Asked by: Steven Collins, by E-mail
You don’t absorb vitamin D from the Sun, your skin synthesises it in response to sunlight. It’s the UVB portion of the ultra-violet spectrum that’s needed. This wavelength range is effectively blocked by glass so you can become deficient in vitamin D if you don’t go out and don’t eat enough oily fish or dairy products to compensate.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA