Iron lifted mood and memory only in people actually short of it
Pooled trials in people without anaemia found gains of 0.34 to 0.53 standard deviations in fatigue, reasoning and memory. Leave out those who were iron deficient and the gains go too.
What the study found
Iron's mood and memory gains vanished in people who were not deficient
Pooled trials in people without anaemia showed iron improving anxiety, fatigue, physical wellbeing, reasoning and short-term memory, by 0.34 to 0.53 standard deviations. Attention and depression did not move. Without the participants who were iron deficient, the benefit vanished.
The caveat · The split was never formally tested
The subgroup finding that carries this claim is reported without a formal test of interaction, so it is suggestive rather than demonstrated. Six of the eighteen studies had no control group, and their effect sizes run two to three times larger, which is what an uncontrolled design predicts. Only 204 of the 1,408 participants were children or adolescents.