Fig. 13

Predicted effects of vitamin D supplementation on 25(OH)D and its metabolites with age. The predicted concentrations of (a, b) extracellular 25(OH)D, (c, d) extracellular 1,25(OH)₂D, (e, f) intracellular 25(OH)D, (g, h) intracellular 1,25(OH)₂D and (i, j) the VDR:1,25(OH)₂D complex with age, from solving Eqs. (3)–(23) with baseline parameters given in Tables 1, 2 and 3 and age functions given in Figs. 2, 3, 4 and 5. A constant (solid lines) or age-dependent (dashed lines) daily intervention of vitamin D supplements is administered from day 0 to day 200 to individuals aged 20 (blue), 40 (red), 60 (green) and 80 (orange) years old. Baseline values (no supplementation) for vitamin D intake in (a,c,e,g,i,k) are \(D^{0}=4.8\) and \(D^{1}=0.5\) nmol/L day⁻¹ and in (b,d,f,h,j,l) are \(D^{0}=3.2\) and \(D^{1}=0.35\) nmol/L day⁻¹. The fixed increase in production of 25(OH)D from supplements across all ages is 1.3 nmol/L day⁻¹ (solid lines) and the age dependent increase is 1.3 nmol/L day⁻¹ at 20 years, 2 nmol/L day⁻¹ at 40 years, 2.7 nmol/L day⁻¹ at 60 years and 3.7 nmol/L day⁻¹ at 80 years (dashed lines).