With the darker nights (& days around here!) should you add vitamin D to your supplements chest..well this study suggests "yes". I first got wind of this study in this article. Obviously the study is on young girls, but seeing as normal intakes (anything up to 5,000 IU's per day) have no bad effects on the human body, do you want to risk lowering your strength levels as the days get colder? I try to take 3-5,000 IU's per day over the winter (D2). Personally I go for the more expensive, but organic viridian brand of vitamin D, yep there is a vegan D3 available these days, but most research points to D2 & D3 being assimilated at a similar level (albeit slightly different pathways I believe-not 100% of those differences in detail, but I seem to remember there being a slight variation in the method they are utilised?). Anyway here's the abstract of the study.
Basically it found the lower the vit D the lower they tended to be able to jump in this test, implying a loss of muscle power (when matched for height/weight with other girls). It could be other factors as they didn't do the obviously follow-up of introducing vit D & seeing if jumping improved in the lower performers, so you can't say for sure with one study that low D = less muscle power, but you can say there was a correlation in this study & it should be looked at again to decide what levels of vit D produce what results & what level will optimise muscle function (if it turns out that this result is correct), so still a lot to do!
But it's something to consider.