I will be taking vit D supplements this winter, starting when the clocks go back and stopping when they go forward. The last few winters I've been constantly ill catching everything going, depressed and lacking energy. Some days I spend half my time at work sitting in the truck feeling completely wiped out. This winter just gone was particularly bad. It cannot be a coincidence that is was also one of the darkest and wettest on record with no sun at all up here for over two months.
I realised I was severely Vit D difficient in March when bright sunlight made an appearance after months of semi-darkness and I instantly felt tons better, like I'd been magically cured of some disease.
That'll be the only sups I take.