Why is vitamin D3 different from vitamin D2?

The vitamin D shelf carries two molecules doing the same job at different efficiencies: D3 — cholecalciferol, the form human skin manufactures — and D2, the plant and fungal form that dominates vegetarian prescriptions for mostly historical reasons. The difference isn't theoretical: per unit, D3 raises and sustains blood levels meaningfully better. This guided journey covers the two molecules, the race between them, and what matters on a label.