Vitamin D
Supplement if levels are low. Test before high-dose supplementation.
What it does
The supplement most often taken without testing. The right protocol is simple: test, dose into the 30 to 50 ng/mL range, retest. Correcting documented deficiency helps; supplementing already-sufficient adults helps less.
Vitamin D regulates calcium absorption and bone metabolism, modulates immune function, and acts as a signaling molecule across many tissues via the vitamin D receptor.
What the evidence shows
Overall evidence strength: Moderate certainty
Who it's for, who should skip
People with low vitamin D. Test first; sun exposure is preferable when feasible. High-dose supplementation without testing is unnecessary and occasionally harmful (hypercalcemia).
Running it
- Form
- oral
- Dose
- 1,000–4,000 IU · daily
- Titration
- adjust to labs; target blood level 30–50 ng/mL
Measuring it
Retest Vitamin D — No guideline-based retest interval — the Endocrine Society (2024) advises against routine follow-up 25(OH)D testing to guide dosing. Retest on a clinician's specific indication, not on a calendar..
Stacking & alternatives
Access & cost
Available over the counter. Look for: vitamin D3.
Last reviewed 27 August 2026.