Fig. 15

Predicted effects of vitamin D supplementation on 25(OH)D, healthy epithelial cells and pro-inflammatory cytokine concentration with age for seasonal dosing regime. The predicted concentration of extracellular 25(OH)D (a), volume fraction of healthy epithelial cells (b) and concentration of pro-inflammatory cytokines (c) 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. An age-dependent daily intervention of vitamin D supplements is administered from day 0 to day 152 to individuals aged 20 (blue), 40 (red), 60 (green) and 80 (orange) years old, followed by a period of no intervention from day 153 to day 365. Baseline values (no supplementation) for vitamin D intake in are \(D^{0}=4.8\) and \(D^{1}=0.5\) nmol/L day⁻¹. The age-dependent increase in production of 25(OH)D from supplements across all ages from day 0-152 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.