Why do vitamin D supplements come in different doses?

The vitamin D shelf runs from 400 IU to 5,000 IU — a twelvefold range that looks like chaos and is actually three different jobs in one aisle: the official prevention dose, the common maintenance middle, and the correction territory that properly belongs to prescriptions and lab results. Add the unit confusion (IU and mcg measuring the same thing two ways) and the shelf becomes unreadable. This guided journey decodes the ladder, then hands you the rule that makes it simple.