Vitamin D: how much is actually enough in winter?

Between October and March, most people living above 40° latitude make almost no vitamin D from sunlight. Stores built over summer drain through winter, and by February a large share of the population is genuinely low.

The number worth knowing

We look for 25-hydroxyvitamin D above 30 ng/mL, and ideally in the 40 to 60 range. Below 20 is a deficiency worth treating properly. Above 100 is not better — it raises calcium and does nothing useful.

Dose follows the result

Someone at 18 ng/mL needs a different protocol from someone at 34, and body weight changes both. That is the whole argument for testing before supplementing: the same pill does very different things to two people.

Retest twelve weeks after starting. If the number has not moved, absorption is the problem, not the dose — and that is worth investigating.