Patients defined as at risk for RFS are those who are malnourished and have recently lost weight. In addition, patients with, eating disorders, uncontrolled diabetes, cancer, malabsorption syndromes, alcoholics, surgical patients, elderly, and patients who are fasting or have undergone bariatric surgery, are high risk groups. It is well known that low levels of electrolytes are fatal, and may cause cardiac-and lung problems, affect the gastrointestinal-and the immune system, and increase the occurrence of sepsis and infections. Even severely malnourished patients have normal electrolyte levels upon admission, but when refeeding, a rapid drop in electrolytes, especially phosphate, may occur, usually within 2-5 days. Electrolytes like potassium and sodium are measured routinely, but magnesium and phosphate are not included in routine laboratory tests, and therefore often overlooked. Studies have found that patients screened as "not at risk" also develop RFS confirming the need for monitoring phosphate levels on a broader group than just patients at risk. Two main challenges regarding RFS are the absence of a clear definition and the uncertainties regarding the optimal refeeding rate. However, new onset hypophosphatemia after refeeding is a good predictor of RFS and will be used in this study when defining RFS.