Aim The reference nutrient intake for vitamin D in people aged ≥4 years is 10 μg/day (400 IU/day) in the UK, but the recommended daily allowance is 15 μg/day (600 IU/day) for people aged 1-70 years in the US. Here, we aim to compare the 25-hydroxyvitamin D3 (25(OH)D3) serum concentration profiles between the two doses. Methods With adult trial data, we constructed a physiologically based pharmacokinetics (PBPK) model of serum concentrations of vitamin D3 and 25(OH)D3 using nonlinear mixed effects (NLME) modelling. We used this model to forecast the mean, 5% and 95% percentiles for serum 25(OH)D3 concentrations. Results Our final model uses bodyweight to adjust volume of each compartment and maximum clearance of 25(OH)D. No other covariate was identified. The model accurately predicted data from trials of a broad range of dose regimens. We simulated subjects of the average UK male and female weights with baseline 25(OH)D < 25 nmol/L. Simulation suggests circulating 25(OH)D concentrations in >5% of men and women taking 10 μg/day for a year might fail to reach 50 nmol/L, while those on 15 μg/day were predicted to attain this threshold. Conclusion The two doses generate significant difference in serum 25(OH)D3 concentrations. This needs to be considered for choosing the right dose for the public health guideline.