Vitamin D kinetics in the acute phase of critical illness: A prospective observational study

•The vitamin D kinetics in critically ill patients was studied.•The vitamin D serum level is changeable in the initial phase of a critical illness.•The median of the vitamin D level decreases until the fourth measurement then stabilizes and finally increases unevenly.•One of the most likely reasons for such an observation could be a hypothesis that the serum vitamin D concentration mirrors the severity of illness.