The immune system was built listening to vitamin D — receptors sit directly on immune cells, and the defense chemistry it regulates is old biology. But the honest version has two halves: correcting a deficiency measurably helps immunity, while megadosing on top of adequate levels does nothing. The supply line is the real story — mostly sun, and modern life is an experiment in avoiding it. This guided journey covers the mechanism, the evidence, and the test-first rule.
3 quick steps~2 minutesEvidence-awareHealth info, not a diagnosis