Iron deficiency can cause vague symptoms, including feeling tired and foggy.
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More people are low in iron than in any other micronutrient, according to the American Society of Hematology. But many live with fatigue, weakness, brain fog and other symptoms for months or years before a deficiency is caught.

Nearly one-third of adults in the United States may be iron deficient, according to a 2024 study. And routine screening among children and pregnant women — who are at high risk — may miss more than 70% of cases.