Hypophosphatemia during nutritional rehabilitation in anorexia nervosa: implications for refeeding and monitoring

To determine the incidence of hypophosphatemia in adolescents with anorexia nervosa (AN) hospitalized for nutritional rehabilitation and to examine factors predisposing to its development. A retrospective chart review of 69 patients (66 female, 3 male) with AN consecutively admitted to an inpatient adolescent medical unit between July 1, 1998 and June 30, 2000. Mean age was 15.5 ± 2.4 (range 8 to 22) years and mean % ideal body weight (IBW) was 72.7 ± 7%. Serum phosphorus was measured daily for 1 week and then biweekly to weekly. Patients were started on 1200–1400 kcal/day and calories were increased by 200 kcal every 24–48 hours. Four (5.8%) patients developed moderate hypophosphatemia (<2.5 and ≥ 1.0 mg/dl) and 15 (21.7%) had mild hypophosphatemia (<3.0 and ≥ 2.5 mg/dl). Patients who developed moderate hypophosphatemia were significantly more malnourished than those who did not (p = 0.02). Phosphorus nadirs were directly proportional to % IBW (r = 0.3, p = 0.01). Over three-quarters of the patients (81%) reached their phosphorus nadir within the first week of hospitalization. The patient with the lowest phosphorus level experienced short runs of ventricular tachycardia. No other severe complications were seen. Overall, 19 (27.5%) patients required phosphorus supplementation. Phosphorus drops to its nadir during the first week of refeeding. We recommend daily monitoring of serum phosphorus with supplementation as needed during the first week of hospitalization, especially in those who are severely malnourished.