Two healthy adults can need a five-fold different vitamin D dose to reach the same blood level. That is not a rounding error - it is why the flat 400 IU in a standard multivitamin leaves so many people low.
What moves your requirement
- Sun exposure. Under 30 minutes of midday sun per day is the single biggest driver of low levels - and an office schedule north of Rome makes winter synthesis nearly zero.
- Body weight. Vitamin D is fat-soluble and dilutes into adipose tissue; requirements scale roughly with body mass.
- Skin tone. Melanin is natural sunscreen. Darker skin can need several times longer in the same sun for the same synthesis.
- Age. Skin synthesis efficiency falls steadily over the decades.
Finding your dose
For most adults the maintenance range lands between 1,000 and 4,000 IU daily, taken with a meal containing fat. The honest answer to "how much exactly" comes from a 25-OH-D test: start, re-test after ten to twelve weeks, adjust. Without a test, your sun habits, weight and latitude give a solid first estimate - which is precisely how the Mythamin quiz sets your starting dose before any lab work.
