×
However, most sunlight-related epidemiologic research to date has relied on observational data that are subject to considerable bias and confounding. Findings from observational studies are far less rigorous and reliable than those of interventional studies. But interventional studies would need to be very large and carried out over several decades (since most UVR-mediated diseases occur later in life). Moreover, it is not at all clear when, over a lifetime, sun exposure/vitamin D is most important. So for now scientists must rely on the results of well-conducted observational analytic studies. - www.ncbi.nlm.nih.gov