The purpose of this study to examine whether vitamin D can reduce the activation of the immune system during dialysis. When activated, the immune cells release certain substances, called cytokines, which can be measured from small blood samples. We want to study to what degree the immune system is activated during a regular dialysis treatment and whether the time point of vitamin D administration, either right before the start or right at the end of a dialysis treatment, has an impact on the activation of the immune system.