Lay summary
Observational studies have shown that low vitamin D levels are associated with an increased risk of cardiovascular disease and respiratory infections. To provide certainty about whether vitamin D protects against these diseases, a large clinical trial is proposed. The aim is to determine if taking a monthly vitamin D dose of 100,000 IU (2.5 g) over 4 years reduces the incidence of both cardiovascular and respiratory infectious diseases in 4000 Auckland adults aged 50-84 years. If vitamin D is beneficial, a number of strategies could be implemented to increase vitamin D levels in the general population, including promoting increased safe sun exposure (without getting sunburnt); increasing vitamin D fortification of food; and increasing availability of higher-dose vitamin D supplements.