more cost-effective (Heaney et al., 2001).

Digestive side effects are real, though. In a review of seven randomized trials, people taking calcium reported gastrointestinal problems more often than those on placebo, 14.1% versus 10.0%. Complaints included constipation, cramping, bloating and upper digestive symptoms. In one study, hospital admissions for functional digestive problems were 6.8% on calcium versus 3.6% on placebo (Lewis et al., 2012).

If calcium gives you trouble, it's worth asking your doctor whether you need a supplement at all, how much calcium you already get from food, and whether a different product or dose might suit you better.

Reading iron and calcium labels

• Find the elemental amount and the form. Both are in the Supplement Facts panel.
• Check the serving size. Calcium products often need two or more tablets to reach the amount shown.
• Count every source. Multivitamins, antacids and fortified foods can contain both minerals.
• Read the iron warning. Iron products carry a warning about accidental overdose in children. Store them out of reach.
• Treat "gentle" as a claim to check, not a promise. Look for the form and the dose.

When to see a doctor

Iron shouldn't be a guess. Anemia or low iron has a cause, and finding it matters, so get tested before you start and follow up afterward. See your doctor promptly for black, tarry stools or blood in your stool, unexplained weight loss, new constipation that doesn't settle, or digestive symptoms that wake you at night.

Sources

1. Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383. PMID 25700159
2. Fischer JAJ, Cherian AM, Bone JN, Karakochuk CD. The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2023;81(8):904-920. PMID 36728680
3. Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533. PMID 29032957
4. Heaney RP, Dowell MS, Bierman J, Hale CA, Bendich A. Absorbability and cost effectiveness in calcium supplementation. J Am Coll Nutr. 2001;20(3):239-46. PMID 11444420
5. Lewis JR, Zhu K, Prince RL. Adverse events from calcium supplementation: relationship to errors in myocardial infarction self-reporting in randomized controlled trials of calcium supplementation. J Bone Miner Res. 2012;27(3):719-22. PMID 22139587

This article is for general information and isn't medical advice. Talk to your healthcare provider before starting or changing iron or calcium supplements, especially if you take thyroid medication or other prescriptions.