Daily iron cuts fatigue in menstruating women, but doubles gut trouble

Iron-deficiency anaemia is highly prevalent among non-pregnant women of reproductive age worldwide, with the heaviest burden falling on lower-income settings. Low iron has been linked to a range of adverse health outcomes, and restoring iron stores with supplements has long been assumed to resolve them. Yet despite decades of trials in menstruating women, that evidence had never been synthesised in a systematic review before this one. The review set out to establish what daily iron actually does for anaemia, iron status, physical performance, psychological health and cognition, and what side effects come with it.

The authors searched major medical databases and trial registries, screening 31,767 records. They included 67 randomised or quasi-randomised trials recruiting 8506 menstruating women, comparing daily oral iron, taken at least five days a week at any dose, against placebo or control. Outcomes spanned anaemia, haemoglobin concentration, iron deficiency, adverse effects, cognitive function and exercise performance. The number of women analysed varied greatly between outcomes; haemoglobin concentration had the most data, drawing on 6861 women.

Women taking iron were far less likely to be anaemic at the end of the intervention than controls, their haemoglobin was higher on high quality evidence, and their risk of iron deficiency fell. Iron also improved maximal and submaximal exercise performance and appeared to reduce symptomatic fatigue. The costs were digestive: hard stools or constipation and loose stools or diarrhoea were each roughly twice as common on iron, both on high quality evidence, with a suggestion of more abdominal pain too. Nausea showed no clear increase, adherence looked similar between groups, and the effect on cognitive performance stayed uncertain because individual studies conflicted and could not be pooled.

As the first systematic synthesis of these trials, this review turned a widely assumed benefit into quantified evidence, and a pooled analysis of this size sits near the top of the evidence hierarchy for a question like this. It confirms that daily iron works for anaemia and iron status in menstruating women and extends the picture to exercise performance and fatigue. Gaps remain: only one small study specifically reported iron-deficiency anaemia as an outcome, none reported mortality, and only 10 of the 67 trials were judged reliably designed. Optimal dosing and long-term effects were beyond what the included trials could answer.

For menstruating women who feel persistently flat or find training performance stubbornly low, this is a reason to take iron status seriously as a possible factor, since restoring iron improved measured exercise performance and reduced symptomatic fatigue in these trials. It is equally a reason not to treat iron supplements as a harmless default, because digestive side effects roughly doubled. The balanced reading is that daily iron has well-established benefits where anaemia or deficiency is in play, alongside a genuine trade-off in gut comfort that is worth knowing about before assuming a supplement is a free win.