Vitamin D and omega-3 fatty acids are promising interventions for kidney disease prevention and treatment, based on results of animal-experimental models and early human studies. Because these interventions are relatively safe, inexpensive, and widely available, they may offer opportunity to substantially reduce the burden of kidney disease in large populations. This VITAL ancillary study will test whether vitamin D3 and/or omega-3 fatty acids prevent loss of glomerular filtration rate, over 4 years of therapy.