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Vitamin D is derived mainly from sun exposure. There is a growing consensus that most people in the UK do not receive enough sun exposure to achieve satisfactory vitamin D blood levels, and that current recommendations on the optimal daily dose of vitamin D should be increased. People with low vitamin D blood levels seem to be at increased risk of several diseases, including heart disease, various infections and some types of cancer. This conclusion is supported by laboratory studies showing that many different types of cell are affected by vitamin D.

However, this evidence is not strong enough for doctors to recommend that everyone should take them, as similar evidence on the expected health benefits for various other vitamins have been disproved in large randomised trials. A trial in which 20,000 people aged 65-84 (at higher risk of deficiency) are randomised, is now needed to show what the real benefits would be.

The Vitamin D and Longevity (VIDAL) Trial is a mixed-design randomised feasibility study sponsored by the London School of Hygiene and Tropical Medicine and funded by the National Institute for Health Research (NIHR). The trial recruited 1615 participants via 20 GP practices across the country, with a view of trying and testing methodology and operating procedures for the main trial.