GC and malnutrition: Several factors may contribute to vitamin D deficiency in critically ill patients with prolonged hospitalization including lack of exposure to sunlight, malnutrition, decreased renal hydroxylation and increased tissue conversion of 25(OH)D3 to 1,25(OH)2D3 however the reasons for the reduced serum level of vitamin D in the first day of hospitalization may be the reduced serum level of albumin or vitamin D binding protein, or receiving intravenous volumes in order to correct hypovolemia or hypotension37.