1. Department of Epidemiology and Population Health, Stanford University School of Medicine , Palo Alto, California , USA
2. Department of Epidemiology and Biostatistics, University of California , San Francisco, California , USA
3. Research Institute, California Pacific Medical Center , San Francisco, California , USA
4. Department of Medicine, University of California , San Francisco, California , USA
5. San Francisco VA Medical Center, Division of General Internal Medicine , San Francisco, California , USA
6. Buck Institute for Research on Aging , Novato, California , USA
7. National Institute on Aging, Intramural Research Program , Baltimore, Maryland , USA
8. Department of Medicine, Urology, Epidemiology and Biostatistics, University of California , San Francisco, California , USA