vitamin D deficiency.
One of the family has been tested as low on this. No, they don't go outside (my suggestion that they do so was met with "Why the fuck would I that, I've got supplements" No, I didn't clobber them with a frying pan of spang)
Reading up on this, it's stated that the sunlight needs to be direct, that incident light through a window isn't enough. Any idea why? Glass blocking some of the spectrum maybe?