Supplementing blind is common and mostly harmless — but testing first tells you the dose, and whether you need one at all.
Aug 5, 2026
5 min read
Dr. Arjun Mehta
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.
Dr. Arjun Mehta
Internal medicine, Verdia
