Vitamin D is an extraordinary nutrient – indeed it is not a ‘vital-amine’ in the true sense of the word but instead a prohormone. It is the only nutrient where our main source is not dietary intake but UVB exposure. In the UK, it is only between April to September that vitamin D can be made from direct exposure to sunlight exposure; during October to March, the Zenith angle of the sun means that the UV wavelength is not strong enough. A ‘shadow shorter than your height’ is a great way of telling whether you are in the right UVB range. Vitamin D has a key role to play in musculo-skeletal disease but its effect on other health outcomes are wide-ranging. There has been much talk in the Media of a link between vitamin D and COVID-19: this presentation will review what vitamin D is, how it is metabolised by the body and whether it is important to the prevention of acute respiratory tract infections and COVID-19. The talk will also focus on the difference between the two forms of vitamin D (plant and animal) and will share some new data on vitamin D2 vs. vitamin D3 from a systems biology angle as well as key work on vitamin D with the UK military including Sub-Mariners.