Iron pills, low iron and your gut: questions answered

Few supplements get as many complaints as iron. Constipation, nausea, stomach cramps and dark stools are all part of its reputation. At the same time, low iron is common in women, and it can leave you feeling drained.

So what do you do when the fix upsets your gut? And when is low iron a sign that your gut itself needs a closer look? Here are answers to the questions women ask most.

Why am I low in iron in the first place?

Iron deficiency has many possible causes, including low intake, poor absorption and blood loss. Because blood is rich in iron, ongoing blood loss, such as from menstrual periods or from the digestive tract, is a common mechanism (Snook et al., 2021).

For women who still have periods, heavy bleeding is often part of the explanation. But it's worth confirming rather than assuming, especially if your periods have become lighter, irregular or have stopped.

Why do iron pills upset my stomach so much?

It isn't in your head. A meta-analysis of 43 randomized trials with 6,831 adults found that ferrous sulfate significantly increased the risk of gastrointestinal side effects. The odds ratio was 2.32 compared with placebo and 3.05 compared with intravenous iron (Tolkien et al., 2015).

Interestingly, the analysis didn't find a relationship between the iron dose and the risk of side effects (Tolkien et al., 2015). So a smaller pill isn't guaranteed to solve the problem, though it may still be worth discussing with your doctor.

Does taking iron every other day help?

It may help your body absorb more of each dose. In two small trials in iron-depleted women aged 18 to 40, iron given on alternate days was absorbed better than iron given on consecutive days. Cumulative fractional absorption was 21.8% with alternate-day dosing and 16.3% with daily dosing. Splitting the dose into two doses a day didn't improve absorption (Stoffel et al., 2017).

The reason appears to be a hormone called hepcidin. Daily and split dosing raised hepcidin, which reduces how much iron you absorb from the next dose (Stoffel et al., 2017).

Better absorption hasn't always meant better results, though. In a randomized trial of 200 adults with iron deficiency anemia, 120 mg of iron every other day and 60 mg every day raised hemoglobin by similar amounts over eight weeks, with no significant difference between the groups (Pasupathy et al., 2023). The right schedule depends on how low your levels are and how you tolerate iron, so decide it with your doctor.

Why would my doctor want to look at my gut?

Because the digestive tract is a common source of hidden blood loss and poor absorption. According to British Society of Gastroenterology guidelines, about a third of men and postmenopausal women with iron deficiency anemia have an underlying abnormality, most commonly in the digestive tract. Digestive cancers can show up this way, often without specific symptoms (Snook et al., 2021).

Celiac disease is another cause worth checking. In a meta-analysis of 18 studies including 2,998 people with iron deficiency anemia, about 1 in 31 had biopsy-confirmed celiac disease (Mahadev et al., 2018).

Depending on your age, history and symptoms, your doctor may suggest a celiac blood test, an upper endoscopy, a colonoscopy or both procedures. Examining both the upper and lower digestive tract, called bidirectional endoscopy, is the standard approach when a digestive cause is being investigated (Snook et al., 2021).

I still have periods. Do I really need all that?

Not necessarily. The decision depends on how heavy your periods are, your age, whether your iron levels recover with treatment, and whether you have other symptoms or a family history of colorectal cancer or celiac disease.

What matters is that the cause is actively considered rather than assumed. If your iron keeps dropping despite treatment, that's a reason to revisit the question.

Can I just start iron if I feel tired?

It's better to get tested first. Tiredness has many possible causes, and a blood test can show whether your iron is actually low and by how much. Starting iron without a diagnosis can also delay finding out why your levels dropped.

When should I call my doctor?

• Black, tarry or sticky stools, or any blood in your stool. Iron can darken stool, but don't assume that's the reason.
• Iron deficiency after menopause
• Low iron that doesn't improve with treatment, or keeps coming back
• Weight loss you can't explain, trouble swallowing or ongoing abdominal pain
• Side effects bad enough that you've stopped taking your iron

Sources

1. Snook J, Bhala N, Beales ILP, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021;70(11):2030-2051. PMID 34497146
2. 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
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. Pasupathy E, Kandasamy R, Thomas K, Basheer A. Alternate day versus daily oral iron for treatment of iron deficiency anemia: a randomized controlled trial. Sci Rep. 2023;13(1):1818. PMID 36725875
5. Mahadev S, Laszkowska M, Sundström J, et al. Prevalence of celiac disease in patients with iron deficiency anemia: a systematic review with meta-analysis. Gastroenterology. 2018;155(2):374-382.e1. PMID 29689265

This article is for general information and isn't medical advice. Talk to your healthcare provider before starting iron, and don't change a prescribed iron regimen without their input.