For how we live.
When adults see a doctor for new fatigue, fewer than one in ten turn out to have clear physical disease. The common, treatable and often-missed exception is iron deficiency, which drains energy before it causes anaemia.
Feeling cold when others are comfortable can point to an under-active thyroid or iron-deficiency anaemia. Fingers and toes that turn white then blue in the cold are usually Raynaud's, a blood-vessel spasm.
In people who are iron-deficient without anaemia, supplements ease self-reported fatigue in trials — but do nothing for objective exercise capacity, and the benefit hinges on being iron-deficient, not just tired.
Classic B12 deficiency is relatively uncommon, but it clusters: in people over 50 whose absorption fails, in vegans whose intake does, and around metformin, acid-lowering drugs and stomach surgery.
Adults need about 2.4 micrograms a day, and most meat-eaters get it easily. Real deficiency clusters in older people, whose absorption fails, and in vegans, whose intake does.
Iron does leach from a cast-iron pan into food, and randomised trials in iron-deficient populations found it raised haemoglobin. The effect depends on what you cook, and means little for people who are iron-replete.
Iron shortfall is the leading cause of anaemia in young children and is linked to poorer development. Yet the evidence that screening symptom-free toddlers or routinely supplementing them helps their brains is thin.
Two large African trials gave anaemic pregnant women intravenous ferric carboxymaltose or oral iron. Neither found less anaemia near term, or heavier babies, with the infusion.
ITACS randomised 955 anaemic adults to intravenous iron or placebo before elective cardiac surgery. Fewer patients needed a transfusion, but the primary benefit was a median of one day alive and at home.
In one study of 62,685 women, iron deficiency prevalence ranged from 3% to 15% depending on the ferritin threshold. On treatment, a recent trial found alternate-day iron caused fewer side effects than daily dosing.
Across 84,348 hospital admissions, 19.8% of children had severe anaemia; as malaria receded the burden moved to older children, and severe anaemia doubled the odds of dying.