Determine the effect of vitamin D repletion on flow mediated dilation (FMD, a measure of endothelial function) in vitamin D deficient systemic lupus erythematosus (SLE) patients. The investigators will enroll vitamin D deficient SLE patients and randomize them to receive either 400 IU or 5,000 IU of cholecalciferol (D3) daily and measure change in FMD as a measure of EC function at baseline and after 16 weeks of repletion.