VitaminWell-studiedVitamin

Vitamin D3

Half-life

15–25 days

What is Vitamin D3?

Hormone-like vitamin produced by skin on UV exposure. Critical for calcium metabolism, immunity, and testosterone.

How Vitamin D3 works

Converted to 1,25-dihydroxyvitamin D3 (calcitriol), which binds nuclear vitamin D receptor (VDR) to regulate >1000 genes — calcium absorption, immune cell differentiation, insulin secretion, and testosterone production.

Benefits & uses

Supports bone density (calcium absorption)
Immune function
Mood/hormonal support
Corrects common deficiency

Common uses: Bone density, immune regulation, testosterone support, mood, cardiovascular health.

LongevityRecovery

Vitamin D3 dosage

Typical dose: Maintenance: 2000–5000 IU/day with fat-containing meal. Deficiency correction: 10,000 IU/day for 8–12 weeks. Target 25-OH-D: 50–80 ng/mL.

Vitamin D3 half-life

15–25 days (25-OH form)

Vitamin D3 side effects

Hypercalcemia at very high doses (>10,000 IU/day long-term). Take with K2 to direct calcium.

Hypercalcemia with chronic overdose (fatigue, confusion, calcium deposits)
Always co-supplement K2 MK-7 (100–200 mcg) to direct calcium to bone
Monitor serum 25-OH-D levels periodically
Well tolerated in normal doses
Excess (very high chronic doses) can raise calcium

Warnings & monitoring

Best paired with K2 so raised calcium goes to bone, not arteries

Vitamin D3 interactions

with Thiazide Diuretics
Caution · moderate
Thiazide diuretics reduce urinary calcium excretion. Combined with high-dose Vitamin D3 (which increases calcium absorption), this can lead to hypercalcemia — elevated calcium causing cardiac and renal complications.
Timing: Monitor serum calcium if taking both; avoid high-dose D3 supplementation
with Testosterone
Compatible
Vitamin D3 acts as a steroid hormone and supports testosterone production via VDR expression in Leydig cells. Deficiency is correlated with low testosterone. Beneficial combination.
Timing: Vitamin D3 taken daily with K2 and a fat-containing meal

Commonly stacked with

Vitamin K2 MK-4 Magnesium Glycinate

Track Vitamin D3 in Protokol.

Log doses, model half-lives, check interactions, and line it all up against your bloodwork. Free on iPhone and Android.