SO, VITAMIN D...

Actually, we need to stop you right there. Vitamin D isn’t really a vitamin but a ‘prohormone’ – a precursor to a hormone, this is an inactive substance with little effect on the body on its own, until it’s converted into an active hormone and can get things done. And, adorably, it’s supplied by sunshine. ‘UVB light within sunlight breaks down a molecule in your skin, which the liver converts into the prohormone form of vitamin D that we talk about and measure,’ says Martin Hewison, professor of molecular endocrinology at the University of Birmingham. This stuff is then converted into the active vitamin D hormone, called calcitriol, wherever there’s an enzyme up for the job – you’ll find most of these in your kidneys.