Current understanding of the benefits and harms of vitamin D (with or without calcium) supplements in the general population is chiefly limited by the difficulties in evaluating true vitamin D status. No methods are currently available to quantify the contribution of endogenous vitamin D synthesis resulting from sun exposure on an individual or a group, and serious limitations remain in accurately estimating dietary vitamin D intake because of the incompleteness of nutrient databases for both vitamin D–fortified food and vitamin D supplements.

Moreover, the addition of vitamin D supplements to vitamin D taken in through all other means may exceed the safe level, resulting in harmful events. This caution is supported by the recent finding from a randomized, double-blind, placebo-controlled trial examining the effects of a single annual megadose of vitamin D3 (500 000 IU, equivalent to approximately 1370 IU/d) on fall and fracture outcomes in community-dwelling elderly women with a history of fall or fracture. This RCT used the highest daily dose of vitamin D3 of all included RCTs and demonstrated that the megadose increased the risk for fractures and falls. Future study is needed to evaluate the appropriate dose and dosing regimens of vitamin D supplementation for bone health outcomes. Evidence is not sufficiently robust to draw conclusions regarding the benefits or harms of vitamin D supplementation for the prevention of cancer.