Patients hospitalized in intensive care units (ICU) are particularly susceptible to vitamin D3 deficiencies. This can be due to the severity of their underlying disease, the type of treatment they are on, malnutrition before and inadequate nutrition during the hospitalisation preceding ICU admission, as well as advanced age. It has also been established that plasma levels of 25(OH)D3 tend to systematically decrease during ICU treatment. Therapeutic interventions administered in ICU settings such as fluid resuscitation or extracorporeal therapies can cause additional vitamin D3 deficiencies. The incidence of deficiency in critically ill patients can reach up to 90%, and even 30% of ICU patients can have undetectable plasma levels. It is impossible to replenish vitamin D3 levels in critically ill patients with traditional enteral and parenteral nutrition treatment regimens, because nutritional products contain too little of the vitamin. Vitamin D3 deficiency in critically ill patients has been associated with acute kidney injury, acute respiratory failure, sepsis, septic shock and increased all-cause ICU mortality. Despite that, assessment of plasma 25(OH)D3 levels is not a routine practice in ICUs. In view of the prevalence of vitamin D3 deficiencies in ICU patients, rapid replenishment of this deficiency with an increased supplementation dose should be considered as a potential means to improve prognosis in this patient population. The current standard therapy is the administration of 500,000 IU of vitamin D3 via the enteral route in ICU patients with severe deficiency (recommended by ESPEN). The NephroD study is meant to help answer the question whether increasing the standard ICU supplementation dose of vitamin D3 by 50% will ensure a more effective replenishment of this vitamin in critically ill patients undergoing CRRT.