Vitamin D Half-Life Explained
"Half-life" is the time for a substance's concentration to fall to 50% of its peak. Vitamin D has three different half-lives depending on which molecule you're measuring — cholecalciferol itself, the circulating 25(OH)D metabolite, or the whole-body pool including adipose stores.
| Molecule | Half-life |
|---|---|
| Cholecalciferol (D3) | ~24 hours |
| 25(OH)D — apparent | 15–25 days |
| 25(OH)D — effective terminal (lean) | ~60 days |
| 25(OH)D — effective terminal (obese) | 80–90+ days |
| 1,25-dihydroxyvitamin D (calcitriol) | ~4–6 hours |
Why body composition matters
Cholecalciferol is fat-soluble and partitions into adipose tissue. In lean adults, the adipose compartment is small and turns over relatively quickly. In obese adults, the compartment is larger and releases stored D3 more slowly — which shows up as a longer terminal half-life on serum 25(OH)D but a lower peak for the same input, because the same D3 molecule is more dilute across a larger fat mass.
Practical implications
- Weekly dosing works. A dose of 10,000 IU once a week gives essentially the same steady-state 25(OH)D as 1,400 IU/day.
- Summer stores sustain winter. Peak summer 25(OH)D can carry you into February at temperate latitudes, but not through spring — see our reserves calculator.
- Change dose, wait 3 months. Any dose change takes ~3 half-lives (roughly 6 months) to fully approach the new steady state; 90 days is when the trend is clear.
- Obese people need more. Roughly 2× the standard dose to reach the same 25(OH)D target.