Latent Vitamin D Reserves Calculator

Pharmacokinetics of cholecalciferol and 25-hydroxyvitamin D

Cholecalciferol (vitamin D₃) synthesised in the epidermis from 7-dehydrocholesterol enters the bloodstream bound to vitamin D-binding protein (DBP). In the liver it is 25-hydroxylated by the CYP2R1 enzyme, producing 25-hydroxyvitamin D [25(OH)D] — the metabolite measured clinically because it best reflects overall vitamin D status. A second hydroxylation in the kidney (CYP27B1) yields the biologically active hormone 1,25-dihydroxyvitamin D (calcitriol), which is tightly regulated by parathyroid hormone, calcium, and phosphorus.

Circulating 25(OH)D has an apparent half-life of 15–25 days when studied in short-duration tracer experiments (Jones 2008; Vieth 1999). However, the whole-body terminal half-life is considerably longer — around 60 days in lean adults and 80–90+ days in obese adults — because cholecalciferol partitioned into adipose tissue re-enters the circulation over months. This adipose sequestration explains why obese individuals often show lower serum 25(OH)D at any given UVB dose (Wortsman 2000; Drincic 2012), and why winter decline curves in temperate populations follow an exponential trajectory with a time constant of roughly two months (Kimlin 2007; Webb 2010).

The two-compartment decay model used here

The calculator assumes a first-order elimination process with an effective terminal half-life scaled to body-mass index. Peak summer serum 25(OH)D is estimated from a pigmentation-, age-, and dose-adjusted anchor point (Fitzpatrick II, 90 min of near-noon UVB per week, exposed fraction ≈ 0.28 → 40 ng/mL, per Holick 2007). The model then projects forward from your entered last-effective-exposure date using C(t) = Cpeak·e−λt where λ = ln(2)/t½. Daily supplemental cholecalciferol asymptotically raises steady-state serum 25(OH)D by ~1 ng/mL per 100 IU/day in lean adults (Heaney 2003), and this contribution is added with the same time constant.

Clinical thresholds

• < 12 ng/mL (30 nmol/L) — severe deficiency; risk of rickets in children and osteomalacia in adults.
• 12–20 ng/mL — deficient by the IOM 2011 threshold; associated with secondary hyperparathyroidism.
• 20–30 ng/mL — insufficient by the Endocrine Society 2011 threshold; may not fully optimise musculoskeletal outcomes.
• 30–50 ng/mL — the range most guidelines consider sufficient for general population health.
• > 100 ng/mL — potential toxicity; hypercalcaemia possible with sustained levels above ~150 ng/mL.

Limitations

This is a population-level model and cannot capture individual variability in DBP polymorphisms, CYP2R1 activity, renal function, malabsorption, or medication interactions (e.g., glucocorticoids and anticonvulsants can accelerate 25(OH)D catabolism). If your estimate suggests insufficiency and you have symptoms — bone pain, unexplained fatigue, frequent infections — please seek a serum 25(OH)D measurement from a healthcare provider rather than relying on a modelled projection.

Key citations

• Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281.
• Holick MF et al. Evaluation, treatment, and prevention of vitamin D deficiency: Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.
• Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press; 2011.
• Wortsman J et al. Decreased bioavailability of vitamin D in obesity. Am J Clin Nutr. 2000;72(3):690–693.
• Drincic AT et al. Volumetric dilution, rather than sequestration, best explains the low vitamin D status of obesity. Obesity (Silver Spring). 2012;20(7):1444–1448.
• Jones G. Pharmacokinetics of vitamin D toxicity. Am J Clin Nutr. 2008;88(2):582S–586S.
• Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr. 1999;69(5):842–856.
• Heaney RP et al. Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr. 2003;77(1):204–210.
• Kimlin MG. Geographic location and vitamin D synthesis. Mol Aspects Med. 2008;29(6):453–461.
• Webb AR et al. The role of sunlight exposure in determining the vitamin D status of the UK white adult population. Br J Dermatol. 2010;163(5):1050–1055.