Institute for Complementary and Integrative Medicine, University Hospital Zurich and University Zurich, Zurich, Switzerland. Faculty of Health Sciences, Universidad Francisco de Vitoria (UFV), Madrid, Spain. Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), Unidad de bioestadística clínica, Hospital Universitario Ramón y Cajal, Madrid, Spain. The Biomedical Research Center Network for Epidemiology and Public Health (CIBERESP), Madrid, Spain. Institute for Social Medicine, Epidemiology and Health Economics, Charité-Universitätsmedizin Berlin, Berlin, Germany. Center for Integrative Medicine, University of Maryland School of Medicine, Baltimore, MD, USA.