A 67-year-old, 60-kg homeless man has been in the intensive care unit (ICU) for a week after an emergency laparotomy and sigmoid resection for perforated diverticulitis. His serum albumin is 1.1 g/dL. He was just weaned from mechanical ventilation. His colostomy is not functioning. You start total parenteral nutrition (TPN) to deliver 1800 kcal/24 h. Two days later, the patient is in respiratory distress and requires reintubation and mechanical ventilation. You should check the level of serum