Arthritis Research UK Centre for Epidemiology, Division of Musculoskeletal and Dermatological Sciences, School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester, M13 9PT, UK. NIHR Manchester Musculoskeletal Biomedical Research Unit, Central Manchester NHS Foundation Trust, Manchester Academic Health Sciences Centre, Manchester, UK. MRC Human Nutrition Research, Elsie Widdowson Laboratory, Cambridge, UK. Gerontology and Geriatrics, Department of Clinical and Experimental Medicine, KU Leuven, Leuven, Belgium. Center for Metabolic Bone Diseases, University Hospitals Leuven, Leuven, Belgium. Radiology and Manchester Academic Health Science Centre, The Royal Infirmary, The University of Manchester, Manchester, UK. Clinical and Experimental Endocrinology, Department of Clinical and Experimental Medicine, KU Leuven, Leuven, Belgium. Andrology Research Unit, Centre for Endocrinology and Diabetes, University of Manchester, Manchester, UK. Arthritis Research UK Centre for Epidemiology, Division of Musculoskeletal and Dermatological Sciences, School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester, M13 9PT, UK. terence.o'neill@manchester.ac.uk. NIHR Manchester Musculoskeletal Biomedical Research Unit, Central Manchester NHS Foundation Trust, Manchester Academic Health Sciences Centre, Manchester, UK. terence.o'neill@manchester.ac.uk.