What is the fine-grained topic of the following text: by Alan R. Gaby, MD 247,574 individuals of all ages from Copenhagen, Denmark, whose serum 25-hydroxyvitamin D level had been measured by their general practitioner between April 2004 and January 2010 participated in a retrospective observational study to find out if there is an association between serum 25-hydroxyvitamin D levels and all-cause mortality. Only return the topic.
Association between serum 25-hydroxyvitamin D levels and all-cause mortality.