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