People said they felt less tired; nothing measurable changed
Eighteen pooled trials found a moderate drop in self-reported tiredness and no change in anything objective. The two trials that raised iron stores furthest found nothing at all.
What the study found
Tiredness without anaemia is where the iron evidence stops agreeing
Eighteen pooled trials showed a moderate drop in how tired people said they felt, and no change in anything measured objectively. The two placebo-controlled trials that raised iron stores furthest, one of them by 114 micrograms per litre, found no change in fatigue at all.
The caveat · Iron darkens stool, so hard to blind
The pooled fatigue result rests on 4 of the 18 trials, on self-reported scales, and iron is hard to blind because it darkens stool. On the other side, both null trials ran in blood donors, a healthy group whose average fatigue was about 4 out of 10, leaving less room to improve. One of them lasted four weeks, and its own authors note the donation had not made these women noticeably tired.
What we don't know
In people low on iron but not anaemic, iron refills the stores. Whether that eases tiredness is open: pooled trials found people reporting less of it, while two placebo-controlled trials, one refilling stores more fully, found nothing.