Acknowledgements
We thank the staff at the Library, Viborg Regional Hospital. This work was supported by “Jacob og Olga Madsens Fond”, the Danish Cancer Society, grant no. DP00027, the grant ENGAGE from the Novo Nordisk Foundation and a grant from the Danish Ministry of Health, Research Centre for Environmental Health's Fund, ENGAGE, and the Danish Research Council (grant no. 09-073597).
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.