Population based studies have shown that over the past few years, in many countries, blood levels of vitamin D are falling over time. It has been speculated that the reasons are multifactorial. It could be due to changes in nutrition but it seems more likely that it is due to the observable changes in lifestyle where people for example spend more time indoors than previously, where the use of suntan blocks are more widespread and where some people use clothes to cover up more than they used to.