A mathematical model is described for estimating changes in plasma 25-hydroxyvitamin D (25(OH)D) levels throughout the year as a consequence of varying the oral intake of vitamin D and the behaviour outdoors of white British adults resident in different regions of the UK. The model yields seasonal and geographical patterns of 25(OH)D concentrations that agree closely with observational studies. Use of the model allows estimates to be easily made of the sun exposure and oral intake necessary to avoid vitamin D deficiency in defined proportions of the population, as well as strategies that would lead to vitamin D sufficiency throughout the year. The analysis demonstrates that addressing concerns about insufficient vitamin D levels, especially during the winter, may be achieved by modifying oral vitamin D intake over the winter, increasing summer sun exposure or a combination of both.